States of Matter

Explore the three states of matter, the particle model, and the phase changes between solid, liquid, and gas.

The Three States

State	Particle Arrangement	Shape	Volume
Solid	Close, ordered, vibrate in place	Fixed	Fixed
Liquid	Close, random, can slide past each other	Not fixed (takes container shape)	Fixed
Gas	Far apart, move freely and fast	Not fixed	Not fixed

Particle (Kinetic) Model

All matter is made of tiny particles in constant motion. Higher temperature → faster particles → more kinetic energy.

Note In a gas, particles are about 10 times farther apart than in a liquid or solid, which is why gases are much less dense.

Phase Changes

Heating or cooling material can change its state:

Solid → Liquid  :  melting
Liquid → Gas    :  evaporation / boiling
Gas → Liquid    :  condensation
Liquid → Solid  :  freezing/solidification
Solid → Gas     :  sublimation (e.g. dry ice)
Gas → Solid     :  deposition

During a phase change the temperature stays constant even though energy is being added or removed. The energy goes into breaking (or forming) bonds between particles.

Latent Heat

Q = m \cdot L

Q = heat transferred (J), m = mass (kg), L = specific latent heat (J/kg).

For water: $L_{fusion} = 334\,000$ J/kg (melting/freezing); $L_{vaporisation} \approx 2\,260\,000$ J/kg (boiling/condensing).

Boiling vs. Evaporation

Evaporation happens at the surface at any temperature.
Boiling happens throughout the liquid at a specific temperature (100 °C for water at sea level).

1
Identify the phase change
Is energy absorbed (melting, boiling) or released (freezing, condensing)?
2
Use Q = mL
Or Q = mcΔT for heating/cooling without a phase change.
3
Remember: temperature is constant during the change
Do not include ΔT in the latent heat calculation.

How much energy is needed to melt 0.5 kg of ice at 0 °C? (L = 334 000 J/kg)

$Q = 0.5 \times 334\,000 = 167\,000 \text{ J} = 167 \text{ kJ}$

Real world: ice in a drink

Ice cubes in a drink absorb latent heat of fusion from the liquid: $Q = m \times 334\,000\text{ J/kg}$ per kilogram of ice melted. During this phase change the temperature stays at 0°C — the ice keeps the drink cold while it melts, not before or after. This is why a drink with ice stays cold much longer than a pre-chilled drink without ice.

Real world: steam iron

A steam iron converts liquid water to steam (gas) by supplying latent heat of vaporisation. The steam condenses on fabric, releasing $Q = m \times 2\,260\,000\text{ J/kg}$ of energy that relaxes textile fibres. The large value of latent heat of vaporisation for water means the steam carries a lot of energy per kilogram — far more than hot water at the same temperature.

Real world: weather formation

When moist warm air rises, it cools and the water vapour condenses into tiny droplets (gas → liquid), forming clouds. The latent heat released during condensation warms the surrounding air, causing it to rise further and potentially triggering thunderstorms. Phase changes drive global weather patterns.

Tip During any phase change, temperature stays constant even as heat is added or removed — all the energy goes into changing the state, not warming the substance. This flat section on a heating curve is called a "plateau".

Practice Problems

What happens to the temperature of water while it is boiling?

Show answer

It stays constant at 100 °C until all the water has turned to steam.

List the three states of matter in order of increasing particle kinetic energy at the same temperature.

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Solid → liquid → gas.

How much energy is released when 2 kg of steam condenses to water? (L = 2 260 000 J/kg)

Show answer

Q = 2 × 2 260 000 = 4 520 000 J = 4 520 kJ.