Matter — Kinetic Theory
1. States of Matter
Matter exists in THREE main states: SOLID, LIQUID, and GAS.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | DEFINITE | Takes the SHAPE of container | Fills the container |
| Volume | DEFINITE | DEFINITE | NO definite volume |
| Compressibility | Negligible | Very slight | HIGHLY compressible |
| Density | HIGH | Medium | VERY LOW |
| Arrangement of particles | Tightly packed, REGULAR pattern | Loosely packed, IRREGULAR | Far apart, RANDOM motion |
| Energy | LOWEST | Medium | HIGHEST |
'Plasma is the FOURTH state of matter — found in stars and fluorescent tubes. It consists of IONISED particles.'
2. Kinetic Theory of Matter
The Kinetic Theory explains the behaviour of matter based on the MOTION of particles.
Postulates:
- All matter is made up of TINY particles (atoms or molecules).
- Particles are in CONTINUOUS random motion.
- Particles experience INTERMOLECULAR forces (attraction and repulsion).
- There are EMPTY SPACES between particles.
- Temperature INCREASES the kinetic energy of particles.
Arrangement and movement:
- Solids: Particles VIBRATE about fixed positions (LATTICE structure)
- Liquids: Particles SLIDE past each other
- Gases: Particles move FREELY at HIGH speeds
3. Interconversion of States
Melting: Solid → Liquid (energy absorbed at melting point) Freezing: Liquid → Solid (energy released) Boiling (Vaporisation): Liquid → Gas (at boiling point) Condensation: Gas → Liquid (energy released) Sublimation: Solid → Gas directly (e.g., dry ice, camphor, iodine) Deposition: Gas → Solid directly (e.g., frost formation)
4. Latent Heat
Latent heat of fusion: The heat energy ABSORBED when 1 kg of a solid melts into liquid WITHOUT a change in temperature.
- For ice: 336 J/g or 336,000 J/kg
Latent heat of vaporisation: The heat energy ABSORBED when 1 kg of liquid changes to vapour WITHOUT a change in temperature.
- For water: 2260 J/g or 2,260,000 J/kg
'Latent heat is HIDDEN heat — it is absorbed but does not raise the temperature. It is used to OVERCOME the intermolecular forces.'
5. Evaporation
Evaporation is the change of a LIQUID into vapour at a temperature BELOW its boiling point.
Factors affecting evaporation:
| Factor | Effect |
|---|---|
| Temperature | Higher temperature → FASTER evaporation |
| Surface area | Larger surface area → FASTER evaporation |
| Wind speed | Higher wind speed → FASTER evaporation |
| Humidity | Higher humidity → SLOWER evaporation |
| Nature of liquid | Volatile liquids (petrol, alcohol) evaporate FASTER |
Cooling effect of evaporation: Particles with HIGHER kinetic energy escape first. The remaining particles have LOWER average energy → TEMPERATURE decreases.
6. Distinction: Evaporation vs Boiling
| Aspect | Evaporation | Boiling |
|---|---|---|
| Temperature | Occurs at ANY temperature | Occurs at FIXED boiling point |
| Location | At the SURFACE only | Throughout the LIQUID |
| Rate | SLOW | RAPID |
| Bubbles | NO bubbles | Bubbles formed |
| Energy change | COOLING effect | STEADY temperature (latent heat) |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Gases have definite volume' | Gases EXPAND to fill their container — NO definite volume |
| 'Melting and boiling happen at any temperature' | Melting and boiling occur at FIXED temperatures (melting point, boiling point) |
| 'Evaporation and boiling are the same' | Evaporation occurs at the SURFACE at ANY temperature. Boiling occurs THROUGHOUT the liquid at the BOILING POINT |
| 'Latent heat raises temperature' | Latent heat changes STATE, not temperature. The temperature stays CONSTANT during phase change |
ICSE Exam Focus (5–6 marks)
- 2-mark questions: States of matter — properties and examples
- 3-mark questions: Differences between evaporation and boiling
- 4-mark questions: Factors affecting evaporation with reasons
- 5-mark questions: Latent heat numericals or kinetic theory applications
Self-Test
Q1. Why does a solid have a definite shape? A1. In solids, particles are TIGHTLY packed in a REGULAR pattern and can only VIBRATE about fixed positions. The strong intermolecular forces keep them in place.
Q2. Why does evaporation cause cooling? A2. During evaporation, particles with HIGHER kinetic energy escape from the surface. The remaining particles have LOWER average kinetic energy, so the temperature DECREASES.
Q3. What is the latent heat of fusion of ice? A3. 336 J/g (or 336,000 J/kg). This is the energy required to melt 1 kg of ice at 0°C into water at 0°C.
Q4. Why does a desert cooler cool better on a hot dry day? A4. On a hot dry day, the temperature is HIGH and humidity is LOW. Both factors INCREASE the rate of evaporation, causing more cooling.
Q5. Why is it easier to compress a gas than a solid? A5. In gases, particles are FAR apart with LARGE empty spaces between them. Compressing pushes particles closer. In solids, particles are already tightly packed.
Q6. Differentiate between evaporation and boiling. A6. Evaporation occurs at the SURFACE at ANY temperature, is SLOW, and causes cooling. Boiling occurs THROUGHOUT the liquid at the BOILING POINT, is RAPID, and absorbs latent heat.
