ScienceCBSE Class 8 · Nature of Matter: Elements, Compounds, and Mixtures

Elements, Compounds and Minerals: Class 8 Science

Updated 17 October 2026 · Fact-checked

An element is a pure substance made of identical atoms that cannot be broken into simpler substances. A compound is formed when different elements combine chemically in a fixed ratio, with properties different from its elements, and it cannot be split by physical methods. A mixture of iron and sulfur keeps both properties and can be separated with a magnet, while heating them gives iron sulfide, a new compound.

Understand Elements, Compounds and Minerals

Splitting water (Activity 8.3). On passing electricity through water (a few drops of dilute sulfuric acid added, 9 V battery, demonstration with a teacher) gas bubbles collect in two test tubes. The gases are not water vapour, since vapour would condense back to water. A burning candle at the mouth of one tube gives a pop sound (hydrogen); at the other the flame glows brighter (oxygen). So water is made of hydrogen and oxygen.

Elements. Hydrogen and oxygen are pure substances called elements. Each element is made up of identical particles called atoms, different from the atoms of any other element. An element cannot be broken into simpler substances; elements are the building blocks of all matter (gold, silver, sulfur, carbon). In many elements atoms exist joined as a molecule: two atoms of hydrogen make one molecule of hydrogen, and two of oxygen make one molecule of oxygen. Elements are classed as metals (gold, silver, magnesium, iron, aluminium) and non-metals (carbon, sulfur, hydrogen, oxygen); silicon and boron have intermediate properties and are metalloids. Facts from the chapter: 118 elements are known and most are solids; eleven are gases at room temperature (all non-metals such as oxygen, helium, nitrogen); only two are liquids at room temperature, mercury (a metal) and bromine (a non-metal); gallium and caesium are solids that become liquid around 30 degrees Celsius (303 K). More than 45 different elements are used in making a mobile phone.

Compounds. Hydrogen and oxygen are so tightly attached in water that they generally cannot be separated by physical means, so water is a compound. Compounds form when different elements combine in fixed ratios to give something entirely new, with properties different from those of the elements, and the elements cannot be separated by physical methods. In water the ratio of hydrogen to oxygen atoms is 2 : 1. Sodium chloride (common salt) has sodium and chlorine in a 1 : 1 ratio; the soft metal sodium and the hazardous gas chlorine form a harmless substance. Dissolved salt can be separated from water by evaporation (a physical method), but not into sodium and chlorine.

Heating sugar (Activity 8.4). Sugar turns brown, then black (charring); droplets of water appear near the mouth of the tube, and carbon (charcoal) is left. Since this water came from the sugar, which gives carbon and water, sugar is a compound of carbon, hydrogen and oxygen.

Iron and sulfur (Activity 8.5). Take 5.6 g of iron filings and 3.2 g of sulfur powder.

Sample A is the simple mixture: both substances can be seen separately; a magnet attracts the iron, so the components can be separated; with dilute hydrochloric acid the iron reacts to form iron chloride and hydrogen (colourless, odourless, burns with a pop), while sulfur is left as a yellow solid.
Sample B: half of the mixture is heated with stirring until a black mass forms; this is cooled and ground. It is the compound iron sulfide, uniform in colour and texture, not attracted by a magnet, and its iron and sulfur can no longer be separated. With dilute hydrochloric acid it gives iron chloride and hydrogen sulfide, a colourless gas with a rotten-egg smell. Iron + hydrochloric acid gives iron chloride + hydrogen; iron sulfide + hydrochloric acid gives iron chloride + hydrogen sulfide. Smell gases only by wafting.

Uses. Chemists use these ideas to make medicines, vaccines and fertilisers; engineers make alloys such as stainless steel. Wood, steel and concrete are mixtures. Graphene aerogel, made from carbon, is said to be the lightest material and can absorb oil spills.

Minerals. Most rocks are mixtures of minerals. Native minerals are pure elements (gold, silver, copper, sulfur, carbon). Most minerals are compounds of more than one element: quartz, calcite, mica, pyroxene, olivine. Cement is made from calcite, quartz, alumina and iron oxide; talcum powder comes from the mineral talc.

The previous topic covers mixtures, air, alloys and pure substances.

