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Particulate Nature of Matter: questions with answers

Class 8 Science, Chapter 7 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

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

Atoms and molecules

Question 1

In ordinary oxygen gas, do oxygen atoms usually remain separate or join into molecules?

  1. (a)They stop being matter when they combine.
  2. (b)They cannot form molecules, because a molecule must contain two different elements.
  3. (c)They must remain separate, because every element normally exists only as independent atoms.
  4. (d)They join in pairs to form oxygen molecules.
Show answer

(d) They join in pairs to form oxygen molecules.

Ordinary hydrogen and oxygen gases consist mainly of two-atom molecules. Atoms of one element can combine into a molecule. This is not a claim that isolated atoms can never exist under special conditions.

Matter is made of constituent particles

Question 2

A visible sand grain came from a larger rock. Is this grain the smallest constituent unit of its material?

  1. (a)No, because an ordinary light microscope can show individual atoms inside the grain.
  2. (b)Yes. Every visible grain is one atom.
  3. (c)No. The grain contains very many smaller constituent particles.
  4. (d)Yes. A visible sand or clay grain is already the smallest constituent unit of rock.
Show answer

(c) No. The grain contains very many smaller constituent particles.

Visible sand grains and clay specks are much larger than their constituent particles. An ordinary light microscope does not resolve individual atoms. Breaking rock into grains does not reach its smallest constituent units.

The word particle means different things in different places

Question 3

Compare a constituent particle with a visible speck of dust. How do their sizes compare?

  1. (a)They are about the same size because both are called particles.
  2. (b)A dust speck is smaller than each of the constituent particles making it up.
  3. (c)They cannot be compared because dust and matter are different substances.
  4. (d)A constituent particle is much smaller; a dust particle contains many constituent particles.
Show answer

(d) A constituent particle is much smaller; a dust particle contains many constituent particles.

Constituent particles are much smaller than an ordinary dust speck. Such a speck contains many constituent particles. Suspended particulate matter includes dust and droplets in air.

Dissolving does not destroy the particles

Question 4

A teacher fully dissolves clean food sugar in drinking water, looks at it, and asks students to taste a small clean sample. What combination of sight and safe taste shows that the sugar remains in the water?

  1. (a)Sugar remains only if a pale cloud of its particles can be seen in the water.
  2. (b)No grains are visible, so the sugar has vanished and the sweet taste cannot come from it.
  3. (c)Sugar grains are not visible, but the sweet taste shows that dissolved sugar is still present.
  4. (d)A ring of dry sugar must appear around the glass before the water can taste sweet.
Show answer

(c) Sugar grains are not visible, but the sweet taste shows that dissolved sugar is still present.

Dissolved sugar grains are no longer visible, but the solution still tastes sweet. The taste shows that sugar remains present as spread-out particles.

The spaces between particles

Question 5

Use the term for an ordinary solid in a jar. What does interparticle space mean here?

  1. (a)It is a hole or bubble visible through an ordinary microscope.
  2. (b)It is the distance a particle travels before striking another particle.
  3. (c)It is the tiny space between constituent particles, not a visible hole or bubble.
  4. (d)It exists only in gases, because solids and liquids have no spaces.
Show answer

(c) It is the tiny space between constituent particles, not a visible hole or bubble.

Interparticle space is the space between constituent particles. Such particles cannot be seen through an ordinary microscope, so the space is not a visible hole or bubble.

Gases can be compressed, liquids practically cannot

Question 6

A syringe contains air and a tight cap seals its outlet. No air leaks in or out. What happens to the amount of air when the plunger is pushed?

  1. (a)The amount decreases because each air particle becomes smaller.
  2. (b)The amount decreases because some air leaves through the seal.
  3. (c)The amount of air stays the same in the sealed syringe.
  4. (d)The amount increases because the air becomes denser.
Show answer

(c) The amount of air stays the same in the sealed syringe.

The seal prevents air from leaving or entering. Pushing reduces the space between the same air particles, not their number.

How particles move in a gas

Question 7

An incense stick is lit in one corner of a room. Its fragrance is later smelt across the room. Which particle motion helps explain this?

  1. (a)Fragrance particles remain at the source until a fan is switched on.
  2. (b)Both fragrance and air particles move randomly. Their collisions help spread the fragrance through the room.
  3. (c)Air particles are stationary, so fragrance particles move across alone.
  4. (d)Fragrance can spread only when a draught carries it.
Show answer

(b) Both fragrance and air particles move randomly. Their collisions help spread the fragrance through the room.

Fragrance particles and air particles move randomly. Collisions between them help spread fragrance through the room.

How particles move in a liquid

Question 8

The coldest tumbler contains ice-cold water and a small grain of potassium permanganate. It is left for a long time. What happens to the colour?

  1. (a)It sinks permanently as a coloured layer at the bottom.
  2. (b)It spreads faster than in warmer water, because cold water pulls colour more quickly.
  3. (c)It spreads through the water slowly, because cold-water particles still move but move more slowly.
  4. (d)It never spreads, because particles in ice-cold water do not move.
Show answer

(c) It spreads through the water slowly, because cold-water particles still move but move more slowly.

