ScienceCBSE Class 8 · Particulate Nature of Matter

Constituent Particles, Interparticle Spaces and Motion: Class 8 Science

Updated 17 October 2026 · Fact-checked

Matter is made of extremely small constituent particles that cannot be seen even with an ordinary microscope. There are spaces between them (interparticle spaces), which is why sugar can dissolve in water and a gas can be compressed. The particles are always moving, and they move faster when heated, which is why a dye or a fragrance spreads on its own.

Sugar particles slip into the empty spaces between water particles, so the sugar seems to vanish; the arrows show particles are always moving.

Illustration drawn for QuestionBuddy.

Understand Constituent Particles, Interparticle Spaces and Motion of Particles

Evidence for particles. In Activity 7.1 a chalk stick is broken and then ground to powder. Each speck is still chalk, so grinding is a physical change (only the size reduces). If grinding could continue, we would reach units that cannot be broken further: the constituent particles, the basic units that make up a piece of substance. Sand and clay grains are also not the smallest units of rock. In Activity 7.2 sugar stirred into water makes the top layer taste sweet though no grains can be seen: the sugar has broken into its constituent particles, millions of which make up one grain of sugar. These particles are too small to be seen even with an ordinary microscope. Acharya Kanad, an ancient Indian philosopher, spoke of the Parmanu (atom), tiny indivisible particles, in the Vaisheshika Sutras.

Spaces between particles. The sugar particles go into the interparticle spaces between the water particles. In Activity 7.7 the water level rises when sugar is added, then falls somewhat after it dissolves, so the volume of the solution is less than the volume of water plus sugar. Sand does not dissolve: its particles settle and take up space, and the volume increases. In Activity 7.6 a syringe full of air can be compressed when the plunger is pushed, since gas particles have much space between them; water is practically incompressible. The space between particles contains nothing at all; it is not air. Even solids have small spaces between their particles.

Particles are always moving (Activities 7.8 and 7.9). A grain of potassium permanganate in water first forms pink streaks, and gradually the whole water turns uniformly pink. The moving water particles pull particles out of the grain and hit them to spread them. Substances such as sand, whose particles are held together strongly, are insoluble. With hot, room-temperature and ice-cold water, the pink spreads fastest in hot water and slowest in cold: particles move faster when heat is provided. Likewise, the fragrance of an incense stick in one corner reaches the whole room because air particles are constantly moving and hit the fragrance particles. Perfume reaches us in the same way.

A word of caution. The word 'particle' has different meanings. Suspended Particulate Matter (SPM) in air pollution means tiny dust particles, which are far larger than the constituent particles (atoms and molecules) of which they are made.

Everyday use. Soap particles attach with one end to oil and with the other to water, lifting the oil off the cloth.

How the particles of solids, liquids and gases differ is in the next topic.

Constituent Particles, Interparticle Spaces and Motion of Particles: formulas and key facts

Constituent particle: Smallest basic unit of a substance that makes up the whole | Too small to see with an ordinary microscope; Grains of sand, clay and sugar are not constituent particles.
Volume on dissolving: Volume of solution < Volume of water + Volume of solute; Solute particles fit into interparticle spaces (Activity 7.7).
Compressibility: Gas: easily compressed | Liquid: practically incompressible; Syringe activity 7.6.
Particle motion and heat: Higher temperature > faster particle motion > faster spreading; Potassium permanganate spreads fastest in hot water.
Spreading of fragrance: Air particles in constant motion hit fragrance particles and spread them through the room; Activity 7.9.

How to solve Constituent Particles, Interparticle Spaces and Motion of Particles questions

Use this method for evidence and explanation questions.

1State the observation in the activity.
2Link it to the idea: particles, spaces between particles, or particle motion.
3For volume questions, compare the actual volume with the simple sum and explain the difference by interparticle spaces.
4For spreading questions, state that particles move all the time and move faster at higher temperature.
5Compare different substances by the strength of attraction between their particles.
6End with a one-line conclusion using the textbook terms.

Constituent Particles, Interparticle Spaces and Motion of Particles in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Volume shrinks on dissolving

Suppose 200 mL of water takes 20 g of sugar, which on its own would occupy 15 mL. After stirring, the level reads 209 mL (invented values).

(a) What volume would you get by simple addition? (b) By how much is the actual volume less? (c) Give the reason. (d) What would happen to the volume if 20 g of sand were added instead?

Try it first, then show the solutionSolution

Simple sum = 200 + 15 = 215 mL.
Contraction = 215 − 209 = 6 mL.
Sugar particles occupy the spaces between water particles.
Sand does not dissolve, settles and occupies space of its own, so the level rises and the volume increases.

Answer: (a) 215 mL; (b) 6 mL; (c) sugar particles fill interparticle spaces; (d) volume increases.