Elements, Compounds and Minerals: formulas and key facts

Element: Identical atoms | Cannot be broken into simpler substances; Examples: hydrogen, oxygen, gold, silver, sulfur, carbon.
Compound: Different elements combined in a fixed ratio
New properties
Cannot be separated by physical methods; Examples: water, sodium chloride, sugar, iron sulfide.
Fixed ratios from the chapter: $W a t er : H : O = 2 : 1∣ S o d i u m c h l or i d e : N a : C l = 1 : 1$; Ratios of atoms.
Electrolysis of water: $W a t er > H y d r o g e n + O x y g e n$; Hydrogen gives a pop sound; oxygen makes a flame glow brighter.
Iron and sulfur: Mixture (Sample A): magnet separates iron | Compound (Sample B): iron sulfide, no magnet attraction; 5.6 g iron and 3.2 g sulfur in the activity.
Reactions with dilute HCl: Iron + HCl > Iron chloride + Hydrogen | Iron sulfide + HCl > Iron chloride + Hydrogen sulfide; Hydrogen sulfide smells of rotten egg.

How to solve Elements, Compounds and Minerals questions

Use this method to decide whether a substance is an element, compound or mixture.

1Can it be separated into parts by a physical method (magnet, evaporation, filtering)? If yes, it is a mixture.
2Does it break down only on a chemical change (electric current, strong heating) into simpler substances? If yes, it is a compound.
3Does it consist of one kind of atom and resist further breakdown? Then it is an element.
4Compare properties: a compound's properties differ from its elements; a mixture keeps the properties of its components.
5Use the fixed ratio idea for compounds; mixtures can be in any proportion.
6Report the evidence (magnet, colour, gas test).

Elements, Compounds and Minerals in real life

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.

Reading the gas volumes

Suppose after passing current through water for 15 minutes, test tube X holds 8 mL of gas and test tube Y holds 4 mL (invented volumes). X gives a pop sound with a lit candle; Y makes the flame glow brighter.

(a) Name the gases in X and Y. (b) Find the ratio of volumes X : Y. (c) Relate this ratio to the 2 : 1 ratio of atoms in water. (d) Why can the gases not be water vapour?

Try it first, then show the solutionSolution

X is hydrogen (pop sound); Y is oxygen (brighter flame).
8 : 4 = 2 : 1.
The volume ratio matches the ratio of atoms in water (hydrogen 2 : oxygen 1); this agreement is noted here as an observation, not a rule the chapter states.
Vapour would condense back to water.

Answer: (a) X hydrogen, Y oxygen; (b) 2 : 1; (c) matches H : O = 2 : 1; (d) vapour would condense.

Memory hook: Water splits: twice the H of O.

Iron and sulfur amounts

Imagine a group uses the book's 5.6 g of iron filings and 3.2 g of sulfur powder, mixes them (Sample A), and heats half of the mixture (Sample B).

(a) Find the total mass of the mixture. (b) Find the mass of iron and sulfur in half the mixture. (c) Express the iron to sulfur mass ratio in simplest form. (d) How can you tell Sample B is a compound?

Try it first, then show the solutionSolution

5.6 + 3.2 = 8.8 g.
Half: iron 2.8 g, sulfur 1.6 g (total 4.4 g).
5.6 : 3.2 = 56 : 32 = 7 : 4.
Sample B is uniform black, not attracted by a magnet and iron and sulfur cannot be separated; it gives hydrogen sulfide with dilute HCl, not hydrogen.

Answer: (a) 8.8 g; (b) 2.8 g iron, 1.6 g sulfur; (c) 7 : 4; (d) new properties and no magnetic separation.

Memory hook: Mixture: separate by magnet. Compound: magnet fails.

Tricks to remember Elements, Compounds and Minerals

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

Elements are alphabet, compounds are words
26 letters make many words; 118 elements make many compounds.
Use it when: Idea of building blocks.
Magnet test
Magnet picks iron out of the mixture, not out of iron sulfide.
Use it when: Mixture versus compound.
Pop for hydrogen, Bright for oxygen
Pop = hydrogen. Brighter flame = oxygen.
Use it when: Gas tests.
2 H, 1 O
Water is H2O: twice as many hydrogen atoms as oxygen.
Use it when: Fixed ratio in water.

