ScienceCBSE Class 10 · Chapter 4

Carbon and its Compounds Class 10: Notes and Worked Examples

Q: Do I need to learn diamond, graphite and fullerenes for the board exam?

These appear in a More to Know box in the chapter. Information in NCERT text boxes is not assessed in the board exam, so treat them as interesting background. Spend your time on the main text topics instead.

Q: Do I need to write position numbers like 1-chloropropane?

The NCERT chapter names compounds such as chloropropane, propanol and butanone without position numbers. Learn the NCERT method: count carbons, add the prefix or suffix, drop the final e before a vowel suffix, and use ene or yne for unsaturation. That is what the syllabus covers.

Q: How do I test whether a hydrocarbon is saturated or unsaturated using the NCERT chapter?

Burn a small amount as in the NCERT burning activity. Saturated hydrocarbons generally give a clean flame, while unsaturated ones give a yellow flame with lots of black smoke and leave soot on a metal plate held above. Remember that a poor air supply can make even a saturated fuel sooty.

Q: Why is changing ethanol into ethanoic acid called oxidation?

Oxygen is added to ethanol by an oxidising agent such as alkaline potassium permanganate or acidified potassium dichromate. Adding oxygen is oxidation. You can also see that ethanol has one oxygen atom while ethanoic acid has two.

Q: What is the main difference between soaps and detergents?

Soaps are sodium or potassium salts of long-chain carboxylic acids, and they form insoluble scum with the calcium and magnesium salts in hard water. Detergents also have a long hydrocarbon tail but a different charged head, of a sulphonate or ammonium type. This head does not combine with calcium or magnesium ions to form an insoluble solid, so detergents keep working in hard water.

Updated 14 October 2026 · Fact-checked

Carbon and its Compounds explains why carbon forms covalent bonds, how catenation and tetravalency give millions of compounds, and how they are named and react. The board asks electron dot structures, naming and isomers, combustion, oxidation, addition and substitution equations, properties and uses of ethanol and ethanoic acid, and how soap cleans and why detergents work in hard water.

What Carbon and its Compounds is about

This chapter is about one element, carbon, and the huge family of compounds it makes. You start with why carbon shares electrons instead of forming ions, then see how catenation and tetravalency let it build chains, branches and rings. From there you learn to sort these compounds into homologous series, name them, and predict how they react: combustion, oxidation, addition and substitution. The chapter ends with two compounds you meet every day, ethanol and ethanoic acid, and with how soaps and detergents clean.

It links closely to earlier chapters. Balancing equations and oxidation come from Chapter 1. The reactions of ethanoic acid with bases, carbonates and hydrogencarbonates follow the acid patterns of Chapter 2, and the pH practical for dilute ethanoic acid sits there too. Covalent compounds are best understood by contrast with the ionic compounds of Chapter 3.

Board questions take several shapes: one-mark MCQs and assertion-reason on bonds, isomers and functional groups; two- and three-mark questions asking you to name a compound, draw an electron dot structure or write an equation with its condition; five-mark puzzles where you identify compounds X, Y and Z from clues; and case-based questions built on a combustion equation, a data table or a soap and hard water experiment.

Weightage: This chapter is part of Unit I, Chemical Substances: Nature and Behaviour, which carries 25 of the 80 theory marks together with Chemical Reactions and Equations, Acids, Bases and Salts and Metals and Non-metals.Source: CBSE Science syllabus 2026-27, Unit I Chemical Substances: Nature and Behaviour

In real board papers: asked in 21 of the 21 board papers we hold (2023 to 2026), on average 6.2 marks per paper. See the questions

Key concepts

Covalent bond: formation and properties of covalent compounds

NCERT section 4.1 Bonding in Carbon – The Covalent Bond

Carbon has atomic number 6, so its electrons are arranged 2, 4. With four valence electrons it is exactly halfway to an octet. Gaining four electrons to form $C^{4 -}$ fails because six protons cannot hold on to ten electrons. Losing four to form $C^{4 +}$ fails because removing four electrons needs a large amount of energy. So carbon takes a third route: it shares electrons.

A shared pair counts towards the outer shell of both atoms, so both reach a noble gas configuration. One shared pair is a single bond (as in $H_{2}$ ), two shared pairs a double bond (as in $O_{2}$ , where each oxygen has six L-shell electrons and needs two more), and three shared pairs a triple bond (as in $N_{2}$ ). In methane, $C H_{4}$ , carbon shares one pair with each of four hydrogen atoms.

This bonding explains the properties. Bonds inside a molecule are strong, but forces between molecules are weak, so covalent compounds have low melting and boiling points. No ions form, so they are generally poor conductors of electricity, unlike the ionic compounds of Chapter 3.

Worked example

The melting and boiling points of four carbon compounds are:

Acetic acid: 290 K and 391 K
Chloroform: 209 K and 334 K
Ethanol: 156 K and 351 K
Methane: 90 K and 111 K

(a) State the physical state of each compound at 300 K. (1)
(b) Which compound may turn solid on a cold day at 285 K? (½)
(c) Why are these values much lower than those of ionic compounds? (1)
(d) Why are these compounds poor conductors of electricity? (½)

Try it first, then show the solutionSolution

(a) A compound is a liquid when its melting point is below 300 K and its boiling point is above 300 K. Acetic acid, chloroform and ethanol are liquids; methane, whose boiling point is only 111 K, is a gas. (1)
(b) Only acetic acid has a melting point (290 K) above 285 K, so it freezes. (½)
(c) The covalent bonds inside each molecule are strong, but the forces of attraction between molecules are weak, so little energy is needed to separate the molecules. (1)
(d) Electrons are shared between atoms, so no ions or charged particles are formed to carry current. (½)

Answer: (a) Acetic acid, chloroform and ethanol are liquids; methane is a gas. (b) Acetic acid. (c) Weak intermolecular forces. (d) No ions are formed.

Versatile nature of carbon: catenation and tetravalency

NCERT section 4.2 Versatile Nature of Carbon

Chemists know the formulae of millions of carbon compounds, more than the compounds of all other elements put together. Two properties explain this.

Catenation: carbon atoms bond to other carbon atoms, making long chains, branched chains and rings, linked by single, double or triple bonds. The carbon–carbon bond is very strong, so these chains are stable. Silicon also forms chains with hydrogen, but only up to seven or eight atoms long, and those compounds are very reactive. No other element catenates to the extent carbon does.

Tetravalency: with a valency of four, one carbon atom can bond to four other atoms, such as carbon or monovalent atoms like hydrogen and chlorine, and it also bonds with oxygen, nitrogen and sulphur. Each different element attached gives a compound with its own properties.

Why are carbon's bonds so strong? Carbon is a small atom, so its nucleus holds the shared electron pairs tightly. Bigger atoms form much weaker bonds. Catenation plus tetravalency plus strong bonds give a huge and stable family of compounds.

Worked example

(a) Name the two properties of carbon that together give rise to the very large number of carbon compounds. (1)
(b) Silicon also forms chains with hydrogen. Why does it not form a comparable number of compounds? (1)
(c) Give the reason why the bonds carbon forms with most other elements are very strong. (1)

Try it first, then show the solutionSolution

(a) Catenation, the ability to bond with other carbon atoms to form long chains, branches and rings (½); tetravalency, a valency of four so each carbon bonds to four other atoms (½).
(b) Silicon chains reach only about seven or eight atoms (½), and these compounds are very reactive, so they are not stable (½).
(c) Carbon is small in size (½), so its nucleus holds the shared pairs of electrons strongly, giving strong, stable bonds (½).

Answer: (a) Catenation and tetravalency. (b) Silicon forms short chains of up to seven or eight atoms that are very reactive. (c) Carbon's small size lets its nucleus hold shared electron pairs strongly.

Saturated and unsaturated compounds; chains, branches, rings and isomers

NCERT section 4.2.2 Chains, Branches and Rings

To draw any hydrocarbon, first join the carbon atoms with single bonds, then use hydrogen atoms to satisfy the remaining valencies. If every valency is satisfied with single bonds, the compound is saturated, like ethane $C_{2} H_{6}$ or propane $C_{3} H_{8}$ . Saturated compounds are not very reactive.

If there are too few hydrogens, carbon must form a double or triple bond: ethene $C_{2} H_{4}$ has one C=C double bond and ethyne $C_{2} H_{2}$ has a triple bond. These are unsaturated and more reactive. Compounds of only carbon and hydrogen are hydrocarbons: saturated ones are alkanes, those with double bonds are alkenes, those with triple bonds are alkynes.

Carbon skeletons can be straight, branched or closed into rings, such as cyclohexane $C_{6} H_{12}$ and benzene $C_{6} H_{6}$ . With four carbons you can make a straight chain or a branched chain, and both have the formula $C_{4} H_{10}$ . Compounds with the same molecular formula but different structures are structural isomers. Closing a chain into a ring removes two hydrogens, because the two end carbons now bond to each other.

Worked example

(a) How many structural isomers are possible for pentane, $C_{5} H_{12}$ ? Describe the carbon skeleton of each. (1½)
(b) A student says cyclopentane is an isomer of pentane. Is the student right? Give a reason, and name an open-chain hydrocarbon that does have the same molecular formula as cyclopentane. (1½)

Try it first, then show the solutionSolution

(a) Three isomers. (½)
Isomer 1: a straight chain of five carbon atoms. Isomer 2: a chain of four carbons with one $C H_{3}$ branch on the second carbon. (½)
Isomer 3: a chain of three carbons with two $C H_{3}$ branches on the middle carbon. Each has the formula $C_{5} H_{12}$ . (½)
(b) No. Cyclopentane is a five-carbon ring with formula $C_{5} H_{10}$ ; isomers must have the same molecular formula, and pentane is $C_{5} H_{12}$ . (1)
Pentene, an alkene following $C_{n} H_{2 n}$ , also has the formula $C_{5} H_{10}$ . (½)

Answer: (a) Three structural isomers: straight chain, one branch on a four-carbon chain, two branches on a three-carbon chain. (b) The student is wrong: cyclopentane is $C_{5} H_{10}$ , not $C_{5} H_{12}$ . Pentene has the same formula as cyclopentane.

Functional groups and homologous series

NCERT section 4.2.4 Homologous Series

Carbon also bonds with halogens, oxygen, nitrogen and sulphur. When one of these replaces a hydrogen in a chain, it is called a heteroatom. A heteroatom, or a group containing it, gives the compound its particular properties whatever the length of the chain, so it is called a functional group. The ones you need: halo (–Cl, –Br), alcohol (–OH), aldehyde (–CHO), ketone (C=O within the chain) and carboxylic acid (–COOH).

A homologous series is a family of compounds that all carry one and the same functional group, while their carbon chains grow by one carbon at a time, for example $C H_{3} O H$ , $C_{2} H_{5} O H$ , $C_{3} H_{7} O H$ , $C_{4} H_{9} O H$ (methanol, ethanol, propanol, butanol). Successive members differ by a $C H_{2}$ unit, which is 12 + 2 = 14 u in molecular mass.

General formulae: alkanes $C_{n} H_{2 n + 2}$ , alkenes $C_{n} H_{2 n}$ (n starts at 2), alkynes $C_{n} H_{2 n - 2}$ .

Why do members behave alike? Chemical properties depend on the functional group, which is the same throughout. Physical properties show a gradation: as molecular mass increases, melting and boiling points rise and solubility in a given solvent changes steadily.

