Class 9 · Science · Chapter 1
Matter in Our Surroundings
This chapter builds the particle model of matter from everyday observation: why matter has mass and occupies space, how the three states of matter differ in the arrangement and motion of their particles, and how heat and pressure drive the changes of state, evaporation and diffusion.
Before you startWhat the board asks from this chapter
Part of the first unit of Class 9 Science (Matter – Nature and Behaviour); expect 1 MCQ, 1 Assertion–Reason and 1–2 short/long questions from this chapter in the term paper, plus regular questions in unit tests.
| Question type | Marks | Usually asked |
|---|---|---|
| MCQ | 1 | Identifying a state/property from a description, units of density, or which factor speeds up evaporation. |
| Assertion–Reason | 1 | Pairs a stated observation (e.g. sponge is compressible) with the particle-theory reason behind it. |
| Very short answer | 2 | Define a term (sublimation, latent heat) or state one/two characteristics of particles. |
| Short answer | 3 | Explain a phenomenon using the particle model, e.g. why we feel cool after a bath, or compare rate of diffusion in the three states. |
| Long answer | 5 | Draw and label the heating/cooling curve of water, or explain all factors affecting evaporation with examples. |
| Case-based | 4 | A short passage about a daily-life situation (perfume spreading, ice melting, sweat cooling) followed by 3–4 sub-questions. |
Hot topics, in order
- Characteristics of particles of matter (five points)
- Heating curve of water and latent heat of fusion/vaporisation
- Factors affecting evaporation
- Evaporation causes cooling
- Sublimation and its examples
- Effect of change of temperature and pressure on states of matter
- Diffusion and why it is faster in gases
- SI unit of temperature, pressure and their conversions
The whole chapter, in orderNotes
01 1.1 Physical Nature of Matter
Anything that has mass and occupies space is called matter. Air, water, a stone, a chair, even the smell of food — all of these are matter because they have mass and take up volume. Early Indian philosophers classified matter into five basic elements: air (vayu), earth (prithvi), fire (agni), sky/void (aakash) and water (jal) — the 'Panch Tatva'. Modern science instead asks a sharper question: is matter continuous, like a block, or is it made of tiny discrete particles with gaps between them?
A simple activity settles this. Dissolve a few crystals of potassium permanganate in water: the colour spreads through the whole beaker even though very little was dissolved, and diluting the solution again and again still gives a faint colour, until an extremely small amount can still colour litres of water. This is only possible if matter is made of very tiny particles that keep dividing among themselves — a continuous, unbroken substance could not do this. The same idea explains why a small crystal of copper sulphate colours a large volume of water. This particle nature of matter is the foundation for the rest of the chapter.
02 1.2 Characteristics of Particles of Matter
Three simple activities establish how particles of matter behave.
Particles of matter have space between them. Dissolving sugar or salt in water and finding the level barely rises shows that the sugar particles have gone into the spaces between the water particles. This is why a mixture like sugar syrup occupies less volume than the sum of the two separate volumes, and why gases can be compressed easily — there is a lot of empty space between gas particles.
Particles of matter are continuously moving (they possess kinetic energy). This is shown by diffusion — the intermixing of particles of two different types of matter on their own. When a few crystals of a coloured solid (like potassium permanganate or copper sulphate) are dropped into water, the colour spreads on its own without stirring — the particles of the solid and of water move and mix. Diffusion is faster in gases than in liquids, and faster in liquids than in solids, because the intermixing depends on the speed of particle motion, which is highest in gases. Diffusion also increases with a rise in temperature, because particles move faster and gain kinetic energy at higher temperature.
Particles of matter attract each other. There are forces of attraction between particles that keep them together, and this force is different for solids, liquids and gases — strongest in solids, weakest in gases. This is why it is difficult to break a solid but easy to compress or flow a gas.
03 1.3 States of Matter
Matter around us exists in three physical states — solid, liquid and gas — because of differences in the arrangement, spacing and motion of the particles, and the strength of the force of attraction between them.
Solids have a definite shape, a distinct boundary and a fixed volume, with negligible compressibility. In a solid, particles are very closely packed (little empty space) because the force of attraction between them is very strong. This strong force holds the particles in fixed positions; particles can only vibrate about their mean position, which is why solids have a definite shape and cannot flow.
Liquids have no fixed shape but a fixed volume — they take up the shape of the container they are poured into. Liquids are almost incompressible and can flow, hence they have a definite volume but not a definite shape. The force of attraction between particles is weaker than in solids, so particles are close together but can move around each other, giving liquids fluidity.
