ScienceCBSE Class 8 · Particulate Nature of Matter

Solids, Liquids and Gases and Change of State: Class 8 Science

Q: Is ice an exception to the usual spacing of particles?

Yes. In most substances the liquid has particles slightly farther apart than the solid, but in ice the particles are farther apart than in water.

Q: What decides the state of a substance?

The thermal energy of its particles, which sets the distance between them and the strength of the attractions.

Q: Is a gas jar full of smoke a solid?

No. The smoke only shows the motion of the invisible gas particles; the gas has no fixed shape or volume.

Q: Why can we move a finger through water but not through a block of iron?

In water the attractions are slightly weaker and the water is temporarily displaced; in a solid the strong attractions hold the particles in fixed positions.

Updated 17 October 2026 · Fact-checked

The state of matter is decided by the interparticle attractions, which depend on the distance between particles and on their thermal energy. Solids have particles closely packed and vibrating about fixed positions (definite shape and volume); liquids have particles that move within limited space (definite volume, no fixed shape); gases have particles moving freely in all directions with negligible attraction (no fixed shape or volume).

Understand Solids, Liquids and Gases: Interparticle Attraction and Change of State

What decides the state? The constituent particles of matter are held together by interparticle attractions. Their strength depends on the nature of the substance and on the distance between particles; even a slight increase in distance decreases the forces drastically. The thermal energy of particles determines the distance, and so the state.

Solids. Strong attractions hold the particles tightly packed in fixed positions; they can only vibrate to and fro about these positions. Hence solids have definite shape and volume (Activity 7.3: iron, rock salt, stone, wood, key, aluminium). On heating, the vibrations become more vigorous; at the melting point, the minimum temperature at which a solid melts to a liquid at atmospheric pressure, the particles leave their fixed positions and the forces weaken. Table 7.1: ice melts at 0 degrees Celsius, urea at 133 degrees Celsius and iron at 1538 degrees Celsius; weak interparticle forces mean a low melting point and strong forces a high one. In most substances the liquid has particles slightly farther apart than the solid; ice is an exception because its particles are farther apart than those of water.

Liquids. In Activity 7.4, 200 mL of water poured into three differently shaped containers takes the shape of each, yet the volume stays 200 mL. So liquids have no fixed shape but a definite volume: particles move freely but only within a limited space. Interparticle attractions are slightly weaker than in solids but still strong enough to keep the particles close. You can move a finger through water because the water is only temporarily displaced. On heating, at the boiling point, the temperature at which a liquid boils and turns into vapour at atmospheric pressure, particles move so vigorously that they move apart and escape; vapour forms fast at the surface and within the liquid as bubbles. Evaporation is the slower formation of vapour at the surface at all temperatures below the boiling point.

Gases. In Activity 7.5, smoke from an incense stick trapped in one gas jar fills the second jar when the plate is removed. Gas particles move freely in all directions, with negligible attraction, so gases have no fixed shape and no fixed volume and occupy all the space available. Liquids and gases both flow and have no fixed shape, so they are called fluids. Iodine vapour can show the same activity (with care; it irritates).

Summary table.

Solid: spacing minimum; attraction maximum; particles only vibrate.
Liquid: spacing a little more; attraction slightly weaker; movement in limited space.
Gas: spacing maximum; attraction negligible; movement through all available space.

Role of energy. In a solid, thermal energy is low, so particles stay close with strong attraction. At the melting point the supplied energy is used to overcome the attractions and the solid becomes a liquid. In a gas the particles have enough energy to overcome the attractions and move freely.

The previous topic dealt with evidence for particles, spaces between them and motion.

Solids, Liquids and Gases: Interparticle Attraction and Change of State: formulas and key facts

Solid: Definite shape
Definite volume
Particles close, strong attraction, vibrate only; Examples: iron, rock salt, stone, wood.
Liquid: No fixed shape
Definite volume
Particles move in limited space; Takes the shape of the container.
Gas: No fixed shape
No fixed volume
Particles move freely, negligible attraction; Fills the available space.
Melting and boiling: 1.Melting point: solid
2.liquid | Boiling point: liquid
3.vapour (at atmospheric pressure); Ice 0 C, urea 133 C, iron 1538 C (Table 7.1).
Evaporation versus boiling: Evaporation: all temperatures, surface only, slow | Boiling: at boiling point, throughout liquid, fast, bubbles; Both make vapour.
Fluids: $L i q u i d s + g a ses$; They flow and have no fixed shape.