Worked example

Consider $C H_{3} O H$ , $C_{2} H_{5} O H$ and $C_{3} H_{7} O H$ (C = 12 u, H = 1 u, O = 16 u).
(a) Find the difference in formula and in molecular mass between the first two. (1)
(b) Why are their chemical properties similar? (½)
(c) How will their boiling points change from the first to the third? Give a reason. (½)
(d) Write the formula and molecular mass of the next homologue of $C_{3} H_{6}$ . (1)

Try it first, then show the solutionSolution

(a) Mass of $C H_{3} O H$ = 12 + 4 + 16 = 32 u; mass of $C_{2} H_{5} O H$ = 24 + 6 + 16 = 46 u. (½)
They differ by a $C H_{2}$ unit and by 46 − 32 = 14 u. (½)
(b) All contain the same functional group, –OH, which decides chemical properties. (½)
(c) Boiling points increase, because boiling point rises with molecular mass in a homologous series. (½)
(d) $C_{3} H_{6}$ is an alkene ( $C_{n} H_{2 n}$ ); add $C H_{2}$ to get $C_{4} H_{8}$ . (½)
Mass = 4 × 12 + 8 × 1 = 56 u. (½)

Answer: (a) $C H_{2}$ and 14 u. (b) Same functional group –OH. (c) They increase with molecular mass. (d) $C_{4} H_{8}$ , 56 u.

Nomenclature of carbon compounds

NCERT section 4.2.5 Nomenclature of Carbon Compounds

Names are built in steps.

Step 1: count the carbons to get the parent alkane: 1 meth-, 2 eth-, 3 prop-, 4 but-, 5 pent-, 6 hex-. The carbon inside –CHO, –COOH or a ketone C=O counts as part of the chain.

Step 2: add the functional group. Halogens go in front as a prefix: chloropropane, bromopropane. Other groups go at the end as a suffix: alcohol –ol, aldehyde –al, ketone –one, carboxylic acid –oic acid.

Step 3: the vowel rule. If the suffix begins with a vowel, drop the final 'e' of the alkane name: propane − e + ol = propanol; propane − e + one = propanone. All four suffixes begin with a vowel, so the 'e' is always dropped for them. Prefixes do not change the parent name.

Step 4: unsaturation. Replace -ane with -ene for a double bond and -yne for a triple bond: propene, propyne.

One useful check: an aldehyde group sits at the end of a chain, but a ketone C=O must have a carbon on each side, so the smallest ketone has three carbons.

Worked example

(a) Name the following: (2)
(i) $C H_{3} C H_{2} C H_{2} C l$
(ii) $C H_{3} C O C H_{2} C H_{3}$
(iii) $C H_{3} C H_{2} C H_{2} C H O$
(iv) $C H_{3} C H_{2} C O O H$
(b) Explain why no ketone can have only two carbon atoms. (1)

Try it first, then show the solutionSolution

(i) Three carbons with a chlorine prefix: chloropropane. (½)
(ii) Four carbons with C=O inside the chain: butane − e + one = butanone. (½)
(iii) Four carbons, counting the –CHO carbon, with an aldehyde group: butanal. (½)
(iv) Three carbons, counting the –COOH carbon: propanoic acid. (½)
(b) In a ketone the C=O carbon must be bonded to a carbon atom on each side (½), so at least three carbons are needed; with two carbons the C=O would be at the end, giving an aldehyde (½).

Answer: (a) (i) Chloropropane (ii) Butanone (iii) Butanal (iv) Propanoic acid. (b) The ketone carbon needs a carbon on both sides, so the minimum is three carbons (propanone).

Chemical properties: combustion, oxidation, addition and substitution

NCERT section 4.3 Chemical Properties of Carbon Compounds

Combustion: carbon and most carbon compounds burn in oxygen to give carbon dioxide, water if hydrogen is present, and heat and light: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ . Saturated hydrocarbons generally give a clean flame; unsaturated ones give a yellow flame with black smoke. Too little air causes incomplete combustion even of saturated fuels, giving a sooty flame, which is why a blackened vessel bottom means blocked air holes and wasted fuel.

Oxidation: alkaline potassium permanganate or acidified potassium dichromate convert ethanol into ethanoic acid. They are oxidising agents because they add oxygen to another substance.

Addition: unsaturated hydrocarbons add hydrogen in the presence of a nickel or palladium catalyst to become saturated. This is used to hydrogenate vegetable oils with nickel.

Substitution: saturated hydrocarbons are fairly unreactive, but in sunlight chlorine replaces their hydrogen atoms one by one: $C H_{4} + C l_{2} \to C H_{3} C l + H C l$ . Higher alkanes usually give several products.

The link: a double or triple bond allows addition; with only single bonds, substitution is the route.

Worked example

A hydrocarbon burns completely according to the equation
$2 C_{x} H_{y} + 7 O_{2} \to 4 C O_{2} + 6 H_{2} O$
(a) Find x and y and name the hydrocarbon. (2)
(b) Is it saturated or unsaturated? Justify using its general formula. (1)
(c) What kind of flame will it give in a good supply of air? (½)
(d) Write the equation for its first reaction with chlorine and state the condition. (1)
(e) Name its next homologue and give its formula. (½)

Try it first, then show the solutionSolution

(a) Carbon: the right side has 4 C atoms and the left has 2x, so 2x = 4 and x = 2. (½)
Hydrogen: the right side has 6 × 2 = 12 H atoms and the left has 2y, so 2y = 12 and y = 6. (½)
Check oxygen: left 7 × 2 = 14; right 4 × 2 + 6 = 14, balanced. (½)
$C_{2} H_{6}$ is ethane. (½)
(b) $C_{2} H_{6}$ fits $C_{n} H_{2 n + 2}$ with n = 2, so it is an alkane with only single bonds: saturated. (1)
(c) A clean flame. (½)
(d) $C_{2} H_{6} + C l_{2} sunlight C_{2} H_{5} C l + H C l$ , a substitution reaction that needs sunlight. (1)
(e) Add a $C H_{2}$ unit: propane, $C_{3} H_{8}$ . (½)

Answer: (a) x = 2, y = 6: ethane, $C_{2} H_{6}$ . (b) Saturated (alkane). (c) Clean flame. (d) $C_{2} H_{6} + C l_{2} sunlight C_{2} H_{5} C l + H C l$ . (e) Propane, $C_{3} H_{8}$ .

Ethanol: properties and uses

NCERT section 4.4.1 Properties of Ethanol

Ethanol, $C H_{3} C H_{2} O H$ , is commonly called alcohol. It is a liquid at room temperature (melting point 156 K, boiling point 351 K) and dissolves in water in all proportions. It does not change the colour of litmus.

Uses: it is the active ingredient of alcoholic drinks, and because it is a good solvent it is used in medicines such as tincture iodine, cough syrups and many tonics.

Health: small amounts of dilute ethanol cause drunkenness; even a small quantity of pure ethanol, called absolute alcohol, can be lethal, and long-term drinking causes many health problems.

Reactions you must write:

With sodium it gives hydrogen gas and sodium ethoxide: $2 N a + 2 C H_{3} C H_{2} O H \to 2 C H_{3} C H_{2} O^{-} N a^{+} + H_{2}$ .
Heated at 443 K with excess concentrated sulphuric acid, it loses water to give ethene. The acid acts as a dehydrating agent.
It burns in air to give carbon dioxide and water, and oxidising agents convert it to ethanoic acid.

So ethanol links to almost every reaction type in the chapter.

Worked example

A colourless liquid X, $C_{2} H_{6} O$ , has no effect on red or blue litmus. A small piece of sodium dropped into X releases a gas that burns with a pop sound. When X is heated at 443 K with excess concentrated sulphuric acid it gives a gas Y. Y adds hydrogen in the presence of nickel to give Z.
(a) Identify X, Y and Z. (1½)
(b) Write the equation for the reaction of X with sodium. (1)
(c) Write the equation for the formation of Y and state the role of sulphuric acid. (1½)
(d) Write the equation for the formation of Z. (1)

Try it first, then show the solutionSolution

(a) X is ethanol, $C H_{3} C H_{2} O H$ (neutral to litmus, gives hydrogen with sodium). (½)
Y is ethene, $C H_{2} = C H_{2}$ . (½)
Z is ethane, $C H_{3} C H_{3}$ . (½)
(b) $2 N a + 2 C H_{3} C H_{2} O H \to 2 C H_{3} C H_{2} O^{-} N a^{+} + H_{2}$ ; the gas is hydrogen and the other product is sodium ethoxide. (1)
(c) $C H_{3} C H_{2} O H hot conc. H_{2} S O_{4}, 443 K C H_{2} = C H_{2} + H_{2} O$ (1)
Concentrated sulphuric acid is a dehydrating agent: it removes water from ethanol. (½)
(d) $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$ , an addition reaction. (1)

Answer: X = ethanol, Y = ethene, Z = ethane. Sulphuric acid acts as a dehydrating agent; the last step is addition (hydrogenation) with a nickel catalyst.

Ethanoic acid: properties and uses

NCERT section 4.4.2 Properties of Ethanoic Acid

Ethanoic acid, $C H_{3} C O O H$ , is commonly called acetic acid and belongs to the carboxylic acids. A 5-8% solution in water is vinegar, widely used as a preservative in pickles. Pure ethanoic acid melts at 290 K, so it often freezes in cold winters, which is why it is called glacial acetic acid.

It is a weak acid. Hydrochloric acid is completely ionised, but carboxylic acids are not, so universal indicator shows dilute ethanoic acid as a less strong acid than dilute HCl, even though both turn blue litmus red.

Reactions:

Esterification: with absolute ethanol and an acid catalyst it forms a sweet-smelling ester, used in perfumes and as flavouring agents.
Saponification: the ester heated with sodium hydroxide gives back the alcohol and the sodium salt of the acid; this is the reaction used to make soap.
With a base: with NaOH it gives sodium ethanoate (sodium acetate) and water.
With carbonates and hydrogencarbonates: it gives sodium acetate, water and carbon dioxide, with brisk effervescence.

The last reaction is the cleanest test to tell ethanoic acid from ethanol.

Worked example

Two unlabelled bottles A and B contain ethanol and ethanoic acid. When sodium hydrogencarbonate is added, A gives brisk effervescence; B shows no change.
(a) Identify A and B and write the equation for the reaction of A. (1½)
(b) How would you confirm the gas? (½)
(c) A and B are warmed together with a few drops of concentrated sulphuric acid. Write the equation and state one use of the product. (2)
(d) What is formed when this product is heated with sodium hydroxide, and what is this reaction called? (1)

Try it first, then show the solutionSolution

(a) A is ethanoic acid and B is ethanol, since only an acid reacts with a hydrogencarbonate to give carbon dioxide. (½)
$C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$ (1)
(b) Pass the gas through freshly prepared lime water; it turns milky, confirming $C O_{2}$ . (½)
(c) $C H_{3} C O O H + C H_{3} C H_{2} O H acid C H_{3} C O O C H_{2} C H_{3} + H_{2} O$ (1)
The product is a sweet-smelling ester (½), used in perfumes or as a flavouring agent (½).
(d) Ethanol and sodium ethanoate are formed: $C H_{3} C O O C_{2} H_{5} + N a O H \to C_{2} H_{5} O H + C H_{3} C O O N a$ (½); the reaction is saponification (½).

Answer: A = ethanoic acid, B = ethanol. The gas is carbon dioxide. A and B form a sweet-smelling ester used in perfumes and flavourings; with NaOH it gives ethanol and sodium ethanoate (saponification).

Soaps and detergents: cleaning action, micelles and hard water

NCERT section 4.5 Soaps and Detergents

Most dirt is oily, and oil does not dissolve in water. Soap solves this. A soap molecule is the sodium or potassium salt of a long-chain carboxylic acid. It has two ends with different behaviour: the ionic end is hydrophilic and interacts with water, while the long carbon chain is hydrophobic and interacts with oil.

In water, soap molecules gather round an oil droplet with their carbon tails pointing into the oil and their ionic ends facing outwards into the water. This cluster is a micelle. The micelles form an emulsion in water, so the oily dirt is pulled into the water and rinsed away.

Hard water contains calcium and magnesium salts. These react with soap to form an insoluble scum, so foam forms with difficulty and more soap is wasted.

Detergents also have a long hydrocarbon tail, but their charged head is of a different kind (a sulphonate or an ammonium type). This head does not combine with $C a^{2 +}$ or $M g^{2 +}$ ions to make an insoluble solid, so detergents keep lathering and cleaning in hard water. Shampoos and products for washing clothes are usually made from detergents.

