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Nature of Matter: Elements, Compounds, and Mixtures: questions with answers

Class 8 Science, Chapter 8 (NCERT). 15 questions by concept, 1 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Heating sugar shows that it is a compound

Question 1

A teacher heats dry sugar. It decomposes, producing water and a carbon-rich black residue. Which method has broken down the compound?

  1. (a)Only electricity can break down a compound; heating never can.
  2. (b)No compound can ever be broken down chemically.
  3. (c)Heating has caused chemical decomposition.
  4. (d)Heating only separated carbon and water already mixed inside the sugar.
Show answer

(c) Heating has caused chemical decomposition.

Sugar can decompose on strong heating. Its products differ from sugar, so this is a chemical change. This does not mean every compound decomposes under the same heating conditions.

Types of mixtures, and what counts as a component

Question 2

A dry baking-powder sample contains baking soda and tartaric acid. No reaction has occurred. How should this supplied sample be classified?

  1. (a)Neither, because powder cannot be classified as matter.
  2. (b)A mixture, because two substances are present without a chemical reaction.
  3. (c)A compound, because the two powders were ground together.
  4. (d)A pure substance, because it has one name and looks uniform.
Show answer

(b) A mixture, because two substances are present without a chemical reaction.

The source table lists baking powder as a solid-and-solid mixture. The stem supplies two substances and no reaction, so it is a mixture.

What a mixture is

Question 3

Yellow sulfur is mixed with iron filings without heating or reaction. Which result shows a component retaining a property?

  1. (a)The sulfur grains retain their yellow colour.
  2. (b)The component stops being matter after mixing.
  3. (c)The component is absent because mixing always produces a new chemical substance.
  4. (d)The component loses all its properties merely because it was mixed.
Show answer

(a) The sulfur grains retain their yellow colour.

In these nonreacting mixtures, components retain the relevant properties: sugar’s sweetness, iron’s magnetic response, or sulfur’s colour. Mixing does not make a component disappear.

What pure means in science

Question 4

A sample of carbon dioxide contains only one type of particle. Each particle has different elements chemically combined in a fixed ratio. How is it classified?

  1. (a)A pure element, because every particle in the sample is the same
  2. (b)A pure compound
  3. (c)A mixture, because any substance containing two elements cannot be pure
  4. (d)A mixture, because a physical process can separate its elements
Show answer

(b) A pure compound

A pure substance contains only one chemical substance. In these examples, that substance consists of identical molecules containing different elements chemically combined.

Air is a uniform mixture

Question 5

A dust-free sample of air has nitrogen, oxygen, argon, carbon dioxide, and water vapour evenly mixed. How should this sample be classified?

  1. (a)It is a compound because its gases have combined chemically.
  2. (b)It is one substance because its gases cannot be picked out by hand.
  3. (c)It is a non-uniform mixture because it contains several gases.
  4. (d)It is a uniform mixture of gases.
Show answer

(d) It is a uniform mixture of gases.

The named gases are evenly mixed and retain their properties. That makes this dust-free air sample a uniform mixture, not a compound or one substance.

Alloys are uniform mixtures

Question 6

A stainless-steel sample contains iron, nickel, chromium, and carbon. It appears the same throughout, and its components cannot be seen separately. How should this supplied sample be classified?

  1. (a)A compound, because named components in an alloy must have combined chemically.
  2. (b)A non-uniform mixture, because components that can be named must be separately visible.
  3. (c)A uniform mixture, because its components are evenly distributed and have not combined chemically.
  4. (d)An element, because stainless steel is a metal.
Show answer

(c) A uniform mixture, because its components are evenly distributed and have not combined chemically.

The source calls stainless steel a mixture. Its components are evenly distributed and cannot be seen separately, so this supplied sample is uniform.

Uniform and non-uniform mixtures

Question 7

A sprout salad has visible green gram, chickpeas, onion, and tomato. How should this mixture be classified?

  1. (a)Non-uniform, because its components can be distinguished.
  2. (b)Uniform, because putting components together and stirring always makes a mixture uniform.
  3. (c)Neither kind, because it has not been stirred.
  4. (d)Its kind depends only on the number of components.
Show answer

(a) Non-uniform, because its components can be distinguished.

The chapter classifies a mixture by whether its components can be distinguished. The supplied components remain visible, so each case is non-uniform.

Dust is suspended in air, not part of it

Question 8

A teacher leaves identical clean black sheets for equal times near a road and in a garden. Can we be certain that both sheets collect the same amount of dust?

  1. (a)Yes. Dust amounts are fixed, so both sheets must collect the same amount.
  2. (b)No. The sheets differ only because one was left out longer.
  3. (c)Yes. Dust is a component of air and has the same proportion everywhere.
  4. (d)No. Dust amounts may vary from place to place, so the deposits can differ.
Show answer

(d) No. Dust amounts may vary from place to place, so the deposits can differ.

The chapter says that dust nature and amount may vary by time and place. Equal exposure time does not guarantee equal deposits at different places.

