Carbon and its Compounds ICSE Class 8: Allotropes, Carbon Dioxide and Carbon Monoxide

What Carbon and its Compounds is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Carbon and its Compounds sits in the Theme: Carbon and its Compounds theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Allotropy

Allotropy is the existence of an element in two or more different forms in the same physical state, which have different physical properties but the same chemical properties. These forms are called allotropes.

Allotropes of carbon:

• Crystalline (atoms in a regular, repeating arrangement; definite shape): diamond, graphite, fullerenes (such as buckminsterfullerene, C₆₀, a football-shaped molecule).
• Amorphous (no regular arrangement; no definite shape): coal, coke, charcoal (wood charcoal, animal charcoal, sugar charcoal), lamp black or soot, gas carbon.

Why are physical properties different? The carbon atoms are arranged differently.

Why are chemical properties the same? All are made of carbon atoms only. Proof: equal masses of diamond, graphite or charcoal, burnt in oxygen, each give only carbon dioxide, and the same mass of it.

$C+O_2\to C{O}_2$ (12 g of any allotrope gives 44 g of CO₂).

Worked example

6 g of diamond and 6 g of graphite are burnt separately in excess oxygen. 12 g of carbon gives 44 g of carbon dioxide. Find the mass of CO₂ from each, and state what this proves.

Try it first, then show the solutionSolution

1. Each gives 6 × 44 ÷ 12 = 22 g of carbon dioxide.
2. Same product and same mass from both: they are made of the same element, carbon, so they are allotropes with the same chemical properties.

Answer: 22 g each; both are pure carbon (allotropes).

Diamond and graphite

Property	Diamond	Graphite
Arrangement	each C joined to 4 others in a rigid 3-D network	each C joined to 3 others in flat layers of hexagons; layers held weakly
Hardness	hardest natural substance	soft, slippery, greasy to touch
Appearance	transparent, brilliant (sparkles)	greyish-black, opaque, shiny
Electricity	non-conductor	good conductor
Heat	good conductor	good conductor

Uses of diamond (from hardness and sparkle):

• cutting glass and marble; drilling through rocks; grinding and polishing tools;
• jewellery (it sparkles because it bends and reflects light strongly);
• fine surgical instruments, for example for eye operations.

Uses of graphite (from softness, slipperiness and conductivity):

• pencil 'lead' (graphite mixed with clay);
• lubricant for machines that get too hot for oil;
• electrodes in electrolysis and in dry cells;
• crucibles for melting metals; moderator in nuclear reactors.

Worked example

Why is graphite used as a lubricant and in electrodes, but diamond is not?

Try it first, then show the solutionSolution

1. Graphite's layers slide over one another, so it is slippery (lubricant); diamond's rigid network makes it hard.
2. Graphite conducts electricity (electrodes); diamond does not.

Answer: Graphite is slippery and conducts electricity; diamond is hard and a non-conductor.

Amorphous carbon: coal, coke, charcoal, soot

• Coal: a fossil fuel formed from plants buried for millions of years. Uses: fuel in homes, industry and power stations; making coke, coal gas and coal tar.
• Coke: the residue left when coal is heated strongly in the absence of air (destructive distillation). It is almost pure carbon, burns without smoke and gives great heat. Uses: smokeless fuel; reducing agent in extracting metals such as iron; making water gas.
• Charcoal: made by heating wood (or bones, or sugar) in a limited supply of air. It is porous and adsorbs (holds on its surface) gases and colouring matter. Uses: in gas masks and water filters; animal charcoal for removing colour from cane sugar; as a fuel.
• Soot (lamp black): formed when oil, kerosene or wax burns in a limited supply of air (the black deposit on a lamp chimney). Uses: printing ink, black paint, shoe polish, carbon paper and in rubber tyres to make them strong and black.

Why soot forms: in limited air, part of the carbon in the fuel does not burn, and settles as fine black particles.

Worked example

Name the form of carbon used (i) to make car tyres black and tough (ii) as a smokeless fuel in a blast furnace (iii) to remove the yellow colour from raw sugar.

Try it first, then show the solutionSolution

1. (i) Soot (lamp black, carbon black).
2. (ii) Coke. (iii) Animal charcoal, which adsorbs colouring matter.

Answer: (i) Soot (ii) Coke (iii) Animal charcoal

Laboratory preparation of carbon dioxide

Reactants: marble chips (calcium carbonate) and dilute hydrochloric acid.