Worked example

A student takes 10 mL of distilled water in test tube P and 10 mL of hand-pump (hard) water in test tubes Q and R. She adds five drops of soap solution to P and Q, and five drops of detergent solution to R, then shakes all three for the same time.
(a) Compare the foam and any solid seen in P, Q and R. (1½)
(b) Explain the observation in Q. (1)
(c) Which cleaning agent should she use with this hand-pump water? Why? (½)

Try it first, then show the solutionSolution

(a) P: plenty of foam, no precipitate. (½)
Q: very little foam and a white curdy precipitate (scum). (½)
R: good foam and no curdy solid. (½)
(b) Hard water contains calcium and magnesium salts (½), which react with soap to form insoluble scum, so soap is used up and little foam forms (½).
(c) Detergent, because its charged head does not combine with calcium and magnesium ions to give an insoluble solid. (½)

Answer: P: lots of foam; Q: little foam with white scum; R: good foam, no scum. Calcium and magnesium salts in hard water react with soap to form scum. Use detergent for hard water.

Science in real life: Carbon and its Compounds

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Chemical properties: combustion, oxidation, addition and substitution

Imagine your family cooks on a biogas connection, whose major component is methane. One week you notice the flame has turned yellow and the bottom of the pressure cooker is getting black on the outside.

(a) What does the blackening tell you about the burning? (b) Write the balanced equation for complete combustion of methane. (c) If 5 molecules of methane burn completely, how many molecules of oxygen are used and how many molecules of water are formed? (d) What should be done to the stove?

Try it first, then show the solutionSolution

(a) Soot is unburnt carbon, so the fuel is burning incompletely because too little air is reaching it; fuel is being wasted.
(b) $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$
(c) One methane needs 2 oxygen molecules, so 5 methane need 5 × 2 = 10 oxygen molecules.
One methane gives 2 water molecules, so 5 methane give 10 water molecules.
(d) Clean the blocked air inlets so an oxygen-rich mixture burns with a clean blue flame.

Answer: (a) Incomplete combustion due to too little air. (b) $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ . (c) 10 oxygen molecules used, 10 water molecules formed. (d) Clean the air holes.

Memory hook: Black pot, yellow flame: not enough air.

Ethanoic acid: properties and uses

Suppose your family uses vinegar to preserve a jar of pickles. Out of curiosity you drop a pinch of baking soda (sodium hydrogencarbonate) into a spoonful of vinegar and it fizzes strongly.

(a) Which compound in vinegar causes the fizz, and roughly what percentage of vinegar is it? (b) Write the balanced equation. (c) How could you prove the gas is carbon dioxide? (d) Why would ethanol not fizz in the same test?

Try it first, then show the solutionSolution

(a) Ethanoic (acetic) acid; vinegar is a 5-8% solution of it in water.
(b) $C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$
(c) Pass the gas through freshly prepared lime water; it turns milky.
(d) Ethanol is not an acid, so it does not react with a hydrogencarbonate to release carbon dioxide.

Answer: Ethanoic acid (5-8% in vinegar) reacts with sodium hydrogencarbonate to give sodium acetate, water and carbon dioxide, which turns lime water milky. Ethanol gives no fizz.

Memory hook: Vinegar plus baking soda: acid meets hydrogencarbonate, out comes $C O_{2}$ .

Soaps and detergents: cleaning action, micelles and hard water

Imagine you visit relatives who take bathing water from a hand-pump. Soap hardly lathers, you use twice as much, and a sticky white layer is left on the bucket.

(a) What is the white layer and why does it form? (b) Why is more soap needed? (c) How could you show in a test tube that this water is hard? (d) Would a detergent-based shampoo work better here? Explain.

Try it first, then show the solutionSolution

(a) It is scum: calcium and magnesium salts in the hard water react with soap to form an insoluble substance.
(b) Soap molecules are used up forming scum, so extra soap is needed before any lather forms.
(c) Shake 10 mL of the water with a few drops of soap solution beside 10 mL of distilled water; the hard water gives less foam and a white curdy precipitate.
(d) Yes; the charged head of a detergent molecule does not combine with calcium or magnesium ions into an insoluble solid, so it lathers and cleans in hard water.

Answer: The layer is scum from calcium and magnesium salts reacting with soap. A soap test shows little foam and a curdy precipitate. A detergent-based shampoo works better in hard water.

Memory hook: Hand-pump scum: calcium and magnesium steal the soap.

Chemical properties: combustion, oxidation, addition and substitution

Suppose your teacher says vegetable oils have unsaturated chains and shows a hydrogenation set-up with nickel catalyst. She asks the class to explain what happens to the oil.

(a) Name the process and state the role of nickel. (b) What change happens to the carbon chains? Show the same change with the simplest example, ethene. (c) Which type of reaction is this, and why can only unsaturated compounds undergo it? (d) Which, as your textbook explains, should be preferred for cooking, oils with unsaturated fatty acids or animal fats, and why?

Try it first, then show the solutionSolution

(a) Hydrogenation. Nickel is a catalyst: it makes the reaction happen or go at a different rate without being affected itself.
(b) Hydrogen adds across the double bonds in the long unsaturated chains, so the chains become saturated.
For ethene: $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$
(c) An addition reaction; hydrogen can only add where there is a double or triple bond, which saturated chains do not have.
(d) Oils with unsaturated fatty acids, because saturated fatty acids, found in animal fats, are said to be harmful for health.

Answer: Hydrogenation, an addition reaction with nickel as catalyst, turns unsaturated chains into saturated ones, as in $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$ . Oils containing unsaturated fatty acids are preferred for cooking.

Memory hook: Oil plus hydrogen on nickel: double bonds become single bonds.

Ethanol: properties and uses

Imagine your school lab has two unlabelled bottles, A and B, each holding a colourless liquid. Your teacher says one is ethanol, the solvent used in medicines such as tincture iodine and cough syrups, and the other is dilute ethanoic acid.

(a) Using sodium hydrogencarbonate, how would you find which bottle is ethanol? Write the equation for the liquid that reacts. (b) Why would dropping sodium into each liquid not settle the question? (c) Write the equation for the reaction of ethanol with sodium and name both products. (d) Ethanol is heated at 443 K with excess concentrated sulphuric acid. Write the equation, name the product and state the role of the acid.

Try it first, then show the solutionSolution

(a) Add a pinch of sodium hydrogencarbonate to a little of each liquid. The one giving brisk effervescence, with a gas that turns lime water milky, is ethanoic acid; the one showing no change is ethanol.
$C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$
(b) Ethanol gives hydrogen with sodium, and acids also give hydrogen with reactive metals, so both liquids would show the same result.
(c) $2 N a + 2 C H_{3} C H_{2} O H \to 2 C H_{3} C H_{2} O^{-} N a^{+} + H_{2}$ ; the products are sodium ethoxide and hydrogen gas.
(d) $C H_{3} C H_{2} O H hot conc. H_{2} S O_{4}, 443 K C H_{2} = C H_{2} + H_{2} O$ ; the product is ethene.
Concentrated sulphuric acid is a dehydrating agent: it removes water from ethanol.

Answer: The bottle with no fizz on adding sodium hydrogencarbonate is ethanol; the other is ethanoic acid. Sodium cannot separate them because both give hydrogen. Ethanol with sodium gives sodium ethoxide and hydrogen; with hot concentrated sulphuric acid at 443 K it gives ethene, the acid acting as a dehydrating agent.

Memory hook: Fizz with baking soda means acid; no fizz means ethanol, the medicine solvent.

Topics in this chapter

Covalent Bonding in Carbon
Versatile Nature of CarbonComing soon
Homologous Series and Nomenclature
Chemical Properties of Carbon Compounds
Ethanol and Ethanoic AcidComing soon
Soaps and DetergentsComing soon

Carbon and its Compounds: formulas and key facts

General formula of alkanes: $C_{n} H_{2 n + 2}$; Saturated hydrocarbons. Use to test whether a formula like $C_{3} H_{8}$ is an alkane, or to find the next member.
General formula of alkenes: $C_{n} H_{2 n} (n \geq 2)$; One double bond. $C_{3} H_{6}$ fits n = 3. Rings like cyclopentane also have this formula, so check the structure.
General formula of alkynes: $C_{n} H_{2 n - 2}$; One triple bond, starting with ethyne $C_{2} H_{2}$ .
Homologous series difference: $Successive members differ by C H_{2} = 14 u$; Use to find the next or previous homologue and the change in molecular mass.
Number of covalent bonds in an alkane: $bonds in C_{n} H_{2 n + 2} = 3 n + 1$; Quick check: ethane has 7 bonds (6 C–H + 1 C–C). Count bonds by hand for alkenes and alkynes.
Complete combustion of methane: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O + heat and light$; Fuel questions and balancing. Every carbon ends in $C O_{2}$ and every hydrogen in $H_{2} O$ .
Combustion of ethanol: $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O + heat and light$; Count the oxygen already inside ethanol when balancing.
Oxidation of ethanol: $C H_{3} C H_{2} O H + 2 [O] alkaline K M n O_{4} + heat C H_{3} C O O H + H_{2} O$; [O] is oxygen supplied by the oxidising agent; acidified potassium dichromate also works. Shows why this is oxidation: oxygen is added.
Addition (hydrogenation): $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$; Only unsaturated compounds. Always write the catalyst (nickel or palladium). Basis of hydrogenation of vegetable oils.
Substitution with chlorine: $C H_{4} + C l_{2} sunlight C H_{3} C l + H C l$; Saturated hydrocarbons. Write sunlight over the arrow; one H is replaced by one Cl.
Ethanol with sodium: $2 N a + 2 C H_{3} C H_{2} O H \to 2 C H_{3} C H_{2} O^{-} N a^{+} + H_{2}$; Products: sodium ethoxide and hydrogen (burns with a pop).
Dehydration of ethanol: $C H_{3} C H_{2} O H hot conc. H_{2} S O_{4}, 443 K C H_{2} = C H_{2} + H_{2} O$; Excess concentrated sulphuric acid is the dehydrating agent; the product is ethene.
Esterification: $C H_{3} C O O H + C H_{3} C H_{2} O H acid C H_{3} C O O C H_{2} C H_{3} + H_{2} O$; Acid plus alcohol gives a sweet-smelling ester; an acid catalyst (a few drops of conc. $H_{2} S O_{4}$ ) is needed.
Saponification: $C H_{3} C O O C_{2} H_{5} + N a O H \to C_{2} H_{5} O H + C H_{3} C O O N a$; Ester plus alkali gives back the alcohol and the sodium salt of the acid; used to make soap.
Ethanoic acid with a base: $N a O H + C H_{3} C O O H \to C H_{3} C O O N a + H_{2} O$; Neutralisation; the salt is sodium ethanoate (sodium acetate).
Ethanoic acid with carbonate and hydrogencarbonate: $2 C H_{3} C O O H + N a_{2} C O_{3} \to 2 C H_{3} C O O N a + H_{2} O + C O_{2}$; With $N a H C O_{3}$ : $C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$ . Brisk effervescence; the test that separates the acid from ethanol.

Carbon and its Compounds: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Electron dot structures of covalent molecules

Draw the electron dot structure of a molecule: for example $H_{2}$ , $C l_{2}$ , $O_{2}$ , $N_{2}$ , $H_{2} O$ , $N H_{3}$ , $C H_{4}$ , $C O_{2}$ , $C H_{3} C l$ , ethane, ethene, ethyne, methanol, ethanoic acid or propanone. A question may also ask you to count the shared pairs.

Draw

Write the symbols of the atoms in the order they are bonded. Put carbon (or the atom with the highest valency) in the centre.
Show only the valence electrons of each atom: dots for one atom and crosses for the other, so the reader can see who gave which electron.
Place each shared pair (one dot and one cross) between the two atoms it joins. Use two pairs for a double bond and three pairs for a triple bond.
Place the unshared electrons (for example on O, N or Cl) around their own atom.
Check: every atom except hydrogen has 8 electrons around it, and each hydrogen has 2.