Gases are highly compressible and have neither a fixed shape nor a fixed volume — they occupy all the space available to them, like a room filled with cooking gas or a scent that spreads everywhere. The force of attraction between particles is the weakest, particles are far apart with large empty spaces between them, and they move randomly at high speed, so gases exert pressure on the walls of their container equally in all directions.
The rigidity, compressibility and fluidity of the three states can each be explained purely from how tightly the particles are packed and how strong the interparticle force of attraction is — this is the single idea examiners test again and again in different wording.
04 1.4.1 Effect of Change of Temperature
Matter can change from one state to another by changing the temperature — this does not create a new substance, it is a physical change. On heating a solid, the particles gain kinetic energy. Because of this increased kinetic energy, the particles start vibrating with greater speed. The energy supplied overcomes the forces of attraction between the particles, the particles leave their fixed positions, and the solid melts to become a liquid. The temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point; the melting point of ice is 273.15 K (0 °C). The process of melting, i.e. change of solid state into liquid state, is also called fusion. On further heating, particles of the liquid gain enough energy to break free completely from one another and change into the gaseous state; the boiling point of water is 373 K (100 °C). Every substance has its own characteristic melting and boiling point, which is why they help identify the purity of a substance — pure water melts and boils sharply at 0 °C and 100 °C, while an impure sample melts/boils over a range.
Latent heat. When ice at 0 °C changes into water at 0 °C, the temperature does not rise even though heat is continuously supplied — this heat is used up in overcoming the force of attraction between particles, not in raising temperature, and is called latent heat. The latent heat of fusion is the heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point without any change in temperature. The latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point without any change in temperature. This is exactly why steam at 100 °C causes more severe burns than water at 100 °C — steam carries extra latent heat of vaporisation.
05 1.4.2 Effect of Change of Pressure
Applying pressure along with lowering the temperature can liquefy gases and even turn them solid. Increasing pressure pushes particles closer together, and lowering temperature slows their motion, so the force of attraction between them can dominate and pull them into a more ordered, closer-packed state. This is how solid carbon dioxide (dry ice) is made and stored: it is kept under high pressure. When the pressure on solid CO2 is reduced to 1 atmosphere, it changes directly into gas without becoming liquid at all — which is exactly why it is called 'dry' ice, since it never wets anything with a liquid stage. This shows that a substance's state is not fixed by temperature alone; both temperature and pressure together decide whether matter exists as solid, liquid or gas, and this is also why LPG and CNG can be stored as compressed liquids in cylinders even though they behave as gases at normal room pressure.
06 1.5 Evaporation
The change of a liquid into vapour at any temperature below its boiling point is called evaporation. Unlike boiling, evaporation is a surface phenomenon — it happens only at the surface of the liquid, at all temperatures, and does not need the liquid to reach its boiling point. Wet clothes dry, water in an open vessel disappears, and a wet floor becomes dry over time — all because of evaporation.
Why evaporation causes cooling. During evaporation, the particles of the liquid that have higher kinetic energy escape from the surface into the vapour phase, taking away this extra energy with them. The particles left behind in the liquid have lower average kinetic energy, so the temperature of the remaining liquid falls, producing a cooling effect. This is why we feel cold when a volatile liquid such as acetone or perfume is put on the palm, why we feel cool after coming out of a bath (water evaporating from the skin), why sweating cools the body, and why water in an earthen pot (matka) stays cool — water seeps through the pores and evaporates from the outer surface, cooling the water inside.
Factors affecting the rate of evaporation:
- Surface area: the larger the surface area, the faster the evaporation — clothes dry faster when spread out.
- Temperature: increasing temperature increases the kinetic energy of particles, so more particles escape as vapour, increasing the rate of evaporation.
- Humidity: humidity is the amount of water vapour present in air. When humidity is high, the air is already close to holding as much water vapour as it can, so the rate of evaporation decreases — this is why clothes take longer to dry on a humid day.
- Wind speed: with an increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding air, which increases the rate of evaporation — this is why clothes dry faster on a windy day.