Elements, Compounds and Minerals: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Element or compound · 4 marks · Short answerBoard level

Classify as element or compound and give a reason: (a) hydrogen, (b) water, (c) sugar, (d) sulfur.

Try it first, then show the solutionSolution

(a) Element: it cannot be broken into simpler substances. (1)
(b) Compound: electricity splits it into hydrogen and oxygen. (1)
(c) Compound: on heating it gives carbon and water. (1)
(d) Element: it is a pure substance made of one kind of atom. (1)

Answer: (a) Element; (b) compound; (c) compound; (d) element.

Example 2: Test for hydrogen · 1 mark · MCQBoard level

A student collects the two gases from the splitting of water. One tube brightens a burning splinter and the other burns with a pop. Which row is correct?
(a) Brightens: hydrogen; pop: oxygen
(b) Brightens: oxygen; pop: hydrogen
(c) Brightens: carbon dioxide; pop: hydrogen
(d) Brightens: oxygen; pop: nitrogen

Try it first, then show the solutionSolution

Hydrogen burns with a pop; oxygen makes the flame glow brighter.

Answer: (b)

Example 3: Mixture versus compound · 5 marks · Long answerHarder board level

Iron filings and sulfur are mixed (Sample A); half is heated to give a black mass (Sample B).
(a) Compare appearance and the effect of a magnet on A and B. (2)
(b) What gas comes from A and from B with dilute HCl? How do the odours differ? (2)
(c) Which is the mixture and which the compound? Give reasons. (1)

Try it first, then show the solutionSolution

(a) A shows black and yellow particles separately and the magnet attracts the iron; B is uniform black and the magnet does not attract it. (2)
(b) A gives hydrogen (colourless, odourless, burns with a pop); B gives hydrogen sulfide (colourless, rotten egg smell). (2)
(c) A is a mixture because iron and sulfur keep their properties and can be separated; B is the compound iron sulfide with new properties. (1)

Answer: A: mixture, magnet separates iron, gives hydrogen; B: iron sulfide compound, no magnetism, gives hydrogen sulfide.

Example 4: Sugar on heating · 4 marks · Case-basedHarder board level

Read the passage and answer.

Sugar is heated gently in a boiling tube. It turns brown, then black, and droplets of water appear near the open end. A black solid remains which burns like coal.
(a) Name the black solid. (1)
(b) Where did the water come from? Explain how we know. (1)
(c) Is sugar an element? Give evidence. (1)
(d) Name the elements present in sugar. (1)

Try it first, then show the solutionSolution

(a) Carbon (charcoal). (1)
(b) From the sugar. The tube was dry before heating, and on heating the sugar changes into black carbon and water. Since these new substances (carbon and water) are formed from the sugar, the sugar itself must contain them. (1)
(c) No: it breaks down into carbon and water, so it is a compound. (1)
(d) Carbon, hydrogen and oxygen. (1)

Answer: (a) Carbon; (b) from the sugar; (c) compound, since it decomposes; (d) carbon, hydrogen, oxygen.

Example 5: Salt, sodium and chlorine · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Sodium is a soft metal and chlorine is a hazardous gas, but together in a 1 : 1 ratio they form common salt. A student argues that salt is just a mixture of sodium and chlorine, since salt dissolves in water and the water can be evaporated off.
(a) What do the evaporation results show? (1)
(b) Give two reasons why salt is a compound and not a mixture of sodium and chlorine. (1)
(c) If a compound has atoms in a 1 : 1 ratio and a sample has 36 atoms of the first element, how many atoms of the second element does it have? (1)
(d) Using the 2 : 1 ratio for water, how many hydrogen atoms go with 36 oxygen atoms? (1)

Try it first, then show the solutionSolution

(a) Evaporation separates the salt from the water, not sodium from chlorine; it is a physical method used on salt solution. (1)
(b) Salt has properties different from sodium and chlorine; sodium and chlorine are in a fixed 1 : 1 ratio and cannot be separated by physical processes. (1)
(c) 36 atoms. (1)
(d) 2 × 36 = 72 hydrogen atoms. (1)

Answer: (a) It separates salt from water only; (b) new properties and fixed ratio; (c) 36; (d) 72.