Label

Atom symbols (C, H, O, N, Cl)
Shared pair of electrons
Unshared (lone) pair where present
Single, double or triple bond where the question asks

Watch out: Do not draw inner-shell electrons, and do not leave out the lone pairs on oxygen. In $C O_{2}$ and ethene, a single pair where a double bond is needed leaves carbon with only 6 electrons. Count to 8 around every atom before you move on.

Structural formulae: isomers of butane, cyclohexane and benzene

Draw the structural isomers of $C_{4} H_{10}$ , or draw the structure of cyclohexane or benzene to show chains, branches and rings.

Draw

Butane, straight chain: four carbons in a row, $C H_{3} - C H_{2} - C H_{2} - C H_{3}$ .
Butane, branched chain: three carbons in a row with the fourth carbon attached to the middle carbon.
In both, fill the remaining valencies with H so that every carbon has four bonds and the formula is $C_{4} H_{10}$ .
Cyclohexane: six carbons joined by single bonds in a ring, each carbon carrying two H atoms ( $C_{6} H_{12}$ ).
Benzene: six carbons in a ring with double and single bonds alternating, each carbon carrying one H atom ( $C_{6} H_{6}$ ).

Label

Names under each structure: straight-chain butane, branched butane, cyclohexane, benzene
Molecular formula under each: $C_{4} H_{10}$ , $C_{4} H_{10}$ , $C_{6} H_{12}$ , $C_{6} H_{6}$

Watch out: Bending a straight chain does not make a new isomer: only a real branch does. Count the bonds on each carbon: every carbon must have exactly four, never three or five.

Micelle formation and cleaning action of soap

Explain the cleaning action of soap with a labelled diagram, or draw a micelle.

Draw

Draw an oil (dirt) droplet as a circle in the centre.
Draw soap molecules as a line (the long hydrocarbon tail) ending in a small circle (the ionic head).
Arrange many soap molecules all round the droplet, with tails pointing into the oil and heads pointing outwards.
Show water around the whole cluster.
Optionally, draw a cloth surface with the micelle being pulled away into the water.

Label

Oily dirt
Hydrocarbon tail (hydrophobic, interacts with oil)
Ionic end (hydrophilic, interacts with water)
Water
Micelle

Watch out: The tails go inside, into the oil, and the heads face the water. Drawing it the other way round is wrong. Always write which end is hydrophilic and which is hydrophobic.

Set-up for preparing an ester (warming in a water bath)

Activity-based question on esterification: show how ethanol and ethanoic acid are warmed together with concentrated sulphuric acid.

Draw

Draw a beaker of water (the water bath) placed over a flame.
Dip a test tube in the water. It holds the mixture of ethanol, glacial acetic acid and a few drops of concentrated sulphuric acid.
Show the test tube held upright by a clamp or holder.

Label

Water bath (beaker of water)
Test tube with ethanol + ethanoic acid + few drops of conc. sulphuric acid
Burner / flame
Holder or clamp

Watch out: The test tube is warmed in water, never directly over the flame, because ethanol burns easily. State that you smell the product only after pouring it into water: the sweet smell shows an ester.

Set-up for testing the gas from ethanoic acid and a carbonate

Activity- or practical-based question on the reaction of ethanoic acid with sodium carbonate or sodium hydrogencarbonate.

Draw

Draw a test tube with solid sodium carbonate (or sodium hydrogencarbonate) and dilute ethanoic acid, closed with a cork.
From the cork, draw a bent delivery tube leading into a second test tube.
The second test tube holds freshly prepared lime water, with the tube dipping into it.
Show bubbles in both test tubes.

Label

Dilute ethanoic acid + sodium carbonate
Cork
Delivery tube
Lime water (turns milky)
Bubbles of carbon dioxide

Watch out: The delivery tube must dip into the lime water, but it must not dip into the acid in the first tube. Write that lime water turns milky. That observation confirms the gas is carbon dioxide.

How the board asks Carbon and its Compounds

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Electron dot structures and bond counting	Draw the electron dot structure of $C O_{2}$ , $N_{2}$ , ethene, ethyne, propanone or cyclopentane; or an MCQ asking how many covalent bonds a molecule has.	Write each atom's valence electrons first. Arrange shared pairs so every atom except hydrogen has eight electrons and hydrogen has two. Show lone pairs on O, N and Cl. To count bonds, count every shared pair as one covalent bond, so a double bond counts as 2 and a triple bond as 3. For example, ethene has 4 C–H + 2 = 6 bonds, and ethyne has 2 C–H + 3 = 5 bonds.
Assertion-reason on bonding and versatility	An assertion about catenation, saturation, isomers or covalent compound properties with a reason that may or may not explain it.	Judge A and R as true or false separately. Then ask: does R answer 'why' for A? For example, low boiling point is explained by weak intermolecular forces, not by strong covalent bonds.
Naming and drawing structures	Name a given condensed formula, or draw the structure of a named compound such as butanone or hexanal; identify the functional group; give the next homologue.	Count every carbon, including the one in –CHO, –COOH or C=O. Apply prefix or suffix, drop the final 'e' before a vowel suffix, change -ane to -ene or -yne for unsaturation. For homologues add or remove $C H_{2}$ .
Equations with conditions	Write the equation for combustion, oxidation, hydrogenation, substitution, esterification, saponification or a reaction of ethanoic acid; or 'what happens when ...'.	Write the balanced equation, put the condition over the arrow (sunlight, Ni, hot conc. $H_{2} S O_{4}$ , acid, alkaline $K M n O_{4}$ ), name the products and the reaction type. Marks are lost for a missing condition.
Identify X, Y and Z	A long answer or case where a compound is described by its formula, litmus behaviour, reaction with sodium or sodium hydrogencarbonate, smell of a product, and you must identify each and write equations.	Start from the strongest clue (formula $C_{2} H_{6} O$ , neutral to litmus, sweet smell, effervescence with $N a H C O_{3}$ ). Identify each compound, then write every equation in order. Check that the chain of reactions is consistent.
Distinguish experimentally	How would you tell ethanol from ethanoic acid, a saturated from an unsaturated hydrocarbon, or soft from hard water?	Give one test that gives different results for the two: what you do, what you see for each, and the equation if one applies. Sodium is not a good choice for alcohol versus acid, since both give hydrogen; use litmus or sodium hydrogencarbonate.
Soaps, micelles and hard water	Explain the cleaning action of soap, describe a micelle, explain scum, or interpret an Activity 4.11 or 4.12 style experiment.	Name the two ends (ionic hydrophilic head, hydrocarbon hydrophobic tail), say which way each faces in a micelle, link to emulsion and rinsing. For hard water mention calcium and magnesium salts, insoluble scum, and why detergents do not form it.
Case-based and data questions	A passage, combustion equation with unknown $C_{x} H_{y}$ , or a data table of melting and boiling points, followed by three or four linked sub-questions.	Extract numbers or clues first, solve the core unknown (atom balance, physical state), then answer each part using the chapter's reasoning, not general knowledge.

Tricks to remember Carbon and its Compounds

Short memory aids. Say each one aloud once, then use it on a question the same day.

Parent names 1 to 4
Monkeys Eat Peeled Bananas: Meth (1), Eth (2), Prop (3), But (4). Then Pent (5) and Hex (6) sound like pentagon and hexagon.
Use it when: Naming any compound: count carbons, then pick the parent name.
General formulae
Ane adds two, ene adds zero, yne loses two: $C_{n} H_{2 n + 2}$ , $C_{n} H_{2 n}$ , $C_{n} H_{2 n - 2}$ .
Use it when: Deciding whether a formula is an alkane, alkene or alkyne, and generating homologues.
Bond count shortcut
For any alkane $C_{n} H_{2 n + 2}$ , bonds = 3n + 1. Ethane 7, propane 10, butane 13.
Use it when: MCQs on number of covalent bonds; check by counting C–C plus C–H.
Suffix vowel rule
ol, al, one, oic acid all start with a vowel, so the 'e' of the alkane always drops: propanol, propanal, propanone, propanoic acid. Prefixes chloro and bromo never change the parent.
Use it when: Spelling names correctly in naming questions.
Double bond adds, single bond swaps
Unsaturated (double or triple bond) = Addition with $H_{2}$ on Ni. Saturated (single bonds) = Substitution with $C l_{2}$ in sunlight.
Use it when: Choosing the reaction type and its condition.
Soap head and tail
Head loves water, tail loves oil. In a micelle the tails hide in the oil drop and the heads face the water.
Use it when: Describing or checking a micelle diagram and explaining cleaning action.

Carbon and its Compounds: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Spot the homologous pair · 1 mark · MCQBoard level

Which of the following pairs of compounds belong to the same homologous series?
(a) $C_{2} H_{6}$ and $C_{3} H_{6}$
(b) $C_{2} H_{4}$ and $C_{2} H_{2}$
(c) $C H_{4}$ and $C_{3} H_{8}$
(d) $C_{3} H_{6}$ and $C_{3} H_{8}$

Try it first, then show the solutionSolution

Use the general formulae: alkanes $C_{n} H_{2 n + 2}$ , alkenes $C_{n} H_{2 n}$ .
(a) $C_{2} H_{6}$ is an alkane but $C_{3} H_{6}$ fits $C_{n} H_{2 n}$ , an alkene. Different series.
(b) $C_{2} H_{4}$ is an alkene and $C_{2} H_{2}$ is an alkyne. Different series.
(c) $C H_{4}$ : n = 1 gives 4 H. $C_{3} H_{8}$ : n = 3 gives 8 H. Both are alkanes.
(d) An alkene and an alkane. Different series.
Members of a series need not be next to each other: $C H_{4}$ and $C_{3} H_{8}$ differ by two $- C H_{2} -$ units.

Answer: (c) $C H_{4}$ and $C_{3} H_{8}$ : both are alkanes.

Example 2: Weak acid and litmus · 1 mark · Assertion-reasonBoard level

Assertion (A): Ethanoic acid is a weak acid.
Reason (R): Ethanoic acid turns blue litmus red.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Try it first, then show the solutionSolution

A is true. Unlike hydrochloric acid, which is completely ionised, carboxylic acids are only weak acids. Universal indicator shows dilute ethanoic acid as less strongly acidic than dilute HCl.
R is true. Every acid, strong or weak, turns blue litmus red.
Litmus only tells you that a substance is acidic, not how strong it is. A strong acid like HCl also turns blue litmus red, so R does not explain why ethanoic acid is weak.

Answer: (b) Both A and R are true but R is not the correct explanation of A.

Example 3: Balancing combustion equations · 3 marks · Short answerBoard level

(a) Write balanced chemical equations for the complete combustion of methane and of ethanol. (2)
(b) Why does a gas stove have air inlets? What does it mean if the bottoms of cooking vessels turn black? (1)

Try it first, then show the solutionSolution

Methane: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ + heat and light. Check: C 1 = 1, H 4 = 4, O 4 = 2 + 2. (1)
Ethanol: $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O$ + heat and light. Check: C 2 = 2, H 6 = 6, O 1 + 6 = 7 = 4 + 3. (1)
The air inlets let in enough oxygen, so the fuel burns completely with a clean blue flame. (½)
Blackened vessels mean the air holes are blocked. Combustion is incomplete, soot (carbon) forms and fuel is wasted. (½)

Answer: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ and $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O$ . Air inlets supply oxygen for complete combustion. Black vessel bottoms show incomplete combustion and wasted fuel.

Example 4: Electron dot structure of ethyne · 3 marks · Short answerHarder board level

(a) Draw the electron dot structure of ethyne, $C_{2} H_{2}$ . (2)
(b) How many covalent bonds does a molecule of ethyne have? Why is ethyne called an unsaturated compound? (1)

Try it first, then show the solutionSolution

Each carbon has 4 valence electrons. It shares 1 electron with one hydrogen atom, so 3 electrons are left over. (½)
These 3 electrons from each carbon are shared with the other carbon, giving three shared pairs between the two carbons: a triple bond. (½)
Draw and label: write H C C H in a row. Put one dot-cross pair between each H and its C, and three dot-cross pairs between the two C atoms. No lone pairs. (½)
Check: each C has 2 (with H) + 6 (with the other C) = 8 electrons, and each H has 2. (½)
Bonds: 1 (C–H) + 3 (C≡C) + 1 (C–H) = 5 covalent bonds. (½)
Ethyne is unsaturated because it has a triple bond between its carbon atoms. (½)

Answer: H : C ⋮⋮⋮ C : H, that is, a triple bond between the carbons and a single bond to each H. Ethyne has 5 covalent bonds. It is unsaturated because of the carbon–carbon triple bond.

