ChemistryICSE Class 8 · Elements, Compounds and Mixtures

Elements, Compounds and Mixtures: Differences, Fixed Ratio and Examples (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

An element has one kind of atom; a compound has elements chemically combined in a fixed ratio by mass (water: H : O = 1 : 8; iron sulphide: Fe : S = 7 : 4); a mixture has substances physically mixed in any proportion. Compounds differ in properties from their elements; mixtures keep the properties of their components.

Understand Elements, Compounds and Mixtures Compared

Matter is sorted into pure substances (elements and compounds) and mixtures.

	Element	Compound	Mixture
Made of	one kind of atom	two or more elements chemically combined	two or more substances physically mixed
Composition	fixed	fixed ratio by mass	variable, any ratio
Properties	its own	different from its elements	those of its components
Separation	cannot be split chemically	only by chemical methods	by physical methods
Examples	iron, oxygen, sulphur	water, sodium chloride	air, soil, salt solution

Fixed ratio in compounds.

Water: hydrogen : oxygen $= 1 : 8$ by mass (so 9 g of water contains 1 g of hydrogen and 8 g of oxygen).
Iron sulphide: $F e + S \to F e S$ ; iron and sulphur combine in the ratio $7 : 4$ by mass, giving 11 parts of FeS.

Mixtures have no fixed ratio: salt and sand can be mixed in any proportion, and the percentage of a component is $\frac{mass of component}{mass of mixture} \times 100$ .

Types of mixtures.

Homogeneous: same throughout (salt solution, air).
Heterogeneous: parts can be seen (sand in water, soil).

Iron and sulphur. A mixture of iron filings and sulphur: iron can be picked out with a magnet; each keeps its properties. When heated, they combine to give iron sulphide, a new substance that the magnet does not attract: a compound.

Elements, Compounds and Mixtures Compared: formulas and key facts

Compound ratios: $W a t er : H : O = 1 : 8 b y ma ss ∣ I r o n s u l p hi d e : F e : S = 7 : 4 b y ma ss$; Fixed for a compound.
Percentage in a mixture: $% = \frac{mass of component}{mass of mixture} \times 100$; Varies from sample to sample.
Iron sulphide: $F e + S \to F e S$; 7 g + 4 g gives 11 g.

How to solve Elements, Compounds and Mixtures Compared questions

Decide from the composition and the method of separation, then use the fixed ratio for compounds.

1One kind of atom: element. Fixed ratio and chemical separation: compound. Variable ratio and physical separation: mixture.
2For a fixed-ratio problem, divide the mass by the sum of the ratio parts to find the mass of one part.
3Find the limiting reactant when masses are not in the ratio.
4Check conservation of mass (total in = total out).

Elements, Compounds and Mixtures Compared 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.

Sea water and salt

Suppose sea water is evaporated in a pan and white solid is left behind.

Is sea water an element, compound or mixture? Give two reasons.

Try it first, then show the solutionSolution

Sea water is a mixture: the amount of salt varies from sample to sample.
Its components can be separated by a physical method (evaporation).

Answer: A mixture.

Memory hook: Varies and separates physically.

Water from hydrogen and oxygen

Imagine 36 g of water is split into its elements by electricity.

Find the masses of hydrogen and oxygen formed.

Try it first, then show the solutionSolution

H : O = 1 : 8, total parts 9. One part $= \frac{36}{9} = 4$ g.
Hydrogen $= 4$ g; oxygen $= 32$ g.

Answer: 4 g of hydrogen and 32 g of oxygen.

Memory hook: 1 : 8, total 9.

Tricks to remember Elements, Compounds and Mixtures Compared

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

C for chemical, P for physical
Compounds break by Chemical methods; mixtures separate by Physical ones.
Use it when: Separation.
Fixed versus any
Compound: fixed ratio. Mixture: any ratio.
Use it when: Composition.
Water 1 : 8
Water: hydrogen 1 part, oxygen 8 parts by mass.
Use it when: Ratio.

Elements, Compounds and Mixtures Compared: solved examples

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

Example 1: Which is a mixture? · 1 mark · MCQBoard level

Which of the following is a mixture?
(a) water
(b) sodium chloride
(c) air
(d) oxygen

Try it first, then show the solutionSolution

Air is a mixture of gases in variable proportion; the others are pure substances.

Answer: (c) air

Example 2: Water and sea water · 2 marks · Short answerBoard level

Pure water and sea water are both clear liquids. Using composition and separation, explain why water is a compound and sea water a mixture.

Try it first, then show the solutionSolution

Water always has hydrogen and oxygen in the fixed ratio 1 : 8 by mass; the amount of salt in sea water varies from sample to sample. (1)
Water can be split only chemically (electricity); sea water can be separated physically (evaporation). (1)

Answer: Fixed vs variable composition; chemical vs physical separation.

Example 3: Iron and sulphur · 4 marks · Long answerHarder board level

Iron filings and sulphur are (i) mixed and (ii) heated together. (a) Which gives a compound? (b) How could you test the difference with a magnet? (c) If 21 g of iron combines completely with sulphur, find the mass of sulphur needed and of iron sulphide formed.