The chapter compares hot, room-temperature, and ice-cold water. In ice-cold water the particles move slowest, so colour spreads slowest but still spreads.

How the spacing is ordered across the three states

Question 9

The general particle model shows a solid and its liquid with particles close together, while its gas has wide gaps. Which comparison describes the liquid?

  1. (a)Its particles are much closer together than in the gas.
  2. (b)They have no separation at all; every liquid is a single continuous particle.
  3. (c)The constituent particles themselves become much larger in the liquid state.
  4. (d)They are spaced like gas particles, because liquids and gases both flow.
Show answer

(a) Its particles are much closer together than in the gas.

Liquid particles remain close together, unlike widely separated gas particles. Most solids are more closely packed than their liquids, but ice is an exception. A change of state does not enlarge the constituent particles themselves.

Interparticle attraction and what decides its strength

Question 10

In a simple particle model, attraction drops sharply as separation increases. If particles move slightly farther apart, what happens to their attraction?

  1. (a)It remains unchanged, because attraction never depends on separation.
  2. (b)It becomes stronger whenever separation increases.
  3. (c)It can weaken only slightly, because a small distance change must always have a small effect.
  4. (d)It becomes much weaker, following the stated distance–attraction relation.
Show answer

(d) It becomes much weaker, following the stated distance–attraction relation.

The model states that attraction falls sharply with separation. Therefore, even a small increase can cause a large decrease. This does not describe ice melting, which has an exceptional spacing change.

The gaseous state, and what a fluid is

Question 11

Water flows in a bottle, and air flows through a tube. Which statement about these fluids is correct?

  1. (a)Only liquids are fluids because a fluid is another word for a liquid.
  2. (b)Liquids and gases are fluids because both flow and do not retain a fixed shape.
  3. (c)Air is a fluid with a fixed volume that it keeps in every container.
  4. (d)All states are fluids because every material can be forced to move.
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(b) Liquids and gases are fluids because both flow and do not retain a fixed shape.

Liquids and gases both flow and do not retain a fixed shape. Those properties distinguish them from solids and classify them as fluids.

The solid state

Question 12

Rice flour is poured into a jar and takes its shape. Is the material a solid or a liquid?

  1. (a)A liquid, because the heap takes the shape of its container.
  2. (b)A solid, but each grain has no definite volume because the heap can be packed down.
  3. (c)A solid. Each grain or speck has its own definite shape and volume.
  4. (d)Neither, because a grain is too small to have a state of matter.
Show answer

(c) A solid. Each grain or speck has its own definite shape and volume.

Rice grains and rice flour can take a jar’s shape, yet they are solids. The test applies to each grain or speck, which has its own shape and volume.

Dissolving changes the total volume

Question 13

A glass of water is marked A. Sand is added, mixed, and allowed to settle without any water loss. How does the final total level compare with A?

  1. (a)It remains above A only because the grains are large; sufficiently fine sand would dissolve.
  2. (b)It remains above A because insoluble sand settles and occupies space in the container.
  3. (c)It returns exactly to A because settled sand no longer occupies space in the mixture.
  4. (d)It falls closer to A after stirring because sand enters interparticle spaces just as dissolved sugar does.
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(b) It remains above A because insoluble sand settles and occupies space in the container.

Sand does not dissolve in water. It settles but still occupies space in the container, so the total level remains above the original water mark.

The liquid state

Question 14

Milk is standing in a glass. Consider particles that remain in the liquid. How can they move?

  1. (a)They stay at fixed positions and do not move.
  2. (b)They move past one another within the liquid.
  3. (c)They only vibrate about fixed positions.
  4. (d)They move freely through all available space and fill the whole container.
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(b) They move past one another within the liquid.

Liquid particles move past one another within the liquid. A particle at the surface can escape by evaporation, but that is not the motion asked here.

Melting and the melting point

Question 15

A small cube and a large block are both pure ice at atmospheric pressure. How do their melting points compare?

  1. (a)The smaller sample has a higher melting point because smaller amounts always need a higher melting temperature.
  2. (b)They have the same melting point under the stated conditions.
  3. (c)The larger sample has a higher melting point because amount alone sets melting temperature.
  4. (d)Their melting points are set only by the container’s shape.
Show answer

(b) They have the same melting point under the stated conditions.

For ordinary-sized samples, melting point depends on substance, purity, and pressure. Amount alone does not change it. A larger amount can require more energy to melt without having a higher melting point.

Common misconception in Particulate Nature of Matter

Open full misconception guide →

Do particles themselves expand when a gas expands?

Wrong

Heating a gas makes each molecule bigger.

Why it feels right. The gas takes more space, so its pieces seem bigger.

Right

At lower density, gas particles are farther apart on average.

Worked example

Both boxes show six equal-size particles. In the larger box, the particles are farther apart.
  1. Compare the same number of drawn particles in a small and a large box.
  2. Keep each dot the same size and increase the spacing.