Example 5: Cleaning action of soap and hard water · 5 marks · Long answerHarder board level

(a) What is a soap molecule? Explain, with a labelled diagram of a micelle, how soap removes oily dirt from clothes. (3)
(b) Why does soap form scum in hard water? How do detergents overcome this problem? (2)

Try it first, then show the solutionSolution

A soap is the sodium or potassium salt of a long-chain carboxylic acid. It has an ionic end, which is hydrophilic and interacts with water, and a long hydrocarbon chain, which is hydrophobic and interacts with oil. (1)
Oily dirt does not dissolve in water. In water, soap molecules cluster round an oil droplet with their tails in the oil and their ionic ends facing the water. This cluster is a micelle. Micelles form an emulsion, so rinsing and agitation carry the dirt away. (1)
Draw and label: oily dirt at the centre; soap molecules drawn as lines with a circle at one end, tails pointing into the droplet and heads outward; water outside. Labels: oily dirt, hydrocarbon tail (hydrophobic), ionic end (hydrophilic), water, micelle. (1)
Hard water contains calcium and magnesium salts. Soap reacts with them to form an insoluble substance, scum, so more soap is needed and little lather forms. (1)
Detergent molecules also have a long hydrocarbon tail, but a different charged head (sulphonate or ammonium type). This head does not combine with $C a^{2 +}$ or $M g^{2 +}$ to form an insoluble solid, so detergents keep lathering in hard water. (1)

Answer: Soap micelles trap oily dirt inside, with tails in the oil and ionic ends in the water, and the dirt is rinsed away. Soap forms scum with calcium and magnesium salts in hard water. A detergent's charged head does not form such an insoluble solid, so it stays effective.

Example 6: Reactions of propene and propane · 5 marks · Long answerHarder board level

A hydrocarbon P, $C_{3} H_{6}$ , is an alkene.
(a) Name P and draw its structure. (1)
(b) P adds hydrogen in the presence of nickel to give Q. Write the equation and name Q. State one industrial use of this type of reaction. (1½)
(c) Q reacts with chlorine in sunlight. Write the equation for the first step, name the organic product and name the type of reaction. (1½)
(d) Write the balanced equation for the complete combustion of P. Compare the flames of P and Q. (1)

Try it first, then show the solutionSolution

P is propene: $C H_{2} = C H - C H_{3}$ , a three-carbon chain with one double bond. (1)
$C H_{2} = C H - C H_{3} + H_{2} N i C H_{3} - C H_{2} - C H_{3}$ . Q is propane. (1)
Use: hydrogenation of vegetable oils with a nickel catalyst. (½)
$C_{3} H_{8} + C l_{2} sunlight C_{3} H_{7} C l + H C l$ (1)
The product is chloropropane. This is a substitution reaction, because a chlorine atom takes the place of a hydrogen atom. (½)
$2 C_{3} H_{6} + 9 O_{2} \to 6 C O_{2} + 6 H_{2} O$ . Check: C 6 = 6, H 12 = 12, O 18 = 12 + 6. (½)
P is unsaturated, so it gives a yellow, sooty flame. Q is saturated, so it generally gives a clean flame. (½)

Answer: P = propene and Q = propane, formed by addition of hydrogen with Ni. With chlorine in sunlight, Q gives chloropropane + HCl by substitution. $2 C_{3} H_{6} + 9 O_{2} \to 6 C O_{2} + 6 H_{2} O$ . P's flame is sooty and Q's is clean.

Example 7: Find the naming errors · 3 marks · Short answerBoard level

A student wrote these names in her notebook. Find the error in each and give the correct name.
(i) $C H_{3} C H_{2} C H_{2} O H$ : propanone
(ii) $C H_{3} C H O$ : ethanol
(iii) $H C \equiv C H$ : ethene

Try it first, then show the solutionSolution

(i) The functional group is –OH, an alcohol, which takes the suffix '-ol', not the ketone suffix '-one'. (½)
(i) Correct name: propanol. (½)
(ii) –CHO is an aldehyde group, which takes the suffix '-al'. The molecule has 2 carbons, counting the carbon of –CHO. (½)
(ii) Correct name: ethanal. (½)
(iii) A triple bond between carbons takes 'yne', not 'ene', which is used for a double bond. (½)
(iii) Correct name: ethyne. (½)

Answer: (i) propanol (alcohol, not ketone). (ii) ethanal (aldehyde, not alcohol). (iii) ethyne (triple bond, not double).

Example 8: Checking a practical record on acetic acid · 3 marks · Short answerBoard level

In the practical on the properties of acetic acid, a student records:

Odour: sweet, fruity smell
Solubility: forms two separate layers with water
Effect on litmus: blue litmus turns red; red litmus unchanged
With sodium hydrogencarbonate: brisk effervescence; the gas turns lime water milky

(a) Which two observations are wrong? Correct them. (2)
(b) Write the balanced equation for the last reaction. (1)

Try it first, then show the solutionSolution

The odour is wrong. Acetic acid smells like vinegar, which is its dilute solution. A sweet smell belongs to an ester, formed only when the acid reacts with an alcohol. (½)
Correction: smell of vinegar. (½)
The solubility is wrong. Acetic acid dissolves in water (vinegar is a 5-8% solution of it), so you see one clear solution, not two layers. (½)
Correction: dissolves in water. The litmus and hydrogencarbonate observations are correct, because ethanoic acid is an acid. (½)
$C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$ (1)

Answer: The wrong observations are the odour (it should be the smell of vinegar) and the solubility (it dissolves in water). $C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$

Example 9: Identify A, B, C and D: from ethanol to soap-making reaction · 5 marks · Long answerHarder board level

A neutral liquid A, $C_{2} H_{6} O$ , is warmed with alkaline potassium permanganate and gives B. B gives brisk effervescence with sodium hydrogencarbonate. A and B are warmed with a few drops of concentrated sulphuric acid to give a sweet-smelling compound C. When C is heated with sodium hydroxide, it gives back A and a salt D.
(a) Identify A, B, C and D with their formulae. (2)
(b) Write the equation for the change of A into B and state the role of alkaline potassium permanganate. (1)
(c) Write the equation for the formation of C and give one use of such compounds. (1)
(d) Write the equation for the reaction of C with sodium hydroxide and name the reaction. (1)

Try it first, then show the solutionSolution

A is ethanol, $C H_{3} C H_{2} O H$ : formula $C_{2} H_{6} O$ and neutral. (½)
B is ethanoic acid, $C H_{3} C O O H$ : it forms from oxidation of ethanol, and effervescence with $N a H C O_{3}$ shows it is an acid. (½)
C is an ester, $C H_{3} C O O C H_{2} C H_{3}$ , formed from an acid and an alcohol; it is sweet-smelling. (½)
D is sodium ethanoate (sodium acetate), $C H_{3} C O O N a$ . (½)
(b) $C H_{3} C H_{2} O H + 2 [O] alkaline K M n O_{4} + heat C H_{3} C O O H + H_{2} O$ (½)
Alkaline potassium permanganate is an oxidising agent: it adds oxygen to ethanol. (½)
(c) $C H_{3} C O O H + C H_{3} C H_{2} O H acid C H_{3} C O O C H_{2} C H_{3} + H_{2} O$ (½)
Esters are used in making perfumes and as flavouring agents. (½)
(d) $C H_{3} C O O C_{2} H_{5} + N a O H \to C_{2} H_{5} O H + C H_{3} C O O N a$ (½)
This is saponification, the reaction used to prepare soap. (½)

Answer: A = ethanol ( $C H_{3} C H_{2} O H$ ), B = ethanoic acid ( $C H_{3} C O O H$ ), C = the ester $C H_{3} C O O C H_{2} C H_{3}$ , D = sodium ethanoate ( $C H_{3} C O O N a$ ). Alkaline $K M n O_{4}$ is an oxidising agent; C forms by esterification and is used in perfumes and flavourings; C with NaOH is saponification.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Burning four hydrocarbon gases

Suppose your chemistry teacher gives your group four gas samples labelled P, Q, R and S, with only their formulae: P is $C_{2} H_{6}$ , Q is $C_{3} H_{6}$ , R is $C_{4} H_{10}$ and S is $C_{2} H_{2}$ . Under her supervision, you burn a little of each, one at a time, in a good supply of air, and hold a metal plate above each flame. P and R burn with a clean flame and leave the plate clean. Q and S burn with a yellow flame, give black smoke and leave a black deposit on the plate. Take the atomic masses as C = 12 u and H = 1 u.

(i) · 1 mark
Which two samples belong to the same homologous series? Find the difference in their molecular masses.
Answer
Answer: P and R are both alkanes. Their molecular masses differ by 28 u.
Both P and R fit $C_{n} H_{2 n + 2}$ . P = 24 + 6 = 30 u and R = 48 + 10 = 58 u, so the difference is 28 u, which is two $- C H_{2} -$ units.
(ii) · 1 mark
Why do Q and S give a sooty flame even with a good supply of air?
Answer
Answer: They are unsaturated hydrocarbons.
Q has a carbon–carbon double bond and S has a triple bond. Unsaturated carbon compounds burn with a yellow flame and lots of black smoke. The black deposit is soot, which is unburnt carbon.
(iii) · 2 marks
Name R and write the balanced equation for its complete combustion. How many structural isomers does R have?
Answer
Answer: R is butane. $2 C_{4} H_{10} + 13 O_{2} \to 8 C O_{2} + 10 H_{2} O$ . It has two structural isomers.
Naming R: 4 carbons, all single bonds, so butane (½). Equation: $2 C_{4} H_{10} + 13 O_{2} \to 8 C O_{2} + 10 H_{2} O$ . Check: C 8 = 8, H 20 = 20, O 26 = 16 + 10 (1). Isomers: there are two, the straight chain and the branched chain with one carbon on the middle carbon (½).

Reading two data tables: an alcohol series and a hard-water test

Suppose a student records two sets of observations in her notebook.

Table 1: four alcohols P, Q, R and S belong to one homologous series, each with one –OH group. Their molecular masses are P = 46 u, Q = 74 u, R = 32 u and S = 60 u. Their boiling points were not labelled.

Table 2: equal volumes of two water samples, W1 and W2, are shaken for the same time with equal drops of cleaning solution.

W1 with soap: plenty of foam, no solid
W2 with soap: very little foam, white curdy solid
W2 with detergent: good foam, no solid

Take C = 12 u, H = 1 u, O = 16 u.