Memory hook: Particles tuck into the gaps.

Dye in hot and cold water

Imagine three tumblers with hot, room-temperature and ice-cold water. Equal grains of potassium permanganate are dropped in; the water is uniformly pink after 40 s, 90 s and 240 s respectively (invented times).

(a) Which tumbler is which? (b) How many times faster is the hot one than the cold one? (c) What does this say about particle motion? (d) Which tumbler would show the steepest streak first?

Try it first, then show the solutionSolution

Hot: 40 s; room temperature: 90 s; ice-cold: 240 s.
240 ÷ 40 = 6 times faster.
Particles move faster at higher temperature.
The hot water, which has the fastest-moving particles.

Answer: (a) 40 s hot, 90 s room, 240 s cold; (b) 6 times; (c) heat speeds particles up; (d) the hot water tumbler.

Memory hook: Heat makes particles hustle.

Tricks to remember Constituent Particles, Interparticle Spaces and Motion of Particles

Short memory aids. Say each one aloud once, then use it on a question the same day.

Chalk and sugar clues
Chalk powder tells us 'tiny pieces', sugar in water tells us 'invisible but there'.
Use it when: Evidence for particles.
Gaps for guests
Water particles leave gaps; sugar particles are the guests who fill them.
Use it when: Volume change on dissolving.
Hot = hurry
Hot water particles hurry; cold ones dawdle.
Use it when: Effect of temperature on spreading.

Constituent Particles, Interparticle Spaces and Motion of Particles: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Why can no sugar be seen · 2 marks · Short answerBoard level

Sugar dissolves in water and cannot be seen, but the water tastes sweet. Explain this using the particle idea.

Try it first, then show the solutionSolution

Sugar breaks into its tiny constituent particles, too small to be seen even with an ordinary microscope. (1)
These particles spread into the spaces between water particles and are sensed by taste. (1)

Answer: Sugar breaks into very small particles that go into the interparticle spaces of water, so they cannot be seen but can be tasted.

Example 2: Sand versus sugar in water · 1 mark · MCQBoard level

When sand is added to water the volume of the water level
(a) decreases
(b) stays the same
(c) increases because sand does not dissolve and takes up space
(d) first falls then rises

Try it first, then show the solutionSolution

Sand does not dissolve; it settles and occupies space in the container.

Answer: (c)

Example 3: Compression in a syringe · 5 marks · Long answerHarder board level

A syringe contains 50 mL of air and the nozzle is closed. The plunger is pushed until the air occupies 20 mL. The same is tried with 50 mL of water.
(a) By how much did the volume of air decrease? (1)
(b) Express this as a percentage of the original volume. (1)
(c) What happens to the volume of water? (1)
(d) Explain both observations using interparticle space. (2)

Try it first, then show the solutionSolution

(a) 50 − 20 = 30 mL. (1)
(b) 30 ÷ 50 × 100 = 60%. (1)
(c) The water is practically incompressible, so the volume stays about 50 mL. (1)
(d) Gas particles have large spaces between them which can be reduced by pressing; liquid particles are close together, so there is very little space to reduce. (2)

Answer: (a) 30 mL; (b) 60%; (c) almost no change; (d) gases have large interparticle spaces; liquids have very little.

Example 4: Spread of fragrance and dye · 4 marks · Case-basedHarder board level

Read the passage and answer.

An incense stick lit at one end of a room is smelt at the other end in a few minutes. In another trial a camphor tablet is heated in a corner and the smell spreads faster.
(a) Why is the smell felt far away? (1)
(b) Why does heating speed it up? (1)
(c) A student says the smell spreads because of the smoke only. Is this right? (1)
(d) Name one related activity with a liquid. (1)

Try it first, then show the solutionSolution

(a) Air particles move constantly and hit the fragrance particles, which spread through the room. (1)
(b) Heating gives particles more energy, so they move faster. (1)
(c) No: the fragrance particles are invisible; the smoke is only a visible sign (the chapter uses smoke to represent the movement of gas particles). (1)
(d) Potassium permanganate crystal spreading in water. (1)

Answer: (a) Constantly moving particles; (b) more energy means faster motion; (c) not only smoke; (d) potassium permanganate in water.

Example 5: Reasoning about missing particles · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

A student argues: 'If we could remove all the particles from a chair, the chair would only weigh less.' Another says: 'The particles in a gas are not in contact with each other, so the space between them must be filled with air.'
(a) Evaluate the first statement. (1)
(b) Evaluate the second statement for gas particles of a gas such as oxygen. (1)
(c) Why does the volume of sugar solution not equal the sum of the volumes of water and sugar? (1)
(d) Why is SPM not the same as the constituent particles? (1)

Try it first, then show the solutionSolution

(a) Wrong: the chair is made of its constituent particles, so with them all removed nothing of the chair would remain. (1)
(b) Wrong: the space between particles is empty; it contains nothing at all (it is not air). (1)
(c) Sugar particles occupy the interparticle spaces of water. (1)
(d) SPM is tiny dust, far larger than constituent particles, and made of very many of them. (1)

Answer: (a) Nothing would remain; (b) the space is empty; (c) sugar fills the gaps; (d) dust is far larger than constituent particles.

