What is the particle model?
A model is a simple picture that helps us explain things. The particle model says:
- All matter is made of very small particles (atoms, molecules or ions). They are far too small to see.
- The particles are always moving.
- There are forces of attraction (a pull) between particles. The pull is strong when they are close.
- Between gas particles there is only empty space, not air.
We draw particles as small balls. Real particles are not coloured balls, but the picture explains a lot. Diffusion (a smell spreading across a room, or ink spreading in water) is good evidence that particles move on their own.
Solids, liquids and gases
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Close, regular rows | Close, no pattern | Far apart, random |
| Movement | Vibrate in place | Slide past each other | Fast, in straight lines, all directions |
| Pull between them | Strong | Medium | Very weak |
| Shape / volume | Fixed shape, fixed volume | Takes container shape, fixed volume | Fills container, no fixed volume |
| Can it be squashed? | No | Almost no | Yes, easily |
Density
Density tells how much mass is packed into each cubic metre: ρ = m ÷ V (kg/m³). Solids and liquids are dense because their particles are close. Gases have low density because their particles are far apart. Water is about 1000 kg/m³; air is about 1.2 kg/m³.
Heating: temperature, internal energy and changes of state
Internal energy is the total energy stored by all the particles: their movement energy (kinetic) plus the energy stored in the pull between them (potential). Heating a substance raises its internal energy. Two things can happen:
- Temperature rises: particles move faster. Energy needed: E = m × c × ΔT, where c is the specific heat capacity (energy to warm 1 kg by 1 °C).
- State changes (melting, boiling): the temperature stays the same. Energy breaks the pull between particles. Energy needed: E = m × L, where L is the specific latent heat (energy to change the state of 1 kg with no temperature change).
Names of changes: melting (solid → liquid), freezing (liquid → solid), boiling/evaporating (liquid → gas), condensing (gas → liquid), sublimation (solid → gas directly, like dry ice). A change of state is a physical change: mass is conserved and it can be reversed.
On a heating graph the flat parts are the changes of state. Evaporation happens at any temperature from the surface, when the fastest particles escape; it cools the liquid left behind (sweat, a clay matka).
Gas pressure and temperature
Gas particles keep hitting the walls of their container. Each hit is a tiny push. Millions of pushes on each square metre make pressure.
- Hotter gas, same volume: particles move faster, hit the walls more often and harder, so pressure goes up.
- Smaller volume, same temperature: particles hit the walls more often, so pressure goes up. For a fixed mass of gas at constant temperature, p × V stays constant.
Absolute zero (−273 °C, or 0 K) is the temperature where particles would have the least possible movement. Kelvin temperature = °C + 273.
Try it at home
Put a few drops of food colour into a glass of cold water and another into hot water. Do not stir. Predict which spreads faster, then watch. (Hot water: particles move faster.)
Key formulas and definitions
- Density: ρ = m / V (kg/m³)
- Heating without change of state: E = m c ΔT
- Change of state: E = m L (specific latent heat)
- Gas at constant temperature: p₁V₁ = p₂V₂
- Kelvin = °C + 273
Worked examples
1. A block has mass 2.7 kg and volume 0.001 m³. Find its density.
ρ = m / V = 2.7 / 0.001 = 2700 kg/m³ (this is aluminium).
2. Why can a gas be squashed but a solid cannot?
Gas particles have large empty spaces between them, so pushing moves them closer. Solid particles already touch, so there is no space to remove.
3. How much energy heats 2 kg of water from 20 °C to 70 °C? (c = 4200 J/kg °C)
E = m c ΔT = 2 × 4200 × 50 = 420 000 J = 420 kJ.
4. How much energy melts 0.5 kg of ice at 0 °C? (L = 334 000 J/kg)
E = m L = 0.5 × 334 000 = 167 000 J = 167 kJ. The temperature stays at 0 °C the whole time.
5. A syringe holds 60 cm³ of air at 100 kPa. It is pushed in to 20 cm³ at the same temperature. Find the new pressure.
p₂ = p₁V₁ / V₂ = 100 × 60 / 20 = 300 kPa. One third of the volume gives three times the pressure.
6. A kettle heater gives 2000 J each second. How long does it take to boil away 0.2 kg of water already at 100 °C? (L = 2 260 000 J/kg)
E = m L = 0.2 × 2 260 000 = 452 000 J. Time = 452 000 / 2000 = 226 s (about 3.8 minutes).
Common mistakes
- Saying particles in a solid do not move. They vibrate all the time.
- Thinking the temperature keeps rising while ice melts. It stays at 0 °C until all the ice has melted.
- Saying there is air between gas particles. There is only empty space.
- Thinking particles themselves expand when heated. The particles stay the same size; the spaces between them grow.