(i) · 1 mark
Arrange P, Q, R and S in order of increasing boiling point and give the reason.
Answer
Answer: R < P < S < Q.
In a homologous series, boiling point rises as molecular mass increases (½). Ordering by mass, 32 u < 46 u < 60 u < 74 u, gives R < P < S < Q (½).
(ii) · 1 mark
Identify R and write the molecular formula of Q. Would P and Q react in the same way with sodium? Give a reason.
Answer
Answer: R is methanol, $C H_{3} O H$ ; Q is $C_{4} H_{9} O H$ (butanol). Yes, they react similarly.
$C H_{3} O H$ = 12 + 4 + 16 = 32 u, so R is methanol; $C_{4} H_{9} O H$ = 48 + 10 + 16 = 74 u, so Q is butanol (½). Both carry the same –OH functional group, and chemical properties depend on the functional group, so both give hydrogen with sodium (½).
(iii) · 2 marks
Which water sample is hard? What is the white curdy solid, and why does detergent still lather in this sample?
Answer
Answer: W2 is hard. The solid is scum. Detergent does not form an insoluble solid with the hardness-causing ions.
W2 is hard, because soap gives little foam and a curdy solid in it (½). The solid is scum, formed when soap reacts with the calcium and magnesium salts that cause hardness (½); soap is used up, so little lather forms (½). A detergent molecule has a different charged head that does not combine with $C a^{2 +}$ or $M g^{2 +}$ to form an insoluble solid, so it lathers well (½).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Covalent compounds such as methane generally have low melting and boiling points and are poor conductors of electricity. Explain both properties. (2 marks)

Step	Marks
The bonds inside each molecule are strong, but the forces between molecules are weak. Little energy is needed to separate the molecules, so melting and boiling points are low.	1
The electrons are shared between atoms, so no ions or other charged particles are formed. With nothing charged to carry current, these compounds are poor conductors.	1

What is a homologous series? State two of its features, and write the names and formulae of the first two members of the alcohol series. (3 marks)

Step	Marks
Definition: a series of compounds in which the same functional group replaces hydrogen in carbon chains of different lengths.	1
Feature 1: successive members differ by a $- C H_{2} -$ unit, which is 14 u in molecular mass.	0.5
Feature 2: chemical properties stay similar because they depend on the functional group. Physical properties such as boiling point change gradually as molecular mass increases.	0.5
Methanol, $C H_{3} O H$ (½) and ethanol, $C_{2} H_{5} O H$ (½).	1

(a) What is a functional group? (b) Draw the structures and write the names of three-carbon compounds containing (i) a bromine atom, (ii) an alcohol group, (iii) an aldehyde group, (iv) a ketone group. (5 marks)

Step	Marks
(a) An atom or group of atoms, such as –OH or –Cl, attached to a carbon chain in place of hydrogen. It gives the compound its characteristic properties whatever the length of the chain.	1
(i) Structure: $C H_{3} - C H_{2} - C H_{2} - B r$ , with every carbon showing four bonds.	0.5
(i) Name: bromopropane.	0.5
(ii) Structure: $C H_{3} - C H_{2} - C H_{2} - O H$ .	0.5
(ii) Name: propanol.	0.5
(iii) Structure: $C H_{3} - C H_{2} - C H O$ , with the end carbon double-bonded to O and single-bonded to H.	0.5
(iii) Name: propanal.	0.5
(iv) Structure: $C H_{3} - C O - C H_{3}$ , with the middle carbon double-bonded to O.	0.5
(iv) Name: propanone.	0.5

Where students lose marks in Carbon and its Compounds

The slips examiners see most often, and what to write instead.

Mistake: Leaving combustion equations unbalanced
It is easy to copy the skeleton equation from the textbook page and forget the instruction to balance it.
Wrong working
$C H_{4} + O_{2} \to C O_{2} + H_{2} O$
Right working
$C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ (C: 1 = 1, H: 4 = 4, O: 4 = 2 + 2)
Do this: Balance C first, then H, then O last; count every atom on both sides before moving on.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Missing the condition over the arrow
Once the atoms balance, the equation can look complete even without its condition.
Wrong working
$C H_{4} + C l_{2} \to C H_{3} C l + H C l$ and $C_{2} H_{4} + H_{2} \to C_{2} H_{6}$
Right working
$C H_{4} + C l_{2} sunlight C H_{3} C l + H C l$ and $C H_{2} = C H_{2} + H_{2} N i C H_{3} C H_{3}$
Do this: Learn each reaction with its condition as one unit: sunlight, Ni or Pd, hot conc. $H_{2} S O_{4}$ at 443 K, acid catalyst, alkaline $K M n O_{4}$ .
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Not counting the functional-group carbon when naming
The carbon in –COOH or –CHO is hidden inside the group, so it is easy to count only the chain before it.
Wrong working
$C H_{3} C H_{2} C O O H$ = ethanoic acid; $C H_{3} C O C H_{3}$ = propanal
Right working
$C H_{3} C H_{2} C O O H$ has three carbons: propanoic acid. $C H_{3} C O C H_{3}$ has C=O in the middle: propanone (ketone), not an aldehyde.
Do this: Circle every C in the formula before naming; C=O at the end is -al, in the middle is -one.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Wrong electron dot structure of $C O_{2}$
A single bond to each oxygen can look enough, and the lone pairs on oxygen are easy to leave out.
Wrong working
One shared pair between C and each O, no lone pairs shown.
Right working
Carbon in the centre with two shared pairs to each oxygen (two double bonds); each oxygen also has two lone pairs. Total 16 valence electrons; every atom has an octet.
Do this: Add up valence electrons (4 + 6 + 6 = 16) and check each atom has eight around it.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Using sodium to tell ethanol from ethanoic acid
Ethanol giving hydrogen with sodium is easy to remember, while the fact that acids also give hydrogen with reactive metals is easy to overlook.
Wrong working
Add sodium: the liquid that gives hydrogen is ethanol.
Right working
Add sodium hydrogencarbonate: ethanoic acid gives brisk effervescence of $C O_{2}$ (turns lime water milky), ethanol gives none. Or use blue litmus: only the acid turns it red.
Do this: Choose a test that gives different results for the two liquids.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Micelle drawn or described the wrong way round
The two ends of a soap molecule are easy to mix up.
Wrong working
The ionic heads point into the oil droplet and the tails stick out into water.
Right working
The hydrocarbon tails point into the oil droplet; the ionic heads face outwards into the water, forming an emulsion that is rinsed away.
Do this: Remember: tail loves oil, head loves water.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.
Mistake: Saying saturated hydrocarbons undergo addition
A hydrocarbon may be picked from a list without checking for a double or triple bond.
Wrong working
$C_{2} H_{6}$ , $C_{3} H_{8}$ and $C H_{4}$ undergo addition reactions.
Right working
Only unsaturated ones, such as $C_{3} H_{6}$ (fits $C_{n} H_{2 n}$ ) and $C_{2} H_{2}$ (fits $C_{n} H_{2 n - 2}$ ), undergo addition; alkanes undergo substitution.
Do this: Test each formula against $C_{n} H_{2 n + 2}$ first; if it fits, it is saturated.
Usually costs: Marks are lost in the step shown; the exact deduction depends on the marking scheme.

NCERT exercise guide: Carbon and its Compounds

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

In-text questions after section 4.1

What it tests: Drawing electron dot structures of covalent molecules, including molecules with double bonds and a molecule whose atoms are joined in a ring.

How to approach it: First write the electronic configuration of each atom and count its valence electrons. Work out how many electrons each atom needs to complete its octet (two for hydrogen), and share exactly that many. Show only valence electrons, using dots for one atom and crosses for the other. Finish by checking that every atom has 8 electrons around it, and every hydrogen has 2.

Mirrors the electron dot structure questions in this box

Draw the electron dot structure of ammonia, $N H_{3}$ (atomic numbers: N = 7, H = 1). State the type of bonds present and the number of shared pairs.

Show the solutionSolution

Nitrogen is 2, 5. It has 5 valence electrons and needs 3 more for an octet. Each hydrogen has 1 electron and needs 1 more.
Nitrogen shares one of its electrons with each of three hydrogen atoms, and each hydrogen gives one electron to its pair.
Write N in the centre with 5 dots. Place an H on three sides, each with one cross, and put each dot-cross pair between N and that H.
The remaining two dots of nitrogen stay as one unshared pair on the fourth side.
Check: nitrogen has 6 shared + 2 unshared = 8 electrons. Each hydrogen has 2 electrons, like helium.

Answer: Ammonia has three shared pairs, that is, three single covalent N–H bonds. One unshared pair remains on nitrogen.

In-text questions after section 4.2.5

What it tests: Structural isomers, the two properties behind carbon's versatility, cyclic compounds, drawing structures from names and naming compounds from structures.

How to approach it: To draw from a name, read the root for the number of carbons (meth 1, eth 2, prop 3, but 4, pent 5, hex 6). Then read the prefix or suffix for the functional group. Fill the remaining valencies with hydrogen so that each carbon has four bonds. To name a structure, count every carbon, including the carbon in –CHO or –COOH. For isomers, keep the molecular formula fixed and change only the arrangement.

Mirrors the drawing-structures and isomer questions in this box

Draw the structures of propanal and propanone. Write their molecular formulae and state whether they are structural isomers.

Show the solutionSolution

Propanal: a three-carbon chain with the aldehyde group at the end, $C H_{3} - C H_{2} - C H O$ . The end carbon is double-bonded to O and also carries one H.
Propanone: a three-carbon chain with the ketone group on the middle carbon, $C H_{3} - C O - C H_{3}$ . The middle carbon is double-bonded to O.
Count the atoms: each has 3 C, 6 H and 1 O, so both are $C_{3} H_{6} O$ .
They have the same molecular formula but different structures (the C=O is at the end in one and in the middle in the other), so they are structural isomers.

Answer: Propanal is $C H_{3} C H_{2} C H O$ and propanone is $C H_{3} C O C H_{3}$ . Both are $C_{3} H_{6} O$ , so yes, they are structural isomers.

In-text questions after section 4.3.4

What it tests: Why changing an alcohol into a carboxylic acid counts as oxidation, and why complete combustion needs a rich supply of oxygen (the welding question on ethyne).

How to approach it: For oxidation, compare the formulae before and after the reaction. If oxygen has been added, the compound has been oxidised, and the reagent that supplied the oxygen is the oxidising agent. For combustion, link the oxygen supply to the result. Enough oxygen gives complete burning, more heat and a clean flame. Too little oxygen gives incomplete burning and soot.

Mirrors the oxidation question in this box

Methanol, $C H_{3} O H$ , is warmed with alkaline potassium permanganate and changes to methanoic acid, $H C O O H$ . Why is this an oxidation reaction? What is the role of alkaline potassium permanganate?

Show the solutionSolution

Compare the formulae: $C H_{3} O H$ has one O atom, while $H C O O H$ has two O atoms.
Oxygen has been added to the starting compound, so the alcohol has been oxidised to a carboxylic acid.
Alkaline potassium permanganate supplies this oxygen, so it is the oxidising agent.

Answer: It is oxidation because oxygen is added to methanol. Alkaline $K M n O_{4}$ acts as the oxidising agent.

In-text questions after sections 4.4.2 and 4.5

What it tests: Telling an alcohol from a carboxylic acid by experiment, the meaning of oxidising agents, whether detergent can show that water is hard, and why clothes need agitation during washing.

How to approach it: For a 'distinguish' question, pick a test that gives different results for the two substances. State what you do, what each sample shows, and the equation for the one that reacts. For hard-water questions, remember the key difference: soap reacts with calcium and magnesium salts to form scum, but detergent does not. Agitation helps the micelles pull oily dirt off the cloth and into the water.

Mirrors the detergent and hard-water question in this box

A student has two water samples, one hard and one soft, but only detergent solution. Can she find out which sample is hard? What should she use instead, and what would she see?

Show the solutionSolution

The charged ends of detergent molecules do not form insoluble precipitates with calcium and magnesium ions, so detergent lathers well in both hard and soft water.
So detergent gives roughly the same foam in both samples. It cannot show which one is hard.
She should use soap solution instead. She adds equal drops to equal volumes of each sample and shakes them for the same time.
Soft water gives plenty of foam. Hard water gives little foam and a white curdy precipitate (scum).

Answer: No. Detergent lathers in both samples. Soap tells them apart: the hard water gives less foam and a white scum.

End-of-chapter exercises

What it tests: Bond counting, identifying functional groups, incomplete combustion, the nature of the covalent bond, electron dot structures, homologous series, distinguishing ethanol from ethanoic acid, micelles, carbon compounds as fuels, scum, soap and litmus, hydrogenation, addition reactions, a test for unsaturation, and the cleaning action of soap.

How to approach it: The MCQs test one idea each, so check every option against the definition. For the 3- to 5-mark questions, answer in points: definition, example, equation with conditions, then a reason. For electron dot structures, count valence electrons and check every octet. For a saturated-versus-unsaturated test, use one from the chapter: the flame on burning, or whether the compound adds hydrogen with a nickel catalyst.

