NCERT Solutions Curiosity Chapter 8 End-of-chapter questions — Keep the curiosity alive

Book page 131 Updated on2026-09-05

Q1.

Consider the following reaction where two substances, A and B, combine to form a product C: A + B → C Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements 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.

Answer

Correct answer: (iv) A and B are elements, C is a compound, and has a fixed composition.

Why: The one fact you are given is that A and B cannot be broken down into simpler substances. That is precisely the definition of an element, so A and B are elements. C is formed by two different elements combining chemically, which makes C a compound — and every compound has its elements combined in a fixed ratio, so C has a fixed composition.

Option	Verdict	Reason
(i)	Wrong	A and B cannot be compounds — a compound can be broken down into simpler substances
(ii)	Wrong	C being a compound is right, but "fixed composition" belongs to C; A and B are single elements, not combinations
(iii)	Wrong	Again treats A and B as compounds, which contradicts the information given
(iv)	Correct	Matches both the definition of an element and the definition of a compound

Q2.

Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change. (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. (ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion. (iii) Assertion is true, but Reason is false. (iv) Assertion is false, but Reason is true.

Answer

Correct answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

Why: The Assertion is true — air is a uniform mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour. The Reason is a correct statement of what a mixture is. And the Reason explains the Assertion: air counts as a mixture precisely because those gases are simply mixed together without any chemical change, each keeping its own properties. Nitrogen in air still refuses to support burning and oxygen in air still supports it, exactly as they would on their own. That is the test the Reason describes, applied to air.

Tip: In an assertion–reason question, first check each statement on its own, then ask the second question separately — does the reason actually explain the assertion, or is it merely another true fact?

Q3.

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

Answer

The properties of water are not a blend of the properties of hydrogen and oxygen — they are entirely new. That is what happens when elements combine chemically to form a compound.

Property	Hydrogen	Oxygen	Water
State at room temperature	Gas	Gas	Liquid
Behaviour with fire	A fuel — burns with a pop sound	Supports combustion — makes a flame glow brighter	Extinguishes fire
Can we drink it?	No	No	Essential for life
Can it be separated physically?	Both are elements — cannot be broken down at all		Cannot be separated into hydrogen and oxygen by any physical method

Why it happens: In water the hydrogen and oxygen are not lying side by side as two gases; they are chemically combined in the fixed ratio of 2 : 1, and their particles are held together so tightly that a new kind of particle exists. A new particle means a new substance, and a new substance has its own set of properties. If water were only a mixture of the two gases, it would still burn and still support burning — but water does the opposite of both, which is exactly why we use it to put out fires.

Q4.

In which of the following cases are all the examples correctly matched? Give reasons in support of your answers. (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.

Answer

Correct answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar.

Why (iii) is right: A pure substance consists of only one type of particle, and every element and every compound qualifies. Carbon dioxide and sugar are compounds; iron and oxygen are elements. All four are therefore pure substances.

Option	The item that spoils it	Reason
(i) Elements	water and air	Water is a compound of hydrogen and oxygen; air is a mixture of gases. Only nitrogen and iron are elements.
(ii) Uniform mixtures	minerals	Minerals are not mixtures at all — most are compounds and a few are pure elements. Seawater, bronze and air are indeed uniform mixtures.
(iii) Pure substances	— none —	All four are elements or compounds. This is the correct set.
(iv) Non-uniform mixtures	air and brass	Air is a uniform mixture and brass is an alloy, which is also uniform. Only sand and muddy water are non-uniform.

Q5.

Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.

Answer

Iron + Moist air → Iron oxide
Magnesium + Oxygen → Magnesium oxide

Substance	Classification	Justification
Iron	Element	Cannot be broken down into simpler substances; made of identical atoms
Moist air	Mixture	Nitrogen, oxygen, argon, carbon dioxide and water vapour mixed together, each keeping its own properties
Iron oxide (rust)	Compound	Iron and oxygen combined chemically in a fixed ratio; its properties — reddish-brown, brittle, not shiny — are quite unlike those of iron
Magnesium	Element	A metal that cannot be broken down further
Oxygen	Element	A non-metal that cannot be broken down further
Magnesium oxide	Compound	Magnesium and oxygen combined chemically in a fixed ratio; a white powder, unlike the shiny metal or the colourless gas

Why it happens: In both reactions an element combines chemically with oxygen and a compound is formed. Note the interesting point in the first one — iron reacts with the oxygen and water vapour that happen to be in the mixture we call air. That is why iron rusts in moist air but hardly at all in dry air.

Q6.

Classify the following as elements, compounds, or mixtures in Table 8.3. 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. Identify pure substances amongst these and list them below.

Answer

Table 8.3

Elements	Compounds	Mixtures
Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen	Carbon dioxide, Magnesium oxide, Rust, Iron sulfide, Glucose, Water, Sodium chloride, Baking soda	Sand, Seawater, Muddy water, Air, Fruit juice

Pure substances — every element and every compound in the list:

Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen,
Carbon dioxide, Magnesium oxide, Rust, Iron sulfide, Glucose, Water, Sodium chloride, Baking soda
— that is all 14 substances from the first two columns.