Where students lose marks in Constituent Particles, Interparticle Spaces and Motion of Particles

The slips examiners see most often, and what to write instead.

Mistake: Saying the spaces between particles are filled with air.
Students feel nothing can be empty.
Wrong working
The space between particles contains air.
Right working
The space between particles contains nothing at all.
Do this: Write 'empty space'.
Usually costs: 1 mark.
Mistake: Writing that sugar disappears when dissolved.
It cannot be seen.
Wrong working
Sugar vanishes in water.
Right working
Sugar breaks into constituent particles that occupy spaces between water particles; we can taste them.
Do this: Use 'particles', not 'disappears'.
Usually costs: 1 mark.
Mistake: Mixing grains with constituent particles.
Both are small.
Wrong working
A grain of sand is a constituent particle.
Right working
A grain is made of a very large number of constituent particles.
Do this: Constituent particles cannot be seen even with an ordinary microscope.
Usually costs: 1 mark.
Mistake: Forgetting temperature in diffusion answers.
Only the spread is described.
Wrong working
The colour spreads because particles move.
Right working
The colour spreads because particles move; it spreads faster in hot water because hot particles move faster.
Do this: Add the effect of heat.
Usually costs: 1 mark.

Exam tips

For evidence questions, name the activity, the observation and the conclusion.
Use the words 'constituent particles' and 'interparticle spaces'.
For volume change, do the addition first and then the subtraction.
Mention that the space between particles is empty.
Link motion with temperature in any spreading question.

Practice questions on Constituent Particles, Interparticle Spaces and Motion of Particles

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
What are constituent particles? Give one activity that suggests that matter is made of particles.
Hint
Chalk or sugar.
Answer
Constituent particles are the basic units that make up a substance (1). Dissolving sugar in water makes the water sweet though nothing can be seen (1).
Q2 · 1 mark · MCQBoard level
The spread of the colour of potassium permanganate through water shows that
(a) particles of water are stationary
(b) particles of water are in constant motion
(c) potassium permanganate is a gas
(d) water particles are very large
Hint
Activity 7.8.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The volume of sugar solution is less than the sum of the volumes of the sugar and the water.
Reason (R): Sugar particles occupy the spaces between water particles.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Interparticle spaces.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
100 mL of water is mixed with a soluble solid whose own volume is 20 mL. After stirring until it dissolves, the level reads 112 mL. (a) What is the sum of the volumes of the water and the solid? (b) By how much did the volume contract? (c) Express the contraction as a percentage of the sum. (d) Explain in particle terms.
Hint
Add the two volumes first.
Answer
(a) 100 + 20 = 120 mL. (b) 120 − 112 = 8 mL. (c) 8 ÷ 120 × 100 ≈ 6.7%. (d) The solid's particles go into the spaces between the water particles (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A student drops identical grains of a coloured crystal into hot and cold water. The colour spreads through the hot water in 30 s and through the cold water in 150 s. A friend says the crystal is insoluble.
(a) How many times faster is the spread in hot water? (b) Explain this using particles. (c) Why can the friend not be right? (d) If sand were used, what would be seen?
Hint
Spreading means the crystal is breaking up in the water.
Answer
(a) 150 ÷ 30 = 5 times. (b) Hot water particles move faster. (c) The colour spread, so the water particles pulled the crystal particles out; it is soluble. (d) Sand would settle at the bottom and not spread (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Can constituent particles be seen with an ordinary microscope?Show answer
No, they are too small.
2.What happens to the water level when sugar dissolves?Show answer
It first rises on adding sugar and then falls somewhat after dissolution.
3.What is between particles of matter?Show answer
Empty space, called interparticle space.
4.Which spreads faster, a dye in hot or cold water?Show answer
Hot water.
5.Who proposed the idea of the Parmanu?Show answer
Acharya Kanad, in the Vaisheshika Sutras.

Constituent Particles, Interparticle Spaces and Motion of Particles: frequently asked questions

Is grinding chalk a chemical change?

No. It is a physical change: only the size of the chalk specks reduces and the substance remains chalk.

Why can gases be compressed but liquids cannot?

Gas particles have much space between them; liquid particles are close together and water is practically incompressible.

Why does sand not dissolve?

Its constituent particles are held together so strongly that water particles cannot pull them out.

How does soap clean oil?

One end of a soap particle attaches to oil and the other mixes with water, lifting the oil off the cloth.