VocabularyKey terms
Keep this on one pageFormula sheet
| Name | Expression | Notes |
|---|---|---|
| Kelvin–Celsius conversion | K = °C + 273 | Used to convert any Celsius temperature to kelvin, the SI unit of temperature. |
| Melting point of ice | 273 K = 0 °C | At atmospheric pressure (1 atm). |
| Boiling point of water | 373 K = 100 °C | At atmospheric pressure (1 atm). |
| Latent heat of fusion of ice | 334 J/g (≈ 3.34 × 10⁵ J/kg) | Heat needed to melt 1 g of ice at 0 °C into water at 0 °C, no temperature change. |
| Latent heat of vaporisation of water | 2260 J/g (≈ 22.6 × 10⁵ J/kg) | Heat needed to convert 1 g of water at 100 °C into steam at 100 °C, no temperature change. |
| Order of strength of interparticle force | Solid > Liquid > Gas | Explains rigidity, fluidity and compressibility of the three states. |
| Order of rate of diffusion | Gas > Liquid > Solid | Depends on the speed/kinetic energy of particles in each state. |
| Order of interparticle space | Gas > Liquid > Solid | Explains why gases are highly compressible and solids are almost incompressible. |
| Factors that increase evaporation | ↑ surface area, ↑ temperature, ↓ humidity, ↑ wind speed | Reversing any of these decreases the rate of evaporation. |
Practise with a pencilDiagrams that earn marks
Diffusion of potassium permanganate (or copper sulphate) in water
Draw a beaker of clear water with a few purple crystals of potassium permanganate settling at the bottom; draw a second beaker (after some time, unstirred) showing the purple colour spread evenly throughout the water with faint upward streaks. Label 'particles diffuse and intermix on their own' and note that repeated dilution still shows colour, proving particles are extremely small and divisible.
Arrangement of particles in solid, liquid and gas
Draw three boxes side by side: an ordered grid of touching circles (solid), a closer but disordered cluster (liquid), and a few widely scattered circles with motion-arrows (gas). Label 'strong force, vibrate only' for solid, 'moderate force, slide past each other' for liquid, 'negligible force, move randomly at high speed' for gas.
Heating curve of water (temperature vs time)
Plot temperature (°C, y-axis) against time (x-axis): rising below 0 °C, a flat segment at 0 °C (label 'latent heat of fusion, ice + water coexist'), rising from 0–100 °C, a flat segment at 100 °C (label 'latent heat of vaporisation, water + steam coexist'), then rising above 100 °C. Mark both plateaus clearly.
Sublimation of camphor/ammonium chloride
Draw an inverted glass funnel over a china dish of camphor or ammonium chloride crystals being gently heated, cotton plug in the funnel's stem. Show wavy vapour rising and re-forming as crystals on the funnel's cooler inner walls. Label 'solid → vapour (sublimation)' and 'vapour → solid (deposition)'.
TextbookNCERT questions the board keeps asking
1.1 Convert the following temperatures to the celsius scale: (a) 300 K (b) 573 K2 m
Use °C = K − 273.
(a) 300 K = 300 − 273 = 27 °C
(b) 573 K = 573 − 273 = 300 °C
1.2 What is the physical state of water at (a) 25 °C (b) 0 °C (c) 100 °C ?3 m
(a) At 25 °C, water is a liquid (between its melting point 0 °C and boiling point 100 °C).
(b) At 0 °C, water is at its melting point, so it can exist as solid (ice), or a mixture of ice and liquid water in equilibrium.
(c) At 100 °C, water is at its boiling point, so it can exist as liquid water, or a mixture of liquid water and vapour (steam) in equilibrium.
1.3 For any substance, why does the temperature remain constant during the change of state even though heat is being continuously supplied?3 m
During a change of state (melting or boiling), the heat supplied is used entirely to overcome the forces of attraction between the particles so that they can separate from each other and move to the next state — this absorbed heat is called latent heat. Since the heat energy is being used to change the state (break interparticle forces) and not to increase the kinetic energy of the particles, the temperature does not rise until the change of state is complete.
1.4 Suggest two methods which will fasten the process of evaporation of a liquid.2 m
(i) Increase the temperature of the liquid — this raises the kinetic energy of the particles, so more of them can escape as vapour.
(ii) Increase the surface area of the liquid (e.g. spread it out) — more particles are exposed at the surface, so more can escape at the same time. (Increasing wind speed or decreasing humidity also fasten evaporation.)
