Matter and Its Composition
1. What Is Matter?
Anything that has MASS and OCCUPIES SPACE (volume) is called MATTER.
Examples: Air, water, stone, wood, food, metals. Non-examples: Light, sound, heat, thoughts (no mass, no volume).
2. Kinetic Theory of Matter
The KINETIC THEORY explains the behaviour of particles that make up matter.
Postulates of Kinetic Theory
- All matter is made up of TINY PARTICLES (atoms, molecules, or ions).
- Particles are in CONSTANT RANDOM MOTION.
- Particles have SPACES between them (intermolecular spaces).
- Particles ATTRACT each other (intermolecular forces).
- Particles COLLIDE with each other and the container walls.
Effect of Temperature
- INCREASING temperature → particles gain KINETIC ENERGY → move FASTER.
- DECREASING temperature → particles lose kinetic energy → move SLOWER.
3. Characteristics of Particles
| Property | Explanation |
|---|---|
| Tiny size | Particles are extremely small (microscopic) |
| Intermolecular space | Gaps exist between particles (largest in gases) |
| Intermolecular force | Force of attraction between particles (strongest in solids) |
| Continuous motion | Particles are always moving (fastest in gases) |
| Kinetic energy | Energy of motion (highest in gases) |
4. States of Matter
Solid
| Property | Characteristic |
|---|---|
| Shape | DEFINITE shape |
| Volume | DEFINITE volume |
| Compressibility | NEGLIGIBLE (almost no compression) |
| Intermolecular space | Very SMALL |
| Intermolecular force | Very STRONG |
| Particle motion | Vibrate about FIXED positions |
| Density | HIGH |
| Examples | Ice, wood, iron, table |
Liquid
| Property | Characteristic |
|---|---|
| Shape | NO definite shape (takes shape of container) |
| Volume | DEFINITE volume |
| Compressibility | Very LOW (almost incompressible) |
| Intermolecular space | Moderate (slightly more than solids) |
| Intermolecular force | Moderate (weaker than solids) |
| Particle motion | Can SLIDE past each other |
| Density | Moderate (lower than solids, higher than gases) |
| Examples | Water, milk, oil, mercury |
Gas
| Property | Characteristic |
|---|---|
| Shape | NO definite shape (fills entire container) |
| Volume | NO definite volume (expands to fill container) |
| Compressibility | HIGH (can be compressed easily) |
| Intermolecular space | Very LARGE |
| Intermolecular force | Very WEAK |
| Particle motion | RANDOM, fast, in all directions |
| Density | VERY LOW |
| Examples | Oxygen, hydrogen, air, carbon dioxide |
5. Interconversion of States
Change of State
| Process | From → To | Condition |
|---|---|---|
| Melting | Solid → Liquid | Heat added (temperature rises to melting point) |
| Freezing | Liquid → Solid | Heat removed (temperature drops to freezing point) |
| Vaporisation | Liquid → Gas | Heat added (boiling point) |
| Condensation | Gas → Liquid | Heat removed |
| Sublimation | Solid → Gas DIRECTLY | Heat added (skip liquid state) |
| Deposition | Gas → Solid DIRECTLY | Heat removed (skip liquid state) |
Sublimation Examples
| Substance | Sublimes? |
|---|---|
| Camphor | YES |
| Ammonium chloride | YES |
| Iodine | YES |
| Naphthalene (mothballs) | YES |
| Dry ice (solid CO₂) | YES |
| Ice | NO (melts first) |
Effect of Pressure
- INCREASING pressure DECREASES the melting point of ice.
- This is why ice skates glide on ice (pressure melts a thin layer of water).
6. Diffusion
The SPONTANEOUS INTERMIXING of particles of different substances.
- Fastest in GASES, slower in LIQUIDS, slowest in SOLIDS.
- Increases with TEMPERATURE.
Examples: Smell of perfume spreading, sugar dissolving in water.
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| States of matter and comparison | 3 marks | Very High |
| Kinetic theory postulates | 2 marks | High |
| Interconversion of states | 2-3 marks | Very High |
| Sublimation examples | 2 marks | High |
| Diffusion | 2 marks | Medium |
Common Mistakes
- Thinking gases have definite volume (they EXPAND to fill any container).
- Confusing condensation and evaporation (condensation = gas→liquid, evaporation = liquid→gas at surface).
- Saying 'molecules are at rest in solids' (they VIBRATE at fixed positions).
- Forgetting that sublimation skips the LIQUID state.
Self-Test (5 Questions)
Q1. Which state has highest intermolecular force of attraction? (1 mark)
- A) Solid
- B) Liquid
- C) Gas
- D) All are equal
Q2. Name the process: Gas → Solid directly. (1 mark)
Q3. 'Why does a gas fill its container completely?' (2 marks)
Q4. Give two examples of substances that sublime. (2 marks)
Q5. 'Why is the smell of burning incense felt even at a distance?' (2 marks)
Answers
A1. A) Solid. A2. Deposition (or solidification from vapour). A3. Gas particles have very LARGE intermolecular spaces and very WEAK forces of attraction. They move RAPIDLY in all directions and spread out to fill the entire container. A4. Camphor, ammonium chloride, iodine, dry ice, naphthalene (any two). A5. Due to DIFFUSION — the gas particles from incense move rapidly and spread throughout the room, reaching our nose.