$CaC{O}_3+2HCl\to CaC{l}_2+H_{2}O+C{O}_2$

Apparatus: marble chips in a Woulfe's bottle (or flat-bottomed flask) with a thistle funnel dipping into the acid and a delivery tube.

Collection: by upward displacement of air (the gas jar stands upright), because carbon dioxide is about 1.5 times heavier than air. It is not usually collected over water because it is fairly soluble in water.

Test that the jar is full: a burning splinter held at the mouth of the jar goes out.

Why not dilute sulphuric acid? It forms calcium sulphate, which is only slightly soluble. It coats the marble chips and stops the reaction after a short time.

Worked example

Why is carbon dioxide collected by upward displacement of air and not over water?

Try it first, then show the solutionSolution

1. It is heavier than air (about 1.5 times), so it stays at the bottom of an upright jar and pushes air up and out.
2. It is fairly soluble in water, so some would dissolve and be lost.

Answer: Heavier than air; fairly soluble in water.

Properties of carbon dioxide

Physical properties:

• Colourless and odourless gas.
• About 1.5 times heavier than air: it can be poured from one jar to another like water.
• Fairly soluble in water; much more soluble under pressure (soda water).
• Under pressure and cooling it forms solid CO₂, dry ice, which turns straight into gas (sublimes) without melting.

Chemical properties:

• Acidic oxide: it dissolves in water to form weak carbonic acid, which turns moist blue litmus light red.

$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3$

• Lime water test: it turns lime water milky, because insoluble white calcium carbonate forms.

$Ca(OH{)}_2+C{O}_2\to CaC{O}_3+H_{2}O$
With excess CO₂, the milkiness disappears, because soluble calcium hydrogencarbonate forms.
$CaC{O}_3+H_{2}O+C{O}_2\to Ca(HC{O}_3{)}_2$

• Reacts with alkalis to form a salt and water: $2NaOH+C{O}_2\to N{a}_{2}C{O}_3+H_{2}O$.
• Does not burn and does not support combustion of most substances. But a burning magnesium ribbon keeps burning in it, leaving white magnesium oxide and black specks of carbon: $2Mg+C{O}_2\to 2MgO+C$.
• Plants use it in photosynthesis.

Uses: fire extinguishers (heavier than air and does not support burning, so it blankets the fire); aerated soft drinks and soda water; dry ice to keep food and ice cream cold and to make stage 'smoke'; photosynthesis in plants.

Worked example

Carbon dioxide is bubbled through lime water for a long time. Describe and explain what you see, with equations.

Try it first, then show the solutionSolution

1. First the lime water turns milky: $Ca(OH{)}_2+C{O}_2\to CaC{O}_3+H_{2}O$ (insoluble white calcium carbonate).
2. On passing excess CO₂, the milkiness disappears: $CaC{O}_3+H_{2}O+C{O}_2\to Ca(HC{O}_3{)}_2$ (soluble).

Answer: Milky, then clear again.

Carbon monoxide

Formation: by burning carbon or carbon fuels in a limited supply of air (incomplete combustion): $2C+O_2\to 2CO$. It is present in car exhaust and in smoke from charcoal stoves (angithi or sigri) and gas geysers in badly ventilated rooms.

Properties:

• Colourless, odourless and almost insoluble in water, so you cannot see or smell it.
• A neutral oxide: no effect on litmus.
• Burns in air with a blue flame: $2CO+O_2\to 2C{O}_2$; so it is a component of fuels such as water gas.
• A strong reducing agent.

Use as a reducing agent in the extraction of iron: in the blast furnace, carbon monoxide removes oxygen from iron ore (iron(III) oxide).

$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$

Fe₂O₃ is reduced; CO is oxidised.

Harmful effects (asphyxia): when inhaled, carbon monoxide combines with haemoglobin in red blood cells far more readily than oxygen does, forming carboxyhaemoglobin. The blood can then carry much less oxygen. The person gets a headache, dizziness and drowsiness, becomes unconscious, and may die of lack of oxygen (asphyxia, suffocation).

Safety: never burn charcoal or run a car engine in a closed room or garage; keep rooms ventilated; fit gas geysers outside the bathroom.

Worked example

In $F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$, which substance is reduced and which is the reducing agent?

Try it first, then show the solutionSolution

1. Fe₂O₃ loses oxygen and becomes iron: it is reduced.
2. CO takes the oxygen and becomes CO₂: it is oxidised, so CO is the reducing agent.

Answer: Fe₂O₃ is reduced; CO is the reducing agent.

Chemistry 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.