1.5 Why does a desert cooler cool better on a hot dry day?2 m
A desert cooler works on the principle of evaporation — water is evaporated by passing dry hot air over wet khus mats, and the cooling effect is felt because evaporation absorbs latent heat from the surrounding air. On a hot dry day, the humidity of the air is low, so the air can hold more water vapour and evaporation happens rapidly, giving a much stronger cooling effect than on a humid day when the air is already close to saturated and evaporation is slow.
1.6 How does water kept in an earthen pot (matka) become cool during summer?2 m
An earthen pot has tiny pores in its walls through which water seeps out slowly and reaches the outer surface, where it evaporates continuously into the surrounding air. Evaporation is a cooling process — the particles of water that evaporate carry away latent heat from the remaining water, lowering its temperature. Because this evaporation from the porous surface happens continuously, the water left inside the pot stays cool.
1.7 Why does a person feel cold immediately after having a bath?2 m
After a bath, a thin film of water remains on the skin. This water evaporates continuously and, being a cooling process, absorbs latent heat from the body's surface to change into vapour. Since this heat is taken from the body, the body loses heat rapidly and the person feels cold, especially just after stepping out where evaporation is fastest.
1.8 Why are we able to sip hot tea or milk faster from a saucer rather than a cup?2 m
A saucer provides a much larger exposed surface area for the same volume of tea/milk compared to a cup. Since evaporation is a surface phenomenon and its rate increases with surface area, the tea in a saucer loses heat and cools faster (more evaporation happens), letting us sip it sooner without burning the tongue.
1.9 What type of clothes should we wear in summer?2 m
We should wear light-coloured, loose cotton clothes in summer. Cotton is a good absorber of sweat and has a large surface area when loosely woven, which helps sweat evaporate quickly from the cloth, and evaporation causes cooling that keeps the body comfortable. Light colours reflect heat rather than absorbing it, which also helps keep the body cool.
E1 Arrange the following in order of increasing force of attraction between the particles: water, sugar, oxygen.2 m
Order of increasing force of attraction: oxygen (gas) < water (liquid) < sugar (solid). The force of attraction between particles is weakest in gases, moderate in liquids and strongest in solids.
E2 Convert the following temperatures into the kelvin scale: (a) 25 °C (b) −5 °C2 m
Use K = °C + 273.
(a) 25 °C = 25 + 273 = 298 K
(b) −5 °C = −5 + 273 = 268 K
E3 Give reasons for the following observations: (a) Naphthalene balls disappear with time without leaving any solid. (b) We can smell perfume sitting several metres away.3 m
(a) Naphthalene undergoes sublimation — it changes directly from the solid state to the gaseous state without passing through the liquid state, so it disappears gradually without leaving any residue.
(b) Perfume particles are volatile and light; due to the property of diffusion, its particles constantly move and intermix with the particles of air, spreading through the room, so we can smell it from a distance even without any air current.
Previous years and frequently askedBoard questions with model answers
MCQ The property to flow is unique to fluids. Which one of the following statements is correct?- Only gases behave like fluids
- Gases and solids behave like fluids
- Gases and liquids behave like fluids
- Only liquids are fluids
Frequently asked1 m
MCQ Which one of the following sets of phenomena would increase on raising the temperature?- Diffusion, evaporation, compression of gases
- Evaporation, compression of gases, solubility
- Evaporation, diffusion, expansion of gases
- Evaporation, solubility, diffusion, compression of gases
Frequently asked1 m
MCQ During summer, water kept in an earthen pot becomes cool because of the phenomenon of- diffusion
- transpiration
- osmosis
- evaporation
Frequently asked1 m
MCQ The SI unit of temperature is- Celsius
- Fahrenheit
- Kelvin
- Joule
Frequently asked1 m
MCQ Solid carbon dioxide (dry ice) is so named because- it is very cold
- it does not contain water
- it changes directly from solid to gas without becoming liquid on decreasing pressure
- it is used to dry other substances
Frequently asked1 m
MCQ Rate of evaporation of a liquid does NOT depend on- surface area
- temperature
- colour of the container
- wind speed
Frequently asked1 m
MCQ The boiling point of water on the kelvin scale, at atmospheric pressure, is- 100 K
- 173 K
- 273 K
- 373 K
Frequently asked1 m
MCQ Which of the following is an example of sublimation?- Melting of wax
- Rusting of iron
- Disappearance of camphor kept in a cupboard, without melting
- Boiling of water
Frequently asked1 m
Assertion–Reason Assertion (A): A gas fills the container it is kept in completely. Reason (R): The force of attraction between the particles of a gas is negligible and the particles move randomly at high speed in all directions.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): Sponge, though a solid, can be compressed. Reason (R): Sponge has minute pores which trap air, and it is the air, not the solid sponge material, that gets compressed.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): We can smell hot cooked food from a distance more easily than cold food. Reason (R): The rate of diffusion increases with a decrease in temperature.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Very short answer Define latent heat of fusion. What happens to the temperature of ice while it is melting even though heat is continuously supplied?Frequently asked2 m
Latent heat of fusion is the heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point, without any change in temperature. While ice at 0 °C melts, the temperature stays constant at 0 °C because the heat supplied is used entirely to overcome the force of attraction between the ice particles (as latent heat), not to raise their kinetic energy/temperature, until all the ice has melted.