Try it first, then show the solutionSolution

(a) Heating, which forms iron sulphide. (1)
(b) The magnet picks iron out of the mixture, but not from iron sulphide. (1)
(c) Fe : S = 7 : 4, so sulphur $= \frac{21 \times 4}{7} = 12$ g. (1)
FeS $= 21 + 12 = 33$ g. (1)

Answer: (a) heating (b) magnet attracts iron in the mixture only (c) 12 g; 33 g

Example 4: Water by mass · 3 marks · Short answerHarder board level

A sample of 90 g of water is decomposed. Find (a) the mass of hydrogen, (b) the mass of oxygen, (c) the percentage of hydrogen by mass in water.

Try it first, then show the solutionSolution

(a) $\frac{90}{9} = 10$ g. (1)
(b) $10 \times 8 = 80$ g. (1)
(c) $\frac{10}{90} \times 100 = 11.1%$ . (1)

Answer: (a) 10 g (b) 80 g (c) 11.1%

Example 5: Sort the materials · 4 marks · Case-basedHOTS / competency

Classify as element, compound or mixture, and the mixtures as homogeneous or heterogeneous: air, water, oxygen, salt solution, sodium chloride, copper, soil, sand in water.

Try it first, then show the solutionSolution

Elements: oxygen, copper. (1)
Compounds: water, sodium chloride. (1)
Homogeneous mixtures: air, salt solution. (1)
Heterogeneous mixtures: soil, sand in water. (1)

Answer: Elements: O₂, Cu; compounds: water, NaCl; homogeneous: air, salt solution; heterogeneous: soil, sand in water

Where students lose marks in Elements, Compounds and Mixtures Compared

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

Mistake: Calling a solution a compound
A solution is a physical mixture.
Wrong working
"Salt solution is a compound."
Right working
"Salt solution is a homogeneous mixture."
Do this: Ask if the ratio is variable.
Mistake: Wrong ratio direction
H : O = 1 : 8 by mass, not 2 : 1.
Wrong working
"Water: 2 g hydrogen to 1 g oxygen."
Right working
"1 g hydrogen to 8 g oxygen by mass."
Do this: Distinguish mass ratio from atom ratio.
Mistake: Forgetting leftover reactant
The ratio fixes how much reacts.
Wrong working
"28 g Fe + 14 g S gives 42 g FeS."
Right working
"Only 38.5 g FeS forms; 3.5 g Fe is left."
Do this: Test which reactant runs out.

Exam tips

Use a table of properties to compare compound and mixture.
Quote the fixed ratios: 1 : 8 and 7 : 4.
Name the separation method (chemical or physical) in each answer.
Check total mass in equals total mass out.

Practice questions on Elements, Compounds and Mixtures Compared

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 · Very short answerBoard level
Define a compound.
Hint
Fixed ratio.
Answer
A compound is a substance made of two or more elements chemically combined in a fixed ratio by mass.
Q2 · 2 marks · Short answerHarder board level
What percentage of water by mass is hydrogen? Show the working.
Hint
H : O = 1 : 8.
Answer
$\frac{1}{1 + 8} \times 100 = 11.1%$ .
Q3 · 3 marks · Short answerHarder board level
Explain why oxygen is an element and carbon dioxide a compound, and say which of the two can be split into simpler substances by chemical means.
Hint
One kind of atom.
Answer
Oxygen contains only one kind of atom, so it cannot be split chemically. Carbon dioxide contains carbon and oxygen combined in a fixed ratio, so chemical means can split it into carbon and oxygen.
Q4 · 4 marks · Long answerHarder board level
28 g of iron is heated with 14 g of sulphur (Fe : S = 7 : 4 by mass). Find the iron sulphide formed and which element is left over, and how much.
Hint
Find which reactant is limiting.
Answer
Sulphur needed for 28 g Fe $= \frac{28 \times 4}{7} = 16$ g, but only 14 g is present, so sulphur is the limiting reactant. Iron used $= \frac{14 \times 7}{4} = 24.5$ g; FeS $= 24.5 + 14 = 38.5$ g; iron left $= 28 - 24.5 = 3.5$ g.
Q5 · 4 marks · Long answerHOTS / competency
A student says: "Air is a compound because it always contains oxygen and nitrogen." Give two arguments against this.
Hint
Think about proportions and separation.
Answer
Air is a mixture: its components are not combined in a fixed ratio by mass, and they keep their own properties (oxygen still supports burning). They can be separated by physical methods such as liquefaction and fractional distillation. A compound would have a fixed composition and could be split only chemically.

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.Ratio H : O in water by mass?Show answer
1 : 8.
2.Ratio Fe : S in FeS?Show answer
7 : 4.
3.Air is?Show answer
A homogeneous mixture.
4.Can an element be split by chemical means?Show answer
No.
5.Mixtures separate by?Show answer
Physical methods.

Elements, Compounds and Mixtures Compared: frequently asked questions

What is the difference between a compound and a mixture?

A compound has elements combined chemically in a fixed ratio and has new properties. A mixture has substances physically mixed in any proportion and keeps their properties.

Is sea water a mixture or a compound?

A mixture: its salt content varies and the components can be separated by physical methods like evaporation.

What are homogeneous and heterogeneous mixtures?

Homogeneous mixtures are the same throughout, like salt solution; heterogeneous mixtures have visible parts, like sand in water.