Topics in this chapter

• Allotropes of Carbon: Diamond, Graphite and Amorphous Forms
• Carbon Dioxide and Carbon Monoxide: Preparation, Properties and Uses

Carbon and its Compounds formula sheet

Complete combustion

$C+O_2\to C{O}_2$

Every allotrope gives only CO₂; 12 g C gives 44 g CO₂.

Incomplete combustion

$2C+O_2\to 2CO$

Limited air; poisonous carbon monoxide.

Laboratory CO₂

$CaC{O}_3+2HCl\to CaC{l}_2+H_{2}O+C{O}_2$

Marble chips and dilute HCl; collect by upward displacement of air.

Lime water test

$Ca(OH{)}_2+C{O}_2\to CaC{O}_3+H_{2}O$

Milky; excess CO₂ clears it.

Carbonic acid

$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3$

Weak acid: moist blue litmus turns light red.

Extraction of iron

$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$

CO is the reducing agent.

Carbon and its Compounds: diagrams to practise

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

How the school exam asks Carbon and its Compounds

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

Tricks to remember Carbon and its Compounds

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

• Diamond 4, Graphite 3

  Diamond has four bonds and is 'four-tress' strong; graphite has three, leaving electrons free to carry current.

  Use it when: Structure, hardness and conductivity.

• Same bricks, different houses

  Allotropes are houses built from the same carbon bricks: they look different but burn the same.

  Use it when: Defining allotropy.

• Milky then clear

  A little CO₂: milky. Too much CO₂: clear again.

  Use it when: Lime water observations.

• CO: Colourless, Odourless, Killer

  It hides from your senses and steals haemoglobin.

  Use it when: Harmful effects of carbon monoxide.

• Marble meets HCl, not H₂SO₄

  Sulphuric acid 'Seals' the marble with calcium Sulphate.

  Use it when: Choice of acid for CO₂.

Carbon and its Compounds: exam-level solved questions

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

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 7-mark question split into 2 + 3 + 2 marks.

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.

Where students lose marks in Carbon and its Compounds

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

• ✗Mistake: Collecting CO₂ over water or in an inverted jar

  It is fairly soluble in water and heavier than air.

  ✓Do this: Upright jar, tube to the bottom: upward displacement of air.

  Usually costs: 1 mark and diagram marks.

• ✗Mistake: Calling coal or coke crystalline

  Coal, coke, charcoal and soot have no regular arrangement of atoms.

  ✓Do this: Crystalline: diamond, graphite, fullerenes. The rest are amorphous.

  Usually costs: 1 mark.

• ✗Mistake: Writing the lime water equation the wrong way round or unbalanced

  Calcium carbonate is the milky product, not a reactant.

  Wrong working

  $CaC{O}_3+H_{2}O\to Ca(OH{)}_2+C{O}_2$ for the milkiness

  Right working

  $Ca(OH{)}_2+C{O}_2\to CaC{O}_3+H_{2}O$

  ✓Do this: Lime water is Ca(OH)₂; the precipitate is CaCO₃.

  Usually costs: 1 mark.

• ✗Mistake: Saying carbon monoxide is acidic

  CO is a neutral oxide; it has no effect on litmus.

  ✓Do this: CO₂ acidic; CO neutral.

  Usually costs: 1 mark.

• ✗Mistake: Saying CO is poisonous 'because it is a gas'

  The mark is for the mechanism: it combines with haemoglobin and blocks oxygen transport.

  ✓Do this: Write haemoglobin → carboxyhaemoglobin → less oxygen → asphyxia.

  Usually costs: 1-2 marks.

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.

1. Q1 · 1 mark · MCQExam level

   Which is an amorphous form of carbon?
   (a) diamond
   (b) graphite
   (c) fullerene
   (d) coke

   Hint

   No regular arrangement.

   Answer

   (d)

2. Q2 · 1 mark · Very short answerExam level

   Name the gas that turns lime water milky.

   Hint

   Laboratory gas from marble.

   Answer

   Carbon dioxide.

3. Q3 · 2 marks · Short answerExam level

   Give two uses each of coke and soot.

   Hint

   Fuel; inks.

   Answer

   Coke: smokeless fuel; reducing agent in extracting iron (or making water gas). Soot: printing ink or black paint; filler in rubber tyres (or shoe polish).

4. Q4 · 2 marks · Short answerExam level

   State two physical properties of carbon dioxide.

   Hint

   Density, solubility.

   Answer

   Any two: colourless and odourless; about 1.5 times heavier than air; fairly soluble in water; solidifies to dry ice.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Diamond is used to cut glass.
   Reason (R): Diamond is a good conductor of heat.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Cutting needs hardness.