Very short answer State two differences between evaporation and boiling.Frequently asked2 m
(i) Evaporation is a surface phenomenon and occurs at any temperature below the boiling point, while boiling occurs throughout the liquid (bubbles form inside) only at the fixed boiling point.
(ii) Evaporation is a slow process, while boiling is comparatively fast; evaporation causes cooling of the liquid left behind, whereas during boiling the temperature stays constant at the boiling point as heat is used as latent heat of vaporisation.
Very short answer Convert 25 °C and −10 °C to the kelvin scale.Frequently asked2 m
K = °C + 273.
25 °C = 25 + 273 = 298 K
−10 °C = −10 + 273 = 263 K
Very short answer Name two substances that undergo sublimation. Define sublimation.Frequently asked2 m
Sublimation is the change of a solid directly into vapour (gas) without passing through the liquid state (and the reverse, gas directly to solid, is called deposition). Examples: camphor and ammonium chloride (naphthalene balls, iodine and solid CO₂/dry ice are also accepted).
Very short answer What is meant by the term 'diffusion'? Give one example.Frequently asked2 m
Diffusion is the intermixing of particles of two different types of matter on their own, due to the motion of particles. Example: the smell of an agarbatti (incense stick) lit in one corner of a room spreads and can be smelled throughout the room because its particles diffuse into the air.
Short answer Explain, using the particle model, why solids are rigid but gases can be compressed easily.Frequently asked3 m
In a solid, the particles are very closely packed with almost no space between them, and the force of attraction between them is very strong. This strong force holds each particle tightly in a fixed position, so particles can only vibrate about a mean position and cannot move freely — this gives solids a definite shape and makes them rigid and almost incompressible.
In a gas, the particles are very far apart with large empty spaces between them, and the force of attraction is negligible. Because of this large empty space, the particles can be pushed much closer together when pressure is applied, so gases are highly compressible, unlike solids.
Short answer List the factors affecting the rate of evaporation and explain any one with an example.Frequently asked3 m
The rate of evaporation is affected by: (i) surface area, (ii) temperature, (iii) humidity, and (iv) wind speed.
Explaining wind speed: an increase in wind speed carries away the water vapour particles that have just evaporated from the liquid's surface, keeping the air near the surface less saturated with vapour. This allows more liquid particles to escape as vapour, increasing the rate of evaporation — this is why clothes dry faster on a windy day than on a still day.
Short answer Why does the temperature of a substance not change during its change of state, even on continuous heating? Explain with reference to the melting of ice.Frequently asked3 m
When ice at 0 °C is heated, its temperature stays at 0 °C until all the ice has melted, even though heat is being supplied continuously. This is because the heat supplied during a change of state is absorbed as latent heat, which is used entirely to overcome the force of attraction between the particles of ice so that they can move apart and become liquid water — the heat does not increase the kinetic energy (and hence the temperature) of the particles during this stage. Only after all the ice has melted does further heating raise the temperature of the resulting water.
Short answer Why does a gas exert pressure equally in all directions on the walls of its container? Explain using the particle model.Frequently asked3 m
The particles of a gas are far apart, have negligible force of attraction between them, and move randomly at very high speed in all possible directions. As these particles keep colliding with the walls of the container from every direction, each collision exerts a tiny force on the wall. Because the motion is random and occurs equally in all directions, the total effect is a pressure that is exerted uniformly and equally on every wall of the container.
Long answer Draw and explain the heating curve obtained on heating ice at −10 °C continuously until it becomes steam at 100 °C and beyond. Mark the melting point and boiling point, and explain what happens at each plateau.Frequently asked5 m
The heating curve is a graph of temperature (y-axis) against time (x-axis) as ice is heated continuously.