Where students lose marks in Elements, Compounds and Minerals

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

Mistake: Calling a compound a mixture because it contains several elements.
Both involve more than one substance.
Wrong working
Water is a mixture of hydrogen and oxygen.
Right working
Water is a compound: hydrogen and oxygen combined chemically in a 2 : 1 ratio, with different properties.
Do this: Check fixed ratio and new properties.
Usually costs: 1 mark.
Mistake: Wrong gas test.
Pop and bright flame are swapped.
Wrong working
Oxygen gives a pop sound.
Right working
Hydrogen gives a pop; oxygen makes the flame glow brighter.
Do this: Pop for Hydrogen.
Usually costs: 1 mark.
Mistake: Smelling gases directly.
Procedure is ignored.
Wrong working
Smell the gas by bringing the tube to the nose.
Right working
Waft the fumes gently towards the nose.
Do this: Never smell anything directly.
Usually costs: ½ mark.
Mistake: Writing that atoms and molecules are the same.
Both are tiny particles.
Wrong working
A molecule of hydrogen is one atom.
Right working
A molecule of hydrogen has two atoms combined.
Do this: Two atoms make a molecule of hydrogen or oxygen.
Usually costs: 1 mark.

Exam tips

Use evidence words: 'magnet attracts', 'new properties', 'cannot be separated by physical methods'.
Say 'fixed ratio' when defining a compound.
Learn the two gas tests and the two 'special' liquid elements.
Quote data exactly: 118 elements, eleven gases, two liquids (mercury and bromine).
For minerals separate native minerals (elements) from compound minerals.

Practice questions on Elements, Compounds and Minerals

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

Q1 · 2 marks · Very short answerBoard level
Define an element and give two examples, one metal and one non-metal.
Hint
Made of identical atoms.
Answer
An element is a pure substance of identical atoms that cannot be broken into simpler substances (1). Iron (metal) and sulfur (non-metal) (1).
Q2 · 1 mark · MCQBoard level
Which of the following is a native mineral?
(a) Quartz
(b) Calcite
(c) Gold
(d) Mica
Hint
Native minerals are pure elements.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Iron sulfide is not attracted by a magnet though iron is.
Reason (R): A compound has properties different from those of the elements that form it.
(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
Think of new substance.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
In the iron and sulfur activity 5.6 g of iron and 3.2 g of sulfur are used. (a) Find the ratio of masses in simplest form. (b) If the same ratio is kept for 2.8 g of iron, find the mass of sulfur needed. (c) Find the mass of iron needed for 6.4 g of sulfur. (d) State why this ratio, unlike in a mixture, must be kept for a compound.
Hint
5.6 : 3.2 = 7 : 4.
Answer
(a) 7 : 4. (b) 2.8 × 4 ÷ 7 = 1.6 g. (c) 6.4 × 7 ÷ 4 = 11.2 g. (d) Elements combine in a fixed ratio in a compound, whereas a mixture can have any proportion (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A student connects the gases collected in Activity 8.3 to a burning candle. In test tube P there is a pop; in Q the flame brightens. P contains twice the gas of Q.
(a) Identify P and Q. (b) Why is this evidence that water is a compound? (c) Write the word equation. (d) Name one use of oxygen that the chapter mentions.
Hint
Oxygen is needed for combustion.
Answer
(a) P hydrogen, Q oxygen. (b) Water breaks into two different substances on passing electricity, so it is not an element and the particles of the elements were combined. (c) Water gives hydrogen + oxygen. (d) It helps in combustion (and most living beings need it) (1 mark each).

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.How many elements are known according to the chapter?Show answer
118.
2.Which two elements are liquid at room temperature?Show answer
Mercury and bromine.
3.What is the ratio of hydrogen to oxygen atoms in water?Show answer
2 : 1.
4.What is Sample B in the iron-sulfur activity?Show answer
Iron sulfide, a compound.
5.Name two native minerals.Show answer
Gold and sulfur (also silver, copper, carbon).

Elements, Compounds and Minerals: frequently asked questions

Are minerals and rocks the same?

No. Most rocks are mixtures of minerals.

Is electrolysis of water a physical change?

The book asks you to decide this using Grade 7; the substance changes into hydrogen and oxygen with new properties, so it is a chemical change.

What is a metalloid?

An element with intermediate properties between metals and non-metals, such as silicon and boron.

Why does Sample B not react with a magnet?

The iron is combined with sulfur in iron sulfide, a new substance with different properties.