How the doubtful ones were decided:

Rust is iron oxide — iron and oxygen chemically combined, so a compound.
Sand is largely quartz but comes with other minerals and grains mixed in, so it is treated as a mixture; its grains can be told apart, making it non-uniform.
Seawater is a uniform mixture — water with salts dissolved in it — because the salt can be recovered by evaporation.
Fruit juice contains water, pulp, sugars and acids together, so it is a mixture.
Baking soda is a single compound. Do not confuse it with baking powder, which is a solid–solid mixture of baking soda and tartaric acid (Table 8.1).

The two questions that classify any sample of matter.

Q7.

What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.

Answer

The new substance is iron sulfide, a black mass — and it is a compound, while the original was a mixture.

Iron + Sulfur (on heating) → Iron sulfide

Point of comparison	Mixture of iron and sulfur (Sample A)	Iron sulfide (Sample B)
Appearance	Black specks among yellow powder; grainy	Uniformly black, same texture throughout
Magnet	Iron is attracted and can be pulled out	Not attracted
With dilute hydrochloric acid	Gives hydrogen — odourless, burns with a pop; sulfur left as a yellow solid	Gives hydrogen sulfide — smells of rotten eggs
Can the components be separated?	Yes, by physical means	No physical method can separate them
Proportion of iron to sulfur	Can be varied at will	Fixed

Why it happens: Heating supplies the energy the two elements need to combine chemically. Once combined, they are no longer iron and sulfur but a single new substance with its own properties — which is why every test in the table gives a different result.

Q8.

Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.

Answer

No — never. The two definitions rule each other out.

Element	Compound
Cannot be broken down into simpler substances	Can be broken down into the elements it is made of
Made of one kind of atom	Made of two or more different elements combined chemically
No question of a ratio — there is only one element	Its elements are combined in a fixed ratio

Why it happens: A substance is an element exactly when it cannot be split into anything simpler. To be a compound it must be splittable into at least two different elements. A single substance cannot both be splittable and not splittable, so no substance can be in both categories. Both, however, are pure substances — that is the one label they share.

Tip: Water is the classic trap. It looks like one simple thing, but Activity 8.3 splits it into hydrogen and oxygen — so water is a compound, never an element.

Q9.

How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?

Answer

There would be no liquid water at all — and therefore no life as we know it.

Why: Hydrogen and oxygen are both gases at room temperature. A mere mixture of them would also be a gas, because in a mixture each component keeps its own properties. Rivers, wells, seas and the water in your bottle exist only because the two elements are chemically combined into a completely new substance that happens to be a liquid.

Following that through:

Nothing to drink, cook with or wash in. You cannot carry a gas in a pot or a bottle.
Everything would be dangerously flammable. A mixture of a fuel (hydrogen) and a gas that supports combustion (oxygen) would explode at the smallest spark, instead of putting fires out.
The proportion would keep changing. A mixture has no fixed ratio, so "water" from one place would not be the same as "water" from another.
No rain, no rivers, no crops. There would be no water cycle, no irrigation and no agriculture.
No life. The bodies of all living things are mostly water; blood, sap and cell fluids all depend on it being a liquid.

Did you know? This is why the girl in the chapter finds it fascinating — hydrogen is a fuel and oxygen supports combustion, yet the compound they form extinguishes fire. Combining chemically really does make a brand new substance.

Q10.

Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

Answer

In Fig. 8.24 iron filings in a test tube have dilute hydrochloric acid added to them, and a gas is coming off through the tube at the top. That gas is hydrogen.

Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

How we know it is hydrogen: This is exactly the reaction you carried out on Sample A in Activity 8.5. The gas released there was colourless, had no smell, and burned with a 'pop' sound when a lighted candle was brought near the mouth of the test tube — the standard test for hydrogen.

Check it yourself: If the tube had contained iron sulfide instead of plain iron filings, the gas would have been hydrogen sulfide, recognisable at once by its rotten egg smell. The smell test alone tells the two reactions apart.

Q11.

Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.

Answer

Two such compounds are water (hydrogen and oxygen) and carbon dioxide (carbon and oxygen). Hydrogen, oxygen and carbon are all non-metals, as the chapter states.

Compound	Non-metals in it	Two uses
Water	Hydrogen, Oxygen	1. Drinking, cooking and washing in every home. 2. Irrigating crops; also used as a coolant and as a solvent in industry.
Carbon dioxide	Carbon, Oxygen	1. Filling fire extinguishers, because it does not support combustion. 2. Making aerated drinks (soda water) fizzy; plants also use it to make their food.

Another good pair: Glucose (carbon, hydrogen and oxygen) — a quick source of energy for the body, and used in medicines and drips for patients.

Q12.

How can gold be classified as both a mineral and a metal?

Answer

Gold fits both labels because the two words answer two different questions — where does it come from? and what kind of element is it?

Called a	Because
Mineral	It is a natural solid substance found in the Earth with a fixed chemical composition. Gold is one of the native minerals — the few minerals that are pure elements rather than compounds.
Metal	As an element it belongs to the class of metals, along with silver, magnesium, iron and aluminium — it is shiny, dense and can be beaten into shape.

Why it happens: Most minerals are compounds made of more than one element — quartz, calcite, mica, pyroxene and olivine. A handful, however, occur in the Earth already in their pure elemental form; these are the native minerals, and they may be metals such as gold, silver and copper, or non-metals such as sulfur and carbon. Gold is simply the best-known example of an element that we dig straight out of the ground.

Tip: "Mineral" describes the source and the composition; "metal" describes the properties of the element. Silver and copper can be classified both ways too.

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