(i) From −10 °C to 0 °C: the temperature rises steadily as heat increases the kinetic energy of the solid ice particles.
(ii) At 0 °C, the curve becomes a flat horizontal line (plateau) — this is the melting point of ice. Here, solid ice and liquid water coexist; the heat supplied is used as latent heat of fusion to overcome the force of attraction between particles, so the temperature does not rise until all the ice has melted.
(iii) From 0 °C to 100 °C: the temperature of the liquid water rises steadily as the particles gain more kinetic energy.
(iv) At 100 °C, the curve becomes flat again — this is the boiling point of water. Here, liquid water and vapour (steam) coexist; the heat supplied is used as latent heat of vaporisation to completely separate the particles from each other, so the temperature stays at 100 °C until all the water has turned to steam.
(v) Beyond 100 °C: once all the water has become steam, further heating raises the temperature of the steam above 100 °C.
The two flat plateaus are the key feature of the graph and are the part examiners check most carefully, along with correct labelling of melting point (0 °C) and boiling point (100 °C).
Long answer (a) State the characteristics of particles of matter (any three) with one activity for each. (b) Arrange solid, liquid and gas in increasing order of (i) interparticle space and (ii) rate of diffusion.Frequently asked5 m
(a) Three characteristics of particles of matter:
(i) Particles of matter have space between them — shown by dissolving sugar in water: the volume of the mixture is less than the sum of the two separate volumes, because sugar particles occupy the spaces between water particles.
(ii) Particles of matter are continuously moving — shown by diffusion: dropping a crystal of potassium permanganate into water colours the whole beaker on its own over time, without stirring, because the particles of both substances are in constant motion and intermix.
(iii) Particles of matter attract each other — shown by trying to break a piece of iron/solid: it needs a hammer and force to break because a strong force of attraction holds its particles together, whereas the same effort easily separates particles of a liquid or a gas.
(b) (i) Interparticle space, increasing order: solid < liquid < gas.
(ii) Rate of diffusion, increasing order: solid < liquid < gas.
Long answer Explain how pressure and temperature together decide the physical state of matter. Illustrate your answer with the example of dry ice, and state why LPG can be stored as a liquid in a cylinder.Frequently asked5 m
The physical state of any given amount of matter depends on the balance between the kinetic energy of its particles (which pushes them apart) and the force of attraction between particles (which pulls them together). Both temperature and pressure can shift this balance.
Raising the temperature increases particle kinetic energy, which can overcome the force of attraction and push matter towards a less ordered state — solid → liquid → gas. Increasing pressure, on the other hand, forces particles closer together, reducing the space between them, so it can push matter towards a more ordered state — gas → liquid → solid — because at smaller separations the force of attraction becomes strong enough to hold the particles together, even without lowering the temperature much.
Dry ice (solid CO2): it is manufactured and stored under high pressure, which keeps carbon dioxide solid. When the pressure is released to normal atmospheric pressure, the particles of CO2 have enough kinetic energy at that pressure to escape directly as gas — it sublimes rather than melts, so it never forms a liquid ('dry' ice).
LPG (liquefied petroleum gas): at room temperature and normal atmospheric pressure, LPG exists as a gas. Inside the gas cylinder, however, it is kept under high pressure, which packs its particles closely enough for the force of attraction to hold it as a liquid, letting a large quantity of gas be stored compactly. When the valve is opened, the pressure drops to atmospheric pressure and the liquid LPG turns back into gas, ready to burn.
This shows the three states are not fixed properties of a substance — they depend on both temperature and pressure.
Case-based Read the passage and answer the questions that follow: Ravi spilt a few drops of acetone (nail polish remover) on his palm on a hot afternoon. He immediately felt a strong cooling sensation on his skin, and within a minute the liquid had completely disappeared from his palm without leaving any residue. His teacher explained that this was because acetone is a highly volatile liquid.
(i) Name the process responsible for the cooling sensation Ravi felt.
(ii) Why did the acetone disappear from his palm without leaving any mark?
(iii) Name two factors, other than the nature of the liquid, that would make this cooling effect even stronger.
(iv) Would Ravi feel the same cooling effect if he used a few drops of a non-volatile oil instead of acetone? Give a reason.Frequently asked4 m
(i) The process is evaporation. As acetone evaporates from the skin, it absorbs latent heat from the palm, producing a cooling sensation.