Mirrors the bond-counting MCQ

Propane, $C_{3} H_{8}$ , has
(a) 8 covalent bonds
(b) 9 covalent bonds
(c) 10 covalent bonds
(d) 11 covalent bonds

Show the solutionSolution

Structure: $C H_{3} - C H_{2} - C H_{3}$ , a chain of three carbons.
C–C bonds: 2.
C–H bonds: 3 + 2 + 3 = 8.
Total = 2 + 8 = 10.

Answer: (c) 10 covalent bonds

Key activities: Activity 4.3 to Activity 4.12

What it tests: Flames of carbon compounds (4.3, 4.4), oxidation of ethanol (4.5), sodium with ethanol (4.6), ethanoic acid compared with hydrochloric acid (4.7), forming an ester (4.8), ethanoic acid with carbonates (4.9), and soap with oil and with hard water (4.10 to 4.12). Practicals 8 and 9 are set on these activities.

How to approach it: For any activity question, write four things: what is done, what is seen, what it shows, and one precaution. Link every observation to the chemistry. For example, the purple colour of permanganate vanishing means it is being used up as it adds oxygen. Use exact words such as 'brisk effervescence', 'lime water turns milky' and 'white curdy precipitate'.

Mirrors Activity 4.5

About 3 mL of ethanol is warmed in a water bath, and 5% alkaline potassium permanganate is added drop by drop.
(a) What happens to the purple colour at first, and what happens later?
(b) Explain both observations.
(c) Why is a water bath used?

Show the solutionSolution

At first, the purple colour disappears as each drop is added.
The permanganate is used up as it adds oxygen to ethanol and oxidises it to ethanoic acid: $C H_{3} C H_{2} O H alk. K M n O_{4} + heat C H_{3} C O O H$ .
Once all the ethanol has been oxidised, any extra permanganate has nothing to react with, so the purple colour stays.
A water bath warms ethanol gently. Ethanol burns easily, so it should not be heated directly over a flame.

Answer: The colour fades at first because permanganate oxidises ethanol to ethanoic acid and is used up. It stays once the ethanol is used up. The water bath gives gentle, safe heating.

Carbon and its Compounds: previous year questions

Asked in 21 of the 21 board papers we hold (2023 to 2026), on average 6.2 marks per paper; 66 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Carbon and its Compounds

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

2026, set 31/1/1, Q20 · 1 mark · MCQ

The formula of functional group for aldehyde is:

(A)–COOH
(B)–CHO
(C)–C(=O)–
(D)–OH

Try it first, then show our solutionSolution

Match each group to its class: –COOH is a carboxylic acid, –C(=O)– inside a chain is a ketone, and –OH is an alcohol.
The aldehyde group is –CHO, for example methanal HCHO. Correct option: (B).
1 mark

Answer: –CHO, option (B)

Where the marks are

Selects –CHO as the aldehyde group (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/1/1, Q29(a) · 5 marks · Short answer

Answer the following.

(i)
Give reasons for the following: (I) Covalent compounds are poor conductor of electricity. (II) Soap does not form lather in hard water. (III) Carbon shows catenation but silicon does not.
3 marks
(ii)
Write chemical equations for the following: (I) Oxidation of ethanol by acidified K₂Cr₂O₇. (II) Hydrogenation of ethene.
2 marks

Try it first, then show our solutionSolution

(i)(I) Covalent compounds are formed by sharing of electrons, so they do not form ions (charged particles). With no ions to carry charge, they are poor conductors of electricity.
1 mark
(i)(II) Hard water contains calcium and magnesium salts. Soap reacts with these salts to form an insoluble precipitate called scum, so the soap is used up and does not form lather easily.
1 mark
(i)(III) C–C bonds are very strong and stable because carbon is a small atom, so carbon can form long chains (catenation). Si–Si bonds are much weaker, so silicon does not form long stable chains.
1 mark
(ii)(I) $C H_{3} C H_{2} OH acidified K_{2} C r_{2} O_{7}, heat C H_{3} COOH$ (ethanol is oxidised to ethanoic acid)
1 mark
(ii)(II) $C H_{2} = C H_{2} + H_{2} Ni catalyst C H_{3} - C H_{3}$ (ethene is hydrogenated to ethane)
1 mark

Answer: (i) (I) No ions formed; (II) Ca/Mg salts form scum with soap; (III) C–C bonds strong, Si–Si bonds weak; (ii) CH₃CH₂OH → CH₃COOH; CH₂=CH₂ + H₂ → CH₃CH₃ (Ni)

Where the marks are

No ions/charged particles in covalent compounds (1 mark)
Scum formed with Ca²⁺/Mg²⁺ salts (1 mark)
Strong C–C bond versus weak Si–Si bond (1 mark)
Oxidation equation (1 mark)
Hydrogenation equation with Ni (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/5/1, Q28 · 4 marks · Long answerCompetency-based

Alcohol forms a homologous series with general formula $C_{n} H_{2n + 1}$ –OH and –OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is good solvent, used in medicines, cough syrups, tonics etc. $1 + 1 + 2$

(a)
Write structural formula and name of 4th member of alcohol homologous series.
1 mark
(b)
What happens when ethanol is heated with alkaline $KMn O_{4}$ ? Write chemical equation involved.
1 mark
(c)
Write the chemical equation of reaction of ethanol with ethanoic acid in the presence of concentrated $H_{2} S O_{4}$ . Write the name of this reaction.
2 marks
OR
(c) OR
What happens when ethanol is heated with excess concentrated sulphuric acid at 443 K ? Write chemical equation involved. What is the role of concentrated sulphuric acid in this reaction ?
2 marks

Try it first, then show our solutionSolution

(a) Fourth member (n = 4): $C H_{3} - C H_{2} - C H_{2} - C H_{2} - OH$ (draw all C–H bonds with –OH on the end carbon).
½ mark
Name: Butanol ( $C_{4} H_{9} OH$ ).
½ mark
(b) Ethanol is oxidised to ethanoic acid by alkaline KMnO₄ (oxidising agent).
½ mark
$C H_{3} C H_{2} OH Alkaline K M n O_{4} + Heat C H_{3} COOH$
½ mark
(c) $C H_{3} COOH + C H_{3} C H_{2} OH Conc. H_{2} S O_{4} C H_{3} COO C_{2} H_{5} + H_{2} O$ (ethyl ethanoate, sweet-smelling ester)
1 mark
This reaction is called esterification.
1 mark
(c) OR – Ethanol is dehydrated to ethene.
½ mark
$C H_{3} C H_{2} OH Hot conc. H_{2} S O_{4}, 443 K C H_{2} = C H_{2} + H_{2} O$
½ mark
Role of conc. H₂SO₄: dehydrating agent – it removes water from ethanol.
1 mark

Answer: (a) CH₃CH₂CH₂CH₂OH, butanol; (b) ethanoic acid formed; (c) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O, esterification. OR: ethene formed, conc. H₂SO₄ is dehydrating agent.

Where the marks are

Structural formula of 4th member (½ mark)
Name butanol (½ mark)
States oxidation to ethanoic acid (½ mark)
Equation with alkaline KMnO₄ (½ mark)
Esterification equation (1 mark)
Names esterification (1 mark)
OR: ethene formed (½ mark), equation at 443 K (½ mark), dehydrating agent (1 mark)

See it in the full 2026 board paper, set 31/5/1

Every Carbon and its Compounds question, by year

For 2027

Sample paper 2026-27: Q19 (1 mark, solved), Q21 (1 mark, solved), Q22 (1 mark, solved), Q25 (2 marks, solved), Q26(A) (3 marks, solved), Q26(B) (3 marks, solved)

2026

Set 31/1/1: Q20 (1 mark, solved), Q24 (1 mark, solved), Q29(a) (5 marks, solved), Q29(b) (5 marks, solved)

Set 31/2/1: Q24 (1 mark, solved), Q29(a) (5 marks, solved), Q29(b) (5 marks, solved)

Set 31/3/1: Q20 (1 mark, solved), Q24 (1 mark, solved), Q29(a) (5 marks, solved), Q29(b) (5 marks, solved)

Set 31/4/1: Q20 (1 mark, solved), Q24 (1 mark, solved), Q29(A) (5 marks, solved), Q29(B) (5 marks, solved)

Set 31/5/1: Q18 (1 mark, solved), Q21 (1 mark, solved), Q28 (4 marks, solved)

Sample paper 2025-26: Q24 (1 mark, solved), Q29(A) (5 marks, solved), Q29(B) (5 marks, solved)

2025

Set 31/1/1: Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/2/1: Q22 (2 marks, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/3/1: Q4 (1 mark, solved), Q5 (1 mark, solved)

Set 31/4/1: Q4 (1 mark, solved), Q34(A) (5 marks, solved), Q34(B) (5 marks, solved)

Set 31/5/1: Q7 (1 mark, solved), Q34(A) (5 marks, solved), Q34(B) (5 marks, solved), Q37 (4 marks, solved)

Set 31/6/1: Q4 (1 mark, solved), Q17 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Sample paper 2024-25: Q34(A) (5 marks, solved), Q34(B) (5 marks, solved)

2024

Set 31/1/1: Q4 (1 mark, solved), Q34(A) (5 marks, solved), Q34(B) (5 marks, solved)

Set 31/2/1: Q7 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/3/1: Q4 (1 mark, solved), Q6 (1 mark, solved), Q37 (4 marks, solved)

Set 31/4/1: Q5 (1 mark, solved), Q22 (2 marks, solved), Q28 (3 marks, solved)

Set 31/5/1: Q17 (1 mark, solved), Q22 (2 marks, solved), Q37 (4 marks, solved)

Sample paper 2023-24: Q34(a) (5 marks, solved), Q34(b) (5 marks, solved), Q37 (4 marks, solved)

2023

Set 31/1/1: Q7 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/2/1: Q7 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/4/1: Q7 (1 mark, solved), Q34(A) (5 marks, solved), Q34(B) (5 marks, solved)

Set 31/5/1: Q7 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/6/1: Q20 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Practice questions on Carbon and its Compounds

