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Nature of Matter: Elements, Compounds, and Mixtures – Class 8 Science notes, important questions and MCQs

NCERT Class 8 Science, chapter 8. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Pure substances are elements or compounds with fixed properties. Mixtures contain two or more substances that are not chemically joined and can be separated physically.

Elements and compounds

Element: made of only one kind of atom; about 118 known (H, O, Fe, Au). Metals, non-metals and metalloids.
Compound: two or more elements chemically combined in a fixed ratio (H₂O, CO₂, NaCl).
A compound's properties are different from its elements (sodium is reactive, chlorine is poisonous, salt is safe to eat).

Mixtures

Components keep their properties and can be present in any proportion.
Homogeneous (air, salt solution) and heterogeneous (sand + iron filings).
Separation: magnet, filtration, evaporation, distillation, chromatography.

Key terms

Element: substance made of only one kind of atom
Compound: substance of two or more elements chemically combined in a fixed ratio
Metalloid: element with properties of both metals and non-metals (silicon, boron)

Formulas

Water = hydrogen : oxygen in a fixed mass ratio of 1 : 8

Important questions with answers

Give two examples each of metals, non-metals and metalloids.2 marks

Metals: iron, copper. Non-metals: carbon, sulphur. Metalloids: silicon, boron (also germanium, arsenic).

Classify as element, compound or mixture: salt solution, carbon dioxide, gold, air, sugar.3 marks

Elements: gold. Compounds: carbon dioxide, sugar. Mixtures: salt solution, air.

Iron filings and sulphur powder are mixed, and some of the mixture is heated strongly. Compare the mixture and the heated product with respect to magnet test, composition and properties.5 marks

Mixture: iron can be removed with a magnet; components can be in any ratio; each keeps its own colour and properties; no heat or light given out. Heated product: iron sulphide (a compound) forms with heat and glow; a magnet no longer attracts it; iron and sulphur are in a fixed ratio; it has new properties and cannot be separated by physical methods.

Quick revision

Elements cannot be broken into simpler substances by chemical means.
Compounds have fixed composition.
Mixtures can be separated physically.

Exam tips

Air is a mixture, not a compound — its composition can vary.
Symbols: first letter capital, second small (Co = cobalt, CO = carbon monoxide).

Practice MCQs

Which is a compound?
1. oxygen
2. iron
3. water
4. gold
Show answer
Answer: (c) water. H₂O.
Air is a
1. compound
2. element
3. metal
4. mixture
Show answer
Answer: (d) mixture. Mixture of gases.
Iron filings can be separated from sand using
1. filtration
2. a magnet
3. evaporation
4. distillation
Show answer
Answer: (b) a magnet. Iron is magnetic.
Which is a metalloid?
1. sodium
2. copper
3. sulphur
4. silicon
Show answer
Answer: (d) silicon. Silicon.
Which is a compound
1. oxygen
2. air
3. water
4. brass
Show answer
Answer: (c) water. H₂O, fixed ratio.
Sodium and chlorine are dangerous, but sodium chloride is edible. This shows that
1. compounds keep the properties of elements
2. compounds have new properties
3. mixtures are safe
4. elements are compounds
Show answer
Answer: (b) compounds have new properties. New substance, new properties.
Which statement about mixtures is correct?
1. they have a fixed ratio
2. they form by chemical reaction only
3. components keep their own properties
4. they cannot be separated
Show answer
Answer: (c) components keep their own properties. That is the key feature.
A mixture differs from a compound because in a mixture
1. components are in a fixed ratio
2. heat is always released
3. a new substance forms
4. components keep their own properties and can be separated physically
Show answer
Answer: (d) components keep their own properties and can be separated physically. No chemical bonding.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.