Lime water shows carbon dioxide is in air

Question 9

Clear limewater is exposed to ordinary air for several hours. Nobody blows into it. Can carbon dioxide in that air cause milkiness?

  1. (a)Yes, but only if all the water evaporates first.
  2. (b)No. A burning candle is always required to create carbon dioxide first.
  3. (c)Yes. Carbon dioxide already present in ordinary air can react with limewater.
  4. (d)No. Carbon dioxide can reach limewater only when someone blows into it.
Show answer

(c) Yes. Carbon dioxide already present in ordinary air can react with limewater.

Ordinary air contains carbon dioxide. With sufficient exposure, it can react with limewater to form tiny white calcium carbonate particles. Directly exhaled breath is not required.

Elements and atoms

Question 10

In this particle model, open and filled circles represent atoms of different elements. What does the container hold?

  1. (a)One element, because all atoms are identical regardless of the open or filled symbols.
  2. (b)A mixture of two elements; no particle joins unlike atoms.
  3. (c)A pure substance, because the box has a boundary.
  4. (d)A compound, because joining two atoms of the same element always makes a compound.
Show answer

(b) A mixture of two elements; no particle joins unlike atoms.

Open and filled circles represent different elements. Both are present without unlike atoms bonded together. Pairs of the same atom type are molecules of an element, not compounds.

Metals, non-metals, and how many elements there are

Question 11

Which statement about metals at 20°C and normal pressure is correct?

  1. (a)Mercury must be a non-metal because liquids cannot be metals.
  2. (b)Gallium and caesium are liquid at 20°C, just like mercury.
  3. (c)Every metal is solid, so mercury cannot be a metal.
  4. (d)Mercury is liquid, so not every metal is solid under these conditions.
Show answer

(d) Mercury is liquid, so not every metal is solid under these conditions.

Mercury is a metal and is liquid at 20°C and normal pressure. It is a direct exception to the claim that every metal is solid.

Compounds combine elements in fixed ratios

Question 12

A water molecule has two hydrogen atoms and one oxygen atom. Could a particle with three hydrogen atoms per oxygen still be that water molecule?

  1. (a)No. A water molecule has the definite hydrogen-to-oxygen atom ratio 2:1.
  2. (b)No. Water contains only hydrogen atoms and no oxygen.
  3. (c)Yes. Water’s hydrogen-to-oxygen ratio can be changed freely without changing the compound.
  4. (d)Yes. Any chosen ratio makes water if the same recipe is repeated.
Show answer

(a) No. A water molecule has the definite hydrogen-to-oxygen atom ratio 2:1.

The composition of a water molecule is definite: two hydrogen atoms for each oxygen atom. Changing the atom ratio does not give the same water molecule.

How we use elements, compounds and mixtures

Question 13

Concrete is selected for a supplied building project. The chapter identifies concrete as a mixture. What conclusion fits this use?

  1. (a)Concrete is unsafe for every use because all mixtures behave unpredictably.
  2. (b)Concrete cannot be useful because only pure substances are useful.
  3. (c)Being a mixture does not prevent this concrete from being useful for the stated project.
Show answer

(c) Being a mixture does not prevent this concrete from being useful for the stated project.

The chapter lists concrete among useful building materials and identifies it as a mixture. Its mixture status does not prevent this stated use.

Passing electricity through water splits it into two gases

Question 14

A teacher collects hydrogen and oxygen from water. At the same temperature and pressure, their volumes are 60 mL and 30 mL. How do they compare?

  1. (a)The oxygen volume is twice the hydrogen volume.
  2. (b)Neither gas occupies any space.
  3. (c)The volumes must be equal because the gases came from the same water.
  4. (d)The hydrogen volume is twice the oxygen volume.
Show answer

(d) The hydrogen volume is twice the oxygen volume.

The measured volumes are 60 mL of hydrogen and 30 mL of oxygen. Hydrogen’s volume is twice oxygen’s, giving a 2:1 ratio.

Minerals

Question 15

How can a naturally occurring sample of native gold be classified?

  1. (a)As a mineral only while underground, becoming a metal only after mining.
  2. (b)As either a mineral or a metal, but never both.
  3. (c)As both a mineral and a metal.
Show answer

(c) As both a mineral and a metal.

Native gold is a naturally occurring mineral and a metallic element. These labels describe different properties, so both apply.

Common misconception in Nature of Matter: Elements, Compounds, and Mixtures

Open full misconception guide →

Is a uniform mixture a compound?

Wrong

Salt water is a compound because it looks uniform.

Why it feels right. I cannot see separate ingredients.

Right

A solution is a mixture with variable composition. A compound has a fixed composition.

Worked example

Glass A
Water1 spoon salt
Glass B
Water2 spoons salt
The water portions are equal. Both solutions look uniform even with different amounts of dissolved salt.
  1. Dissolve 1 spoon of salt in one glass of water and 2 in another equal glass.
  2. Both solutions look uniform, yet their different salt-to-water ratios show why uniformity alone does not make a compound.