Keep the Curiosity Alive — Chapter Exercises — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 12 marksCuriosity Grade 8, Chapter 8, page 131

   Consider the reaction A + B → C. Assume A and B cannot be broken down into simpler substances by chemical reactions. 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.

   Hint. The condition given about A and B is the definition of one particular category.

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

   Step by step.

   • A and B cannot be broken down into simpler substances — that is exactly the chapter's definition of an element. So A and B are elements.
   • C is formed when two different elements combine chemically. Compounds are formed when different elements combine in fixed ratios to form something entirely new. So C is a compound.
   • And in fixed ratios is part of that same definition, so C has a fixed composition.

   ✦ Why each of the others fails:

   Option	Fault
   (i)	Calls A and B compounds, which contradicts the condition that they cannot be broken down
   (ii)	Right about C, but 'A and B have a fixed composition' is the wrong claim — an element has no composition to fix, being one kind of atom
   (iii)	Again calls A and B compounds, contradicting the given condition

   The single line to write in an exam: A and B cannot be broken down, so they are elements; C is formed by their chemical combination, so it is a compound with a fixed composition.

2. 22 marksCuriosity Grade 8, Chapter 8, page 131

   Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change. Choose: (i) both true, R explains A; (ii) both true, R does not explain A; (iii) A true, R false; (iv) A false, R true.

   Hint. Check each statement separately, then ask whether the second accounts for the first.

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

   The Assertion is true. Air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour.

   The Reason is true. It restates the chapter's definition: When two or more substances are mixed, where each substance retains its properties, it is called a mixture ... The components of a mixture do not react chemically with each other.

   And the Reason explains the Assertion. Air is called a mixture precisely because several gases are present together and none of them has reacted with the others — oxygen in air still supports combustion, nitrogen still does not. Apply the Reason to air and you arrive at the Assertion, which is what makes it the correct explanation rather than merely a true statement standing nearby.

   ✦ How to handle assertion-reason questions generally. Three separate checks, in order: is A true? is R true? does R, applied to the case in A, produce A? Skipping the third check is the usual way these are lost — plenty of true reasons sit beside true assertions without explaining them.

3. 33 marksCuriosity Grade 8, Chapter 8, page 131

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

   Hint. The chapter puts the sharpest version of this in a speech bubble.

   The justification is the definition of a compound, backed by a striking example.

   The properties of compounds are different from those of elements forming that compound.

   And the example the chapter gives beside Activity 8.3: Hydrogen is a fuel, oxygen supports combustion, whereas water extinguishes fire.

   	Hydrogen	Oxygen	Water
   State at room temperature	Gas	Gas	Liquid
   Behaviour with fire	Burns — a fuel	Makes things burn better	Puts fire out

   ✦ Look at how complete the reversal is. Both elements are gases that help fire in one way or the other; the compound is a liquid that stops it. This is not a mild difference of degree — it is the opposite behaviour, and it is why the chapter can say a compound is something entirely new.

   Why it happens. In water the hydrogen and oxygen are so tightly attached to each other that it is generally impossible to separate them apart using physical methods. They are no longer present as hydrogen and oxygen at all, so their properties are simply not available. A compound's properties belong to the compound, not to a mixture of its parents'.

4. 44 marksCuriosity Grade 8, Chapter 8, page 131

   In which of the following cases 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.

   Hint. One wrong item spoils a set; check every item in every row.

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

   Why (iii) is correct. Carbon dioxide is a compound; iron is an element; oxygen is an element; sugar is a compound. Pure substances are either an element or a compound, so all four qualify.

   Why each of the others fails:

   Set	The offending items
   (i) Elements	Water is a compound (Activity 8.3 breaks it into two elements) and air is a mixture. Nitrogen and iron are fine
   (ii) Uniform mixtures	Minerals are not mixtures at all — most often they are compounds but rarely, they can also be pure elements. Seawater, bronze and air are genuine uniform mixtures
   (iv) Non-uniform mixtures	Air is a uniform mixture and brass is a uniform alloy. Sand and muddy water are non-uniform

   ✦ Set (ii) is the interesting mistake, and it is easy to fall for. Rocks are mixtures of minerals — but a mineral itself is a pure substance with a fixed chemical composition. Swap 'minerals' for 'rocks' and set (ii) would be correct. So the trap is one level of the hierarchy, not a wild error, which is exactly why it is worth being able to state the rock–mineral–element chain cleanly.

5. 54 marksCuriosity Grade 8, Chapter 8, page 131

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

   Hint. There are six substances across the two reactions; do not overlook the air.