   Answer

   (d)

6. Q6 · 2 marks · Short answerHarder exam level

   3 g of graphite is burnt in excess oxygen. 12 g of carbon gives 44 g of CO₂. Find the mass of CO₂ and name the law that tells you 8 g of oxygen was used.

   Hint

   Scale; then subtract.

   Answer

   3 × 44 ÷ 12 = 11 g of CO₂; oxygen = 11 − 3 = 8 g by the law of conservation of mass.

7. Q7 · 2 marks · Short answerHarder exam level

   What happens when excess carbon dioxide is passed through milky lime water? Write the equation.

   Hint

   Soluble hydrogencarbonate.

   Answer

   The milkiness disappears: $CaC{O}_3+H_{2}O+C{O}_2\to Ca(HC{O}_3{)}_2$ (soluble calcium hydrogencarbonate).

8. Q8 · 2 marks · Short answerHarder exam level

   Show by an equation that CO₂ is an acidic oxide, using sodium hydroxide.

   Hint

   Base + acidic oxide → salt + water.

   Answer

   $2NaOH+C{O}_2\to N{a}_{2}C{O}_3+H_{2}O$: it reacts with a base to form a salt and water, so it is acidic.

9. Q9 · 3 marks · Short answerHarder exam level

   State three properties of carbon monoxide that make it dangerous.

   Hint

   Senses, blood, flame.

   Answer

   It is colourless and odourless, so it cannot be detected; it combines with haemoglobin much more readily than oxygen, causing asphyxia; it is combustible, so it can catch fire.

10. Q10 · 2 marks · Short answerHarder exam level

    Write the equation for the reduction of copper(II) oxide by carbon monoxide and name the substance oxidised.

    Hint

    Like iron oxide.

    Answer

    $CuO+CO\to Cu+C{O}_2$; carbon monoxide is oxidised.

11. Q11 · 3 marks · Short answerHarder exam level

    Give the use of each allotrope and the property behind it: (i) graphite in dry cells (ii) diamond in eye surgery (iii) wood charcoal in gas masks.

    Hint

    Conducts, hard and sharp, adsorbs.

    Answer

    (i) Electrode: conducts electricity. (ii) Fine, sharp cutting edge: hardest substance. (iii) Adsorbs poisonous gases: porous with a large surface.

12. Q12 · 3 marks · Short answerHOTS / competency

    A student collects CO₂ over water from 50 g of marble and gets much less gas than expected (100 g of marble gives 44 g of CO₂). Find the expected mass and give two reasons why less was collected.

    Hint

    Think about solubility and acid choice.

    Answer

    Expected: 50 × 44 ÷ 100 = 22 g. Reasons: CO₂ is fairly soluble in water, so some dissolved; if dilute sulphuric acid was used, a calcium sulphate coat stopped the reaction (or the funnel was not dipped in the acid, so gas escaped).

13. Q13 · 3 marks · Short answerHOTS / competency

    Why is a carbon dioxide fire extinguisher preferred over water for an electrical or oil fire? Give two reasons.

    Hint

    Water conducts; oil floats.

    Answer

    Water conducts electricity, so it can cause electric shock, and oil floats on water and spreads the fire. Carbon dioxide does not conduct, does not support burning, and being heavier than air it blankets the fire and cuts off oxygen.

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. 1.Define allotropy.Show answer

   The existence of an element in two or more forms with different physical but the same chemical properties.

2. 2.Two crystalline allotropes of carbon?Show answer

   Diamond and graphite (also fullerenes).

3. 3.Which form of carbon is used in pencils?Show answer

   Graphite (mixed with clay).

4. 4.Reactants for laboratory CO₂?Show answer

   Marble chips and dilute hydrochloric acid.

5. 5.Effect of CO₂ on moist blue litmus?Show answer

   Turns it light red (weak carbonic acid).

6. 6.What is dry ice?Show answer

   Solid carbon dioxide.

7. 7.Colour of the flame of burning CO?Show answer

   Blue.

8. 8.Compound formed when CO is inhaled?Show answer

   Carboxyhaemoglobin.

Quick revision checklist

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

• ☐Can you prove that diamond and graphite are both carbon?
• ☐Can you give two structure and two property differences between diamond and graphite?
• ☐Can you name the form of carbon for any given use?
• ☐Can you draw the CO₂ preparation with the jar the right way up?
• ☐Can you describe lime water with a little and with excess CO₂?
• ☐Can you explain asphyxia caused by carbon monoxide?

Carbon and its Compounds: frequently asked questions