How to solve Solids, Liquids and Gases: Interparticle Attraction and Change of State questions

Use this method for states-of-matter questions.

1Identify what is given: shape, volume, packing, movement of particles or a temperature.
2Compare with the three descriptions to decide the state.
3For melting or boiling, compare the temperature with the melting or boiling point to decide the state at that temperature.
4Use interparticle attraction to explain high or low melting points.
5Describe changes of state as a change in thermal energy and spacing of particles.
6Use the words: vibrate, limited space, free, fluid.

Solids, Liquids and Gases: Interparticle Attraction and Change of State 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.

State at a given temperature

Use Table 7.1 of the chapter: ice melts at 0 degrees Celsius, urea at 133 degrees Celsius and iron at 1538 degrees Celsius. Suppose a metalworker heats an iron rod to 1500 degrees Celsius and a chemist heats urea to 150 degrees Celsius.

(a) State whether the iron is solid or liquid at 1500 degrees Celsius. (b) State the state of urea at 150 degrees Celsius. (c) By how many degrees must the iron be heated further to melt? (d) Which of the three has the strongest interparticle forces?

Try it first, then show the solutionSolution

1500 is below 1538, so iron is still solid.
150 is above 133, so urea has melted and is a liquid (below its boiling point as far as the table tells us).
1538 − 1500 = 38 degrees.
Iron has the highest melting point, so it has the strongest interparticle forces among the three.

Answer: (a) Solid; (b) liquid; (c) 38 degrees Celsius; (d) iron.

Memory hook: Melting point is the temperature where the particles let go of their places.

Spilled water disappears

Imagine water spilled on a table at room temperature is dry after an hour, though no one heated it, while a pan on a stove boils away with bubbles.

(a) Name the process on the table and in the pan. (b) Give two differences. (c) Which has particles with greater energy?

Try it first, then show the solutionSolution

Evaporation on the table; boiling in the pan.
Evaporation occurs at all temperatures, only at the surface, slowly; boiling occurs at the boiling point, throughout the liquid, fast, with bubbles.
The boiling water: its particles move so vigorously that they escape throughout the liquid.

Answer: (a) Evaporation; boiling; (b) any two of the listed differences; (c) boiling water.

Memory hook: Evaporation is quiet and surface-only; boiling is noisy and everywhere.

Tricks to remember Solids, Liquids and Gases: Interparticle Attraction and Change of State

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

Solid shuffles, liquid slides, gas goes
Solid particles vibrate in place, liquid particles slide past each other, gas particles go everywhere.
Use it when: Particle movement in the three states.
S-L-G ladder
Spacing: least, little more, most. Attraction: most, less, least.
Use it when: Comparison table.
Fluids flow
Liquid and gas both flow: fluids.
Use it when: Definition of fluids.

Solids, Liquids and Gases: Interparticle Attraction and Change of State: solved examples

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

Example 1: Compare the three states · 3 marks · Short answerBoard level

Tabulate shape, volume, interparticle space and interparticle attraction for solids, liquids and gases.

Try it first, then show the solutionSolution

Solid: fixed shape, fixed volume, minimum space, maximum attraction. (1)
Liquid: no fixed shape, fixed volume, a little more space, slightly weaker attraction. (1)
Gas: no fixed shape or volume, maximum space, negligible attraction. (1)

Answer: See the three rows: solid (fixed shape and volume), liquid (fixed volume only) and gas (neither).

Example 2: What is a fluid · 1 mark · MCQBoard level

Which pair of states of matter are both called fluids?
(a) Solid and liquid
(b) Solid and gas
(c) Liquid and gas
(d) Only gases

Try it first, then show the solutionSolution

Both liquids and gases flow and do not keep a fixed shape.