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A student shakes equal volumes of a water sample X, first with soap solution and then, in a fresh test tube, with detergent solution. Soap gives very little lather and a white curdy solid; detergent lathers easily. Which inference is correct?
(a) X is soft water, and the soap sample is impure.
(b) X contains calcium and magnesium salts, which form an insoluble substance with soap but not with detergent.
(c) Detergent reacts with calcium and magnesium ions to form scum, which helps it lather.
(d) Sodium ions in X react with soap to form scum, while detergent does not react with them.
Hint
Think about which salts cause hardness in water, and which cleaning agent forms scum with them.
Answer
(b) X contains calcium and magnesium salts, which form an insoluble substance with soap but not with detergent. The curdy solid is scum, which shows X is hard water; detergents do not form scum, so they lather.
Q2 · 2 marks · Very short answerBoard level
Carbon has four valence electrons. Give a reason why it does not form a $C^{4 -}$ ion and a reason why it does not form a $C^{4 +}$ ion. How does carbon reach a noble gas configuration instead?
Hint
Compare the number of protons with the number of electrons in each case.
Answer
$C^{4 -}$ : a nucleus with 6 protons would find it hard to hold on to 10 electrons (½). $C^{4 +}$ : removing four electrons would need a large amount of energy (½). Instead, carbon shares its four valence electrons with other atoms, forming covalent bonds (1).
Q3 · 1 mark · Assertion-reasonBoard level
Assertion (A): Methane reacts with chlorine in sunlight to give chloromethane and hydrogen chloride.
Reason (R): Methane is a saturated hydrocarbon, so in sunlight chlorine replaces its hydrogen atoms rather than adding across a bond.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does methane have a double bond for chlorine to add across?
Answer
(a). $C H_{4} + C l_{2} sunlight C H_{3} C l + H C l$ . Methane has only single bonds, so there is no bond for chlorine to add across; instead a chlorine atom takes the place of a hydrogen atom. R explains why the products are chloromethane and HCl.
Q4 · 3 marks · Short answerBoard level
Draw the electron dot structure of methanol, $C H_{3} O H$ . How many shared pairs of electrons does it have? How many unshared pairs are there on oxygen?
Hint
Carbon makes four bonds and oxygen makes two. Then check the octet on oxygen.
Answer
Carbon shares one pair with each of three H atoms and one pair with O. Oxygen shares one pair with C and one pair with H. That gives 5 shared pairs (three C–H, one C–O, one O–H), all single bonds. Oxygen keeps 2 unshared pairs, so it has 4 shared + 4 unshared = 8 electrons.
Q5 · 3 marks · Short answerHarder board level
You are given ethanol and ethanoic acid in unlabelled bottles. Describe two tests to tell them apart, and write the equation for the reaction in one of the tests.
Hint
Only one of the two is an acid.
Answer
Test 1: blue litmus turns red with ethanoic acid. Ethanol has no effect on litmus (1). Test 2: add sodium hydrogencarbonate. Ethanoic acid gives brisk effervescence of a gas that turns lime water milky, while ethanol shows no change (1). Equation: $C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2}$ (1).
Q6 · 5 marks · Long answerHarder board level
An organic compound X, $C_{2} H_{4} O_{2}$ , turns blue litmus red. A 5-8% solution of X in water is used to preserve pickles.
(a) Identify X, give its common name, and name its dilute solution. (1)
(b) Why does pure X often freeze in cold winters, and what is it then called? (1)
(c) Write balanced equations for the reaction of X with sodium hydroxide and with sodium carbonate. Name the salt formed. (2)
(d) Dilute X and dilute hydrochloric acid are tested with universal indicator. Which shows the lower pH, and why? (1)
Hint
Pure X has a melting point of 290 K.
Answer
(a) X is ethanoic acid, commonly called acetic acid. Its dilute solution is vinegar. (b) Its melting point is 290 K, so it freezes in cold weather; it is called glacial acetic acid. (c) $N a O H + C H_{3} C O O H \to C H_{3} C O O N a + H_{2} O$ and $2 C H_{3} C O O H + N a_{2} C O_{3} \to 2 C H_{3} C O O N a + H_{2} O + C O_{2}$ . The salt is sodium ethanoate (sodium acetate). (d) Hydrochloric acid shows the lower pH. It is completely ionised, while ethanoic acid is a weak acid.
Q7 · 3 marks · Short answerBoard level
The first member of the alkyne series is ethyne, $C_{2} H_{2}$ . (a) Write the general formula of alkynes. (b) Write the name and formula of the third member. (c) Find its molecular mass (C = 12 u, H = 1 u) and state by how much it differs from the second member.
Hint
Each alkyne has two fewer hydrogens than the alkene with the same number of carbons.
Answer
(a) $C_{n} H_{2 n - 2}$ (1). (b) The members are ethyne $C_{2} H_{2}$ , then propyne $C_{3} H_{4}$ , then butyne $C_{4} H_{6}$ , so the third member is butyne, $C_{4} H_{6}$ (1). (c) Mass = 48 + 6 = 54 u. It differs from propyne (40 u) by 14 u, one $- C H_{2} -$ unit (1).
Q8 · 3 marks · Short answerHarder board level
What is hydrogenation? Write the condition needed, and state why the reaction works on vegetable oils but not on animal fats. Which of the two is advised for cooking, and why?
Hint
Addition needs a double or triple bond to add across.
Answer
Hydrogenation is the addition of hydrogen to an unsaturated compound to give a saturated one, using a catalyst such as nickel or palladium (1). Vegetable oils have long unsaturated carbon chains, so they can add hydrogen. Animal fats have saturated chains, so they cannot (1). Oils with unsaturated fatty acids are advised for cooking, because saturated fatty acids in animal fats are said to be harmful to health (1).
Q9 · 3 marks · Short answerHOTS / competency
For welding, ethyne is burnt with pure oxygen rather than with air. Write the balanced equation for the complete combustion of ethyne, and justify the choice of oxygen over air.
Hint
Ethyne is unsaturated. Think about what a limited supply of oxygen does to burning.
Answer
$2 C_{2} H_{2} + 5 O_{2} \to 4 C O_{2} + 2 H_{2} O$ . Check: C 4 = 4, H 4 = 4, O 10 = 8 + 2 (1). Air supplies far less oxygen than pure oxygen does. With limited oxygen, an unsaturated fuel like ethyne burns incompletely, giving a sooty flame and less heat (1). Pure oxygen lets ethyne burn completely, giving the very hot, clean flame that welding needs (1).
Q10 · 4 marks · Short answerHOTS / competency
An open-chain hydrocarbon with only one multiple bond burns completely according to the equation
$2 C_{x} H_{y} + 11 O_{2} \to 8 C O_{2} + 6 H_{2} O$
(a) Find x and y and write the molecular formula. (1½)
(b) Is the compound saturated or unsaturated? Justify using a general formula, and name it. (1)
(c) Predict the kind of flame it gives. (½)
(d) Write the name and formula of its next higher homologue and find that homologue's molecular mass (C = 12 u, H = 1 u). (1)
Hint
Two molecules burn, so halve the atom counts on the right to get one molecule.
Answer
(a) Carbon: 2x = 8, so x = 4. Hydrogen: 2y = 6 × 2 = 12, so y = 6. Check oxygen: left 11 × 2 = 22; right 8 × 2 + 6 = 22. Formula $C_{4} H_{6}$ (1½). (b) $C_{4} H_{6}$ fits $C_{n} H_{2 n - 2}$ with n = 4, so it is an alkyne with a triple bond: unsaturated, butyne (1). (c) A yellow flame with black smoke (½). (d) Add a $C H_{2}$ unit: pentyne, $C_{5} H_{8}$ , molecular mass 5 × 12 + 8 × 1 = 68 u (1).
Q11 · 3 marks · Short answerHarder board level
An organic compound X has the molecular formula $C_{2} H_{4} O_{2}$ . Its solution turns blue litmus red. When sodium carbonate is added to it, a gas is given off that turns lime water milky. When X is warmed with ethanol and a few drops of concentrated sulphuric acid, a sweet-smelling substance Y is formed.
(a) Identify X and Y. (1)
(b) Write the balanced equations for the reaction of X with sodium carbonate and with ethanol. (2) (3 marks)
Hint
Which carboxylic acid has two carbon atoms? What class of compound smells sweet?
Answer
(a) X is ethanoic acid (acetic acid), $C H_{3} C O O H$ ; Y is the ester ethyl ethanoate, $C H_{3} C O O C_{2} H_{5}$ . (1)
(b) $2 C H_{3} C O O H + N a_{2} C O_{3} \to 2 C H_{3} C O O N a + H_{2} O + C O_{2}$ (1)
$C H_{3} C O O H + C_{2} H_{5} O H acid C H_{3} C O O C_{2} H_{5} + H_{2} O$ (1)
Q12 · 3 marks · Short answerHOTS / competency
Suppose three test tubes each hold 10 mL of the same water sample. In tube A a few drops of soap solution are added; in tube B the same amount of detergent solution is added; tube C gets soap solution after the water has been made soft. All three are shaken equally. The results: A gives very little foam and a white curdy layer; B and C give plenty of foam.
(a) Is the water sample hard or soft? Give the evidence. (1)
(b) Explain why A forms a curdy layer while B foams well. (2) (3 marks)
Hint
Compare A with C first: same soap, different water.
Answer
(a) Hard: the same soap foams well in softened water (C) but forms scum in the sample (A), so the sample contains the salts that make water hard. (1)
(b) Soap reacts with the calcium and magnesium salts in hard water to form an insoluble substance (scum), so much of the soap is wasted and little foam forms. (1) Detergents do not form insoluble precipitates with these salts, so they lather and clean well even in hard water. (1)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Why can carbon not form $C^{4 +}$ easily?Show answer
Removing four electrons needs a large amount of energy, leaving six protons holding only two electrons.
2.What is catenation?Show answer
The ability of carbon atoms to bond with other carbon atoms to form long chains, branched chains and rings.
3.Define structural isomers.Show answer
Compounds with the same molecular formula but different structures, for example the two forms of $C_{4} H_{10}$ .
4.By how much do successive members of a homologous series differ?Show answer
By a $C H_{2}$ unit, which is 14 u in molecular mass.
5.What is an oxidising agent? Name two.Show answer
A substance that adds oxygen to others; alkaline potassium permanganate and acidified potassium dichromate.
6.What catalyst is used to hydrogenate vegetable oils?Show answer
Nickel.
7.What is glacial acetic acid and why is it so called?Show answer
Pure ethanoic acid; its melting point is 290 K, so it often freezes in cold winters.
8.What is a micelle?Show answer
A cluster of soap molecules with hydrocarbon tails inside, holding the oily dirt, and ionic ends facing the water.

Best order to study it

Start with covalent bonding: why carbon shares, then practise electron dot structures of $H_{2}$ , $O_{2}$ , $N_{2}$ , $C H_{4}$ , $C O_{2}$ and ethane until you can do them without looking.
Learn catenation and tetravalency, then saturated versus unsaturated, chains, branches, rings and isomers by drawing butane and pentane isomers.
Learn the functional groups and homologous series together with the general formulae, then master the four naming steps with many examples.
Learn the four reaction types (combustion, oxidation, addition, substitution) as balanced equations with conditions, and link each to saturated or unsaturated compounds.
Study ethanol and ethanoic acid: properties, uses and every equation; then practise identify-X and distinguish questions.
Finish with soaps and detergents: micelle, scum and the hard water activities, and revise by explaining cleaning action aloud.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you explain in two lines why carbon forms covalent bonds and not $C^{4 -}$ or $C^{4 +}$ ions?
Can you draw the electron dot structures of $N_{2}$ , $C O_{2}$ , ethene and ethyne with all lone pairs?
Can you state why covalent compounds have low melting points and do not conduct electricity?
Can you draw the three isomers of pentane and explain why cyclopentane is not one of them?
Can you write the general formulae of alkanes, alkenes and alkynes and find the next homologue of any member?
Can you name $C H_{3} C H_{2} C H O$ , $C H_{3} C O C H_{2} C H_{3}$ , $C H_{3} C H_{2} B r$ and $C H_{3} C \equiv C H$ ?
Can you write balanced equations, with conditions, for combustion of methane, hydrogenation of ethene and chlorination of methane?
Can you write ethanol's reactions with sodium and with hot concentrated sulphuric acid at 443 K?
Can you write esterification, saponification and the reactions of ethanoic acid with NaOH, $N a_{2} C O_{3}$ and $N a H C O_{3}$ ?
Can you explain micelle formation, scum in hard water and why detergents work in hard water?

Carbon and its Compounds: frequently asked questions

Do I need to learn diamond, graphite and fullerenes for the board exam?

These appear in a More to Know box in the chapter. Information in NCERT text boxes is not assessed in the board exam, so treat them as interesting background. Spend your time on the main text topics instead.

Do I need to write position numbers like 1-chloropropane?

The NCERT chapter names compounds such as chloropropane, propanol and butanone without position numbers. Learn the NCERT method: count carbons, add the prefix or suffix, drop the final e before a vowel suffix, and use ene or yne for unsaturation. That is what the syllabus covers.

How do I test whether a hydrocarbon is saturated or unsaturated using the NCERT chapter?

Burn a small amount as in the NCERT burning activity. Saturated hydrocarbons generally give a clean flame, while unsaturated ones give a yellow flame with lots of black smoke and leave soot on a metal plate held above. Remember that a poor air supply can make even a saturated fuel sooty.

Why is changing ethanol into ethanoic acid called oxidation?

Oxygen is added to ethanol by an oxidising agent such as alkaline potassium permanganate or acidified potassium dichromate. Adding oxygen is oxidation. You can also see that ethanol has one oxygen atom while ethanoic acid has two.

What is the main difference between soaps and detergents?

Soaps are sodium or potassium salts of long-chain carboxylic acids, and they form insoluble scum with the calcium and magnesium salts in hard water. Detergents also have a long hydrocarbon tail but a different charged head, of a sulphonate or ammonium type. This head does not combine with calcium or magnesium ions to form an insoluble solid, so detergents keep working in hard water.

Based on: CBSE Science (086) syllabus 2026-27; NCERT Science, Textbook for Class X (Reprint 2026-27)