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NCERT Solutions · Class 8 Science · Chapter 8

Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures (Chemistry)

Answers to all "Keep the curiosity alive" questions of Chapter 8, Nature of Matter: Elements, Compounds, and Mixtures (NCERT Class 8 Science, Curiosity, 2026-27): elements, compounds and mixtures, uniform and non-uniform mixtures, pure substances, iron and sulfur, iron with hydrochloric acid, why water is a compound, and gold as a mineral and a metal. All 12 questions are answered, with the key answer highlighted.

An element cannot be broken into simpler substances by chemical reactions (iron, oxygen, gold). A compound is formed when elements combine chemically in a fixed ratio, and it has new properties (water, carbon dioxide). A mixture contains substances mixed without any chemical change, in any proportion, and each keeps its own properties (air, seawater). Elements and compounds are pure substances.

Keep the curiosity alive

A and B, which cannot be broken down into simpler substances, combine to form C. Which statement is correct? (i) A, B and C are all compounds and only C has a fixed composition. (ii) C is a compound and A and B have a fixed composition. (iii) A and B are compounds and C has a fixed composition. (iv) A and B are elements, C is a compound and has a fixed composition.

Solution

A and B cannot be broken down into simpler substances, so they are elements. When elements combine chemically they form a compound, and a compound always has a fixed composition.

(iv)

Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed without any chemical change.

Solution

Both are true. Air is a mixture because its gases (nitrogen, oxygen, carbon dioxide, water vapour and others) are simply mixed, not chemically combined. That is exactly what the reason says.

(i) Both are true and the Reason correctly explains the Assertion.

Water, a compound, has properties different from those of hydrogen and oxygen, from which it is formed. Justify this statement.

Solution

Property	Hydrogen	Oxygen	Water
State at room temperature	gas	gas	liquid
Burning	burns with a "pop"	helps other things burn	does not burn; puts out fire
Other	lightest gas	needed for breathing	dissolves many substances; boils at 100 °C

When hydrogen and oxygen combine chemically, a new substance forms with completely new properties. This is what happens whenever a compound forms.

Hydrogen (a flammable gas) and oxygen (a gas that supports burning) form water, a liquid that neither burns nor supports burning: a compound has new properties.

In which list are all the examples correctly matched? Give reasons. (i) Elements: water, nitrogen, iron, air (ii) Uniform mixtures: minerals, seawater, bronze, air (iii) Pure substances: carbon dioxide, iron, oxygen, sugar (iv) Non-uniform mixtures: air, sand, brass, muddy water

Solution

(iii) is correct. Carbon dioxide and sugar are compounds, and iron and oxygen are elements. All of these are pure substances.

The others are wrong:

(i): water is a compound and air is a mixture.
(ii): minerals are not always uniform mixtures. Many are elements or compounds, such as gold or quartz.
(iv): air and brass are uniform mixtures.

(iii) Carbon dioxide, iron, oxygen and sugar are all pure substances.

Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify every substance as an element, compound or mixture.

Solution

Substance	Type	Reason
Iron	Element	made of only one kind of atom; cannot be broken down
Moist air	Mixture	oxygen, nitrogen, water vapour and other gases mixed without combining
Oxygen (in air, and in the second reaction)	Element	cannot be broken down further
Water (vapour in moist air)	Compound	hydrogen and oxygen chemically combined
Iron oxide (rust)	Compound	iron and oxygen combined, with new properties
Magnesium	Element	a metal made of one kind of atom
Magnesium oxide	Compound	magnesium and oxygen combined in a fixed ratio

Elements: iron, oxygen, magnesium. Compounds: iron oxide, magnesium oxide (and the water in moist air). Mixture: moist air.

Classify as elements, compounds or mixtures: carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda. List the pure substances.

Solution

Elements	Compounds	Mixtures
aluminium, gold, oxygen, nitrogen, sulfur, hydrogen	carbon dioxide, magnesium oxide, rust (iron oxide), iron sulfide, glucose, water, sodium chloride, baking soda	sand, seawater, muddy water, air, fruit juice

Pure substances: all the elements and compounds, i.e. aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride and baking soda.

Elements: aluminium, gold, oxygen, nitrogen, sulfur, hydrogen. Compounds: carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda. Mixtures: sand, seawater, muddy water, air, fruit juice. The pure substances are all the elements and compounds.

What new substance forms when a mixture of iron filings and sulfur powder is heated? How is it different from the mixture? Write the word equation.

Solution

A new substance, iron sulfide, is formed. It is a black solid compound.

Mixture of iron and sulfur	Iron sulfide (compound)
iron filings can be pulled out with a magnet	not attracted by a magnet in the same way
yellow sulfur and grey iron can be seen separately	one uniform black substance
any proportion; can be separated physically	fixed proportion; cannot be separated physically
no new properties	new properties, e.g. gives a bad-smelling gas with dilute acid

Word equation: iron + sulfur → iron sulfide

Iron sulfide forms: a compound that a magnet cannot separate, with new properties. Iron + sulfur → iron sulfide.

Can a substance be both an element and a compound? Explain.

Solution

No. An element cannot be broken down into simpler substances by chemical means, but a compound can be broken down into the elements it is made of. A substance cannot be both breakable and unbreakable in this way, so it must be one or the other.

No: an element cannot be broken into simpler substances, while a compound always can.

How would our daily lives change if water were a mixture of hydrogen and oxygen, not a compound?

Solution

In a mixture, each part keeps its own properties. So "water" would be a mixture of two gases: hydrogen, which burns explosively, and oxygen, which makes burning fiercer.

We could not drink it, cook with it or wash with it.
It would cause fires and explosions instead of putting them out.
There would be no rivers, oceans, rain or ice. Plants and animals, including us, could not exist, because life depends on liquid water.

Water would be an explosive mixture of two gases: undrinkable and dangerous, with no liquid water for rain, oceans or life.

In Fig. 8.24, dilute hydrochloric acid is added to iron filings and a gas (A) is given off. Identify gas A and write the word equation.

Solution

Gas A is hydrogen. It burns with a "pop" sound when a lighted matchstick is brought near.

Word equation: iron + hydrochloric acid → iron chloride + hydrogen

Hydrogen. Iron + hydrochloric acid → iron chloride + hydrogen.

Name two compounds made only of non-metals, and give two uses of each.

Solution

Water (hydrogen and oxygen):

for drinking and cooking
as a solvent for cleaning, and in farming and industry

Carbon dioxide (carbon and oxygen):

used by plants in photosynthesis, and in fire extinguishers
used to make soda drinks fizzy, and as dry ice for keeping things cold

(Other examples: ammonia, from nitrogen and hydrogen, used in fertilisers and cleaning liquids; methane, from carbon and hydrogen, used as a fuel.)

E.g. water (drinking, cleaning) and carbon dioxide (fire extinguishers, fizzy drinks).

How can gold be both a mineral and a metal?

Solution

Metal: gold is an element with all the properties of metals. It is shiny, malleable and ductile, and it conducts heat and electricity.
Mineral: gold occurs naturally in the Earth's crust, often as pure grains or nuggets in rocks and river sands, with a definite composition. It is mined from the Earth like other minerals.

So "metal" describes its properties, and "mineral" describes how it is found.

Gold is a metal by its properties and a mineral because it is found naturally in the Earth's crust with a definite composition.

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