(ii) Acetone is a volatile liquid, so at room temperature its particles gain enough kinetic energy to escape from the surface as vapour continuously; being volatile, it evaporates completely and quickly, leaving no residue.
(iii) The cooling effect would be stronger with (a) an increase in surface area (spreading the drops over a larger area of the palm) and (b) an increase in wind speed / lower humidity around the palm (e.g. blowing air on it), both of which increase the rate of evaporation.
(iv) No, Ravi would not feel the same cooling effect with a non-volatile oil. A non-volatile substance has particles with a much stronger force of attraction and does not evaporate readily at room temperature, so it will not produce the same cooling sensation.
Case-based Read the passage and answer the questions that follow: A science teacher heated a beaker of ice from −10 °C continuously and plotted a graph of temperature against time. The class observed that the graph rose steadily, then became flat for a while at 0 °C, rose steadily again, and became flat once more at 100 °C before rising further.
(i) What is this graph called, and what does each flat part represent physically?
(ii) Name the heat absorbed during the first flat part and the second flat part.
(iii) Why does the temperature not rise during the flat parts even though heating continues?
(iv) What would the graph look like beyond the second flat part, and why?Frequently asked4 m
(i) This is called the heating curve of water. Each flat part represents a change of state at constant temperature — the first flat part (at 0 °C) is ice melting into water, and the second flat part (at 100 °C) is water boiling into steam.
(ii) The first flat part absorbs the latent heat of fusion; the second flat part absorbs the latent heat of vaporisation.
(iii) During a change of state, the heat supplied is used entirely to overcome the force of attraction between the particles so that they can move into the next, less ordered state, rather than to increase their kinetic energy — so the temperature stays constant until the change of state is complete.
(iv) Beyond the second flat part, the graph rises steadily again, showing the temperature of the steam increasing above 100 °C as further heat now raises the kinetic energy of the gas particles, since there is no more change of state left to undergo.
Test yourselfPractice quiz
Marks lost every yearCommon mistakes
- Writing that evaporation only occurs at the boiling point — students confuse evaporation (surface phenomenon, any temperature below boiling point) with boiling (throughout the liquid, at boiling point only).
- Saying 'heat is lost' during melting/boiling instead of explaining that heat is absorbed as latent heat to overcome interparticle forces — examiners look for this exact mechanism, not just 'temperature stays constant'.
- Confusing sublimation with evaporation or melting — sublimation is specifically solid directly to gas (and back), no liquid stage.
- Forgetting to convert Celsius to Kelvin correctly — students often use K = °C − 273 instead of K = °C + 273.
- Stating diffusion is fastest in liquids — the correct order is gas > liquid > solid, based on particle speed and interparticle space.
- Explaining 'evaporation causes cooling' without saying which particles leave (the higher-energy ones) and why that lowers the temperature of what is left behind — a vague answer loses marking-scheme keywords.
- Mixing up latent heat of fusion and latent heat of vaporisation, or giving one value for both — they are different processes (solid↔liquid vs liquid↔gas) with different values.
- Thinking pressure alone (without lowering temperature) is enough to liquefy every gas — both increased pressure and decreased temperature usually work together.
How to prepareStrategy for this chapter
This chapter rewards precision, not memorisation of paragraphs. First lock the three characteristics of particles of matter (space between them, continuous motion, force of attraction) with the one activity that proves each — this single list answers a large share of VSA and SA questions if you can attach the correct activity to the correct characteristic.
Second, master the heating curve of water: be able to draw it blind, label both plateaus, and explain in your own words why temperature does not rise during melting or boiling (heat becomes latent heat, used to break interparticle forces, not to raise kinetic energy). This single diagram covers melting point, boiling point, latent heat of fusion and latent heat of vaporisation together, and is the most common 5-mark question from this chapter.
Third, evaporation deserves its own revision block: the four factors (surface area, temperature, humidity, wind speed) and the cooling mechanism (higher-energy particles leave, average energy of what remains falls) show up in almost every daily-life 'why' question — sweating, matka water, saucer tea, cotton clothes, desert coolers. Practise applying the same mechanism to a new, unfamiliar daily-life scenario, since case-based questions usually invent a fresh situation rather than reusing the NCERT examples.
Finally, keep a clean comparison table of solid/liquid/gas (shape, volume, compressibility, density, rigidity, interparticle space, force of attraction) memorised — it is the fastest way to answer MCQs and Assertion–Reason questions that describe a property and ask you to identify the state or the reason behind it.