   Substance	Category	Justification
   Iron	Element	Cannot be broken into anything simpler; made of iron atoms only
   Moist air	Mixture	Nitrogen, oxygen, argon, carbon dioxide and water vapour together, none of them reacting with the others
   Iron oxide	Compound	Two different elements, iron and oxygen, chemically combined in a fixed ratio, with properties of its own
   Magnesium	Element	Named by the chapter among the metals; cannot be broken down further
   Oxygen	Element	Obtained from water in Activity 8.3 and not further divisible
   Magnesium oxide	Compound	Magnesium chemically combined with oxygen, giving a white solid unlike either

   ✦ Moist air is the one to be careful with. It is tempting to write 'oxygen and water', but the reaction happens in air, and air is a mixture of several gases of which only some take part. The classification asks what the substance is, not which part of it reacts, so 'mixture' is the answer.

   Both reactions are element + element → compound, which is the pattern of exercise 1 and of Activity 8.5. That matters because, once you see the pattern, the classification of a whole reaction takes one line: the reactants are elements (with the air as their mixture), and the products are compounds.

6. 65 marksCuriosity Grade 8, Chapter 8, page 132

   Classify the following as elements, compounds or mixtures, then list the pure substances: 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.

   Hint. Nineteen substances. Sort them first, then remember which two columns make up the pure substances.

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

   Pure substances — every element and every compound above, that is: aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda. Fourteen of the nineteen.

   The mixtures are the other five: sand, seawater, muddy water, air, fruit juice.

   ✦ The last part of the question is the part most often missed. Pure substance is not a fourth category alongside the three — it is the name for the first two columns taken together. Pure substance can be either an element or a compound. So you build that list by combining, not by re-sorting.

   Two entries worth a note. Sand is classed as a mixture here, consistent with §8.5 — rocks and the sand made from them are mixtures of minerals. Rust is taken as iron oxide, the compound named in exercise 5 on the previous page; that is the level at which this chapter works with it.

7. 74 marksCuriosity Grade 8, Chapter 8, page 132

   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? Write the word equation.

   Hint. This is Activity 8.5 asked as an exercise — use every test you performed.

   The new substance is iron sulfide.

   Iron + Sulfur → Iron sulfide

   How it differs from the mixture, test by test:

   	Mixture (Sample A)	Iron sulfide (Sample B)
   Appearance	Black and yellow particles both visible	Uniformly black; same colour and texture throughout
   Magnet	Iron filings are attracted and can be pulled out	Not attracted at all
   Separation	The components can be separated	Iron and sulfur can no longer be separated
   With dilute HCl	Hydrogen — colourless, odourless, burns with a pop	Hydrogen sulfide — colourless, smells of rotten eggs
   Sulfur	Left as a yellow solid, unreacted	None left free
   Ratio of components	Any amounts may be mixed	Fixed

   ✦ Give more than one difference. A single test could always be explained away — but appearance, magnetism, separability and two different gases all pointing the same way cannot be. That is why the activity runs three separate tests instead of stopping at the colour change.

   The word equation the chapter does print is the acid reaction: Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide. The formation equation above follows directly from the activity's own account of what is heated and what results.

8. 83 marksCuriosity Grade 8, Chapter 8, page 132

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

   Hint. Put the two definitions side by side and see whether anything could satisfy both.

   No. The two definitions exclude each other.

   	Element	Compound
   Definition	Cannot be further broken down into simpler substances	Different elements combine in fixed ratios to form something entirely new
   Kinds of atom	One kind only	Two or more kinds
   Can it be decomposed chemically	No	Yes

   Why nothing can be both. To be a compound, a substance must contain at least two different elements and be capable of being broken down into them. To be an element, it must contain only one kind of atom and be incapable of being broken down. A substance cannot both contain two kinds of atom and contain only one, so no substance can satisfy both definitions.

   ✦ A common confusion worth clearing up. A molecule of hydrogen has two atoms in it — does that make hydrogen a compound? No: both atoms are hydrogen atoms. The test is whether the atoms are of different kinds, not how many there are. Fig. 8.10 (two identical circles) and Fig. 8.11 (three circles of two kinds) are drawn to make exactly this contrast.

   What a substance can be at once is a compound and a mineral, or an element and a mineral, as with gold — but that is a different pair of categories entirely, and the next exercise turns on it.

9. 94 marksCuriosity Grade 8, Chapter 8, page 132

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

   Hint. Take the properties of the two gases seriously and follow them through.

   It would not be water at all, and almost nothing about life would survive the change.