Answer: (c)

Example 3: Explain with the particle model · 5 marks · Long answerHarder board level

(a) Why does a liquid take the shape of its container yet keep a fixed volume? (2) (b) Why does a gas fill the whole of a closed room? (1) (c) Why does heating a solid at its melting point change its state? (2)

Try it first, then show the solutionSolution

(a) Liquid particles move freely but only within a limited space, because attractions are still strong enough to keep them close; so there is a definite volume but no fixed shape. (2)
(b) Gas particles have negligible attraction and move freely in all directions, so they spread to all available space. (1)
(c) At the melting point the added thermal energy overcomes the attractions between particles; they leave their fixed positions and move around, so the solid becomes a liquid. (2)

Answer: (a) Limited free movement gives volume but no shape; (b) free movement in all directions; (c) added energy overcomes attractions.

Example 4: Reading melting points · 4 marks · Case-basedHarder board level

Read the passage and answer.

Table 7.1 gives melting points: ice 0 degrees Celsius, urea 133 degrees Celsius, iron 1538 degrees Celsius. A student heats urea and iron, both starting at 25 degrees Celsius, to 140 degrees Celsius.
(a) State the state of each at 140 degrees Celsius. (1)
(b) Arrange the three in order of increasing interparticle attraction. (1)
(c) By how many degrees does the temperature of the iron need to rise from 140 to melt? (1)
(d) Ice is an exception in the usual spacing of particles; explain. (1)

Try it first, then show the solutionSolution

(a) Urea: liquid (140 is above 133); iron: solid (140 is far below 1538). (1)
(b) Ice < urea < iron, since a higher melting point means stronger attraction. (1)
(c) 1538 − 140 = 1398 degrees. (1)
(d) In most substances the liquid has particles farther apart than the solid, but in ice the particles are farther apart than in water. (1)

Answer: (a) Urea liquid, iron solid; (b) ice < urea < iron; (c) 1398 degrees; (d) ice particles are more widely spaced than those of water.

Example 5: Rice grains and flour · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Grains of rice and rice flour both take the shape of the jar when poured into it, and a student wonders whether they are liquids. A teacher also asks why the glass tumbler stays in shape while spilled milk spreads on a table.
(a) State the properties of a solid. (1)
(b) Explain whether grains of rice are solids. (1)
(c) Why does spilled milk spread while the tumbler keeps its shape? (1)
(d) In the gas jar activity with smoke from an incense stick, what does the smoke show about gas particles? (1)

Try it first, then show the solutionSolution

(a) A solid has a fixed shape and a fixed volume. (1)
(b) Each grain keeps its own shape and volume, so rice grains and flour specks are solids; the heap takes the container's shape because the grains can be arranged differently, not because the grain particles can flow (an inference from the definition). (1)
(c) Milk is a liquid with weaker attraction between particles, so it flows; glass is a solid with strong attractions. (1)
(d) The smoke makes the movement of the invisible gas particles visible; it shows that gas particles move freely and spread to fill all the space available. (1)

Answer: (a) Fixed shape and volume; (b) each grain is a solid; (c) liquid flows, solid does not; (d) the smoke shows that gas particles move freely and spread to fill all the available space.

Where students lose marks in Solids, Liquids and Gases: Interparticle Attraction and Change of State

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

Mistake: Saying liquids have no fixed volume.
Liquids change shape.
Wrong working
Liquids have no fixed shape or volume.
Right working
Liquids have no fixed shape but a definite volume.
Do this: Activity 7.4: 200 mL stays 200 mL.
Usually costs: 1 mark.
Mistake: Saying solid particles do not move at all.
Solids look still.
Wrong working
Particles in a solid are completely stationary.
Right working
They vibrate about fixed positions but cannot move past each other.
Do this: Write 'vibrate about fixed positions'.
Usually costs: 1 mark.
Mistake: Mixing up evaporation and boiling.
Both produce vapour.
Wrong working
Evaporation happens only at 100 degrees Celsius.
Right working
Evaporation happens at all temperatures below the boiling point, only at the surface.
Do this: Learn the two lists.
Usually costs: 1 to 2 marks.
Mistake: Using 'melting point' for liquids turning to gas.
Terms are confused.
Wrong working
The melting point of water is 100 degrees Celsius.
Right working
Melting point of ice is 0 degrees Celsius; boiling point of water is 100 degrees Celsius.
Do this: Melt: solid to liquid; boil: liquid to vapour.
Usually costs: 1 mark.