   Working through the consequences:

   1. It would be a gas, not a liquid. Hydrogen and oxygen are both gases at room temperature, and in a mixture each keeps its own properties. There would be no rivers, no seas, nothing to pour into a glass.
   2. It would be dangerously flammable. Hydrogen is a fuel, oxygen supports combustion — a fuel mixed with the very gas that helps things burn. Instead of putting out fires it would feed them explosively.
   3. It would separate. The components of a mixture can be separated by physical means, so it would not stay a fixed thing.
   4. Nothing would dissolve in it as it does now, so no way to carry nutrients in plants or in blood, and no way to wash anything.
   5. The proportions would vary from place to place, since a mixture has no fixed ratio — so no two samples would even behave the same.

   ✦ The exercise is really asking you to prove that the compound-versus-mixture distinction is not a technicality. Every one of these consequences follows from a single sentence in the chapter — the properties of compounds are different from those of elements forming that compound. Because water is a compound, it has properties neither of its elements has, and every one of those properties is something life depends on.

10. 103 marksCuriosity Grade 8, Chapter 8, page 132

    Analyse Fig. 8.24, which shows iron filings with dilute hydrochloric acid producing a gas A. Identify Gas A and write the word equation.

    Hint. You produced this gas twice in the chapter and tested it the same way both times.

    Gas A is hydrogen.

    Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

    How you know. This is exactly the Sample A test from Activity 8.5: iron in the mixture reacts with dilute hydrochloric acid to form iron chloride and hydrogen gas. The gas is colourless, has no smell, and burns with a 'pop' sound.

    How you would confirm it in the lab. Bring a burning splinter to the mouth of the tube — hydrogen gives the characteristic pop. That is the same test that identified hydrogen in Activity 8.3, where the tube over one terminal of the battery popped and the other made the flame glow brighter.

    ✦ Watch out for the trap in this figure. Iron sulfide with the same acid gives hydrogen sulfide, which smells of rotten eggs. Here the flask contains iron filings, not iron sulfide — no sulfur is present anywhere, so the gas cannot contain sulfur. Reading exactly what is in the flask is the whole of this question.

    Both products belong in the equation. Iron chloride is easy to forget because it stays dissolved and unseen, but it is as much a product as the gas.

11. 114 marksCuriosity Grade 8, Chapter 8, page 132

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

    Hint. Both of the chapter's own headline compounds qualify — check their elements against the metals-and-non-metals list.

    The chapter's non-metals include carbon, sulfur, hydrogen and oxygen, so a compound made only from these is made only from non-metals.

    1. Water — hydrogen and oxygen, both non-metals.

    • Drinking and all household use — it is essential for life.
    • Extinguishing fires, which the chapter highlights as a property none of its elements has.
    • (Also: irrigation, and dissolving substances so they can be carried and used.)

    2. Carbon dioxide — carbon and oxygen, both non-metals.

    • Used by plants to make food, and detected in air by the lime-water test of Activity 8.1.
    • Dissolved in aerated water (soda water), the chapter's example of a gas-and-liquid mixture.
    • (Also: in fire extinguishers, since it does not support combustion.)

    ✦ Check your candidates before you commit to them. Magnesium oxide, iron sulfide, sodium chloride and rust all contain a metal, so none of them can be used here. Glucose (carbon, hydrogen, oxygen) would be another good answer, and so would hydrogen sulfide — though its use is harder to state at this level.

    The examiner is testing two things at once: that you can recall which elements are non-metals, and that you know what each compound is made of. Naming the elements explicitly in your answer shows both.

12. 123 marksCuriosity Grade 8, Chapter 8, page 132

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

    Hint. These are two different classifications, not two answers to the same question.

    Because 'mineral' and 'metal' answer two different questions, and gold satisfies both.

    Question	Category	Gold
    Where does it come from and what is it?	Mineral — natural, solid substances found on the Earth with a fixed chemical composition	Found naturally in the Earth as a solid, so it is a mineral
    What kind of element is it?	Metal	Named by the chapter among the metals

    And more precisely, gold is a native mineral. Some of the minerals are called native minerals, which are pure elements and not compounds. These can be metals, such as gold, silver, copper.

    ✦ The Snapshot allows for exactly this case: Most often they are compounds but rarely, they can also be pure elements. Gold is one of the rare ones — a mineral that is a single element rather than a combination. So no contradiction arises, because being a mineral says nothing about whether the substance is an element or a compound.

    Contrast with exercise 8. There, element and compound genuinely exclude each other, because both are answers to the same question. Here they are not, which is why gold can hold both labels comfortably. Ask what question a classification is answering before assuming two labels must clash.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 8 (hecu108.pdf), Reprint 2026-27, pages 116-133. Questions are referenced from the NCERT textbook for identification.