Exam tips

Describe each state by shape, volume, spacing, attraction and movement.
Explain a change of state by thermal energy and interparticle attraction.
Quote Table 7.1 values with units (degrees Celsius).
Write that the melting point and boiling point are defined at atmospheric pressure.
Use 'fluids' for liquids and gases together.

Practice questions on Solids, Liquids and Gases: Interparticle Attraction and Change of State

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 is the melting point of a solid? Give two examples with their values from the chapter.
Hint
Table 7.1.
Answer
The minimum temperature at which a solid melts to a liquid at atmospheric pressure (1). Ice 0 degrees Celsius; iron 1538 degrees Celsius (urea 133 degrees Celsius) (1).
Q2 · 1 mark · MCQBoard level
A sample moves in all the available space of a closed jar and has no fixed volume. It is most likely
(a) a solid
(b) a liquid
(c) a gas
(d) a powder
Hint
Think of smoke spreading.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A wooden block keeps its shape when moved from one table to another.
Reason (R): Its particles are held in fixed positions by strong attractions and can only vibrate about them.
(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
Which part gives volume and which gives shape?
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
A student pours 200 mL of water from container A into B and then into C, and notes the level each time. (a) What volume is expected in B and C if the containers are clean? (b) If 3 mL clings to the walls of each container after pouring, what volumes remain in B and C? (c) What percentage of the original is lost in C? (d) Which property of liquids does the first part show?
Hint
The chapter notes a little water may stick to walls.
Answer
(a) 200 mL each. (b) B: 200 − 3 = 197 mL; C: 197 − 3 = 194 mL. (c) 6 ÷ 200 × 100 = 3%. (d) Liquids have a definite volume but no fixed shape (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A beaker of water is heated. At first a few bubbles stick to the sides, and then, at its boiling point, bubbles rise from throughout the liquid. A tumbler of water kept on a table loses a little water in a day.
(a) Name the two processes. (b) Give one difference. (c) What happens to the particles at the boiling point? (d) Complete the definition: the boiling point is the temperature at which a liquid boils and turns into vapour at ...?
Hint
The chapter defines boiling point 'at atmospheric pressure'.
Answer
(a) Boiling and evaporation. (b) Boiling occurs throughout the liquid at the boiling point with bubbles; evaporation occurs only at the surface at any temperature, slowly. (c) They move so vigorously that they move apart, the attractions weaken and they escape as vapour. (d) Atmospheric pressure (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.Which state has the strongest interparticle attraction?Show answer
Solid.
2.Which state has the maximum interparticle space?Show answer
Gas.
3.What is the melting point of ice?Show answer
0 degrees Celsius.
4.What are fluids?Show answer
Liquids and gases, which flow and have no fixed shape.
5.What is the difference between evaporation and boiling?Show answer
Evaporation is slow, at the surface and at any temperature; boiling is fast, throughout the liquid, at the boiling point.

Solids, Liquids and Gases: Interparticle Attraction and Change of State: frequently asked questions

Is ice an exception to the usual spacing of particles?

Yes. In most substances the liquid has particles slightly farther apart than the solid, but in ice the particles are farther apart than in water.

What decides the state of a substance?

The thermal energy of its particles, which sets the distance between them and the strength of the attractions.

Is a gas jar full of smoke a solid?

No. The smoke only shows the motion of the invisible gas particles; the gas has no fixed shape or volume.

Why can we move a finger through water but not through a block of iron?

In water the attractions are slightly weaker and the water is temporarily displaced; in a solid the strong attractions hold the particles in fixed positions.