Change of state

Substances can change state, usually when they are heated or cooled. For example, liquid water turns into steam when it is heated enough, and it turns into ice when it is cooled enough.

The closeness, arrangement and motion of the particles in a substance change when it changes state. Simple diagrams of particles in a solid, liquid and a gas are shown like this:

Table showing diagrams of solid, liquid and gas, their particle arrangements, movement and closeness.

Gaining energy

The table summarises what happens to the particles in a substance when it gains energy, and it melts or boils (ie changes state):

MeltingEvaporating or boiling
DescriptionSolid to liquidLiquid to gas
Closeness of particlesStay close togetherBecome much further apart
Arrangement of particlesRegular to randomStay random
Motion of particlesStart to move around each otherBecome able to move quickly in all directions

Evaporation happens below the boiling point of a liquid. When the liquid reaches its boiling point, evaporation happens very quickly and the liquid boils.

How substances change state

Losing energy

The table summarises what happens to the particles in a substance when it loses energy, and it freezes or condenses (ie changes state):

DescriptionGas to liquidLiquid to solid
Closeness of particlesBecome much closer togetherStay close together
Arrangement of particlesStay randomRandom to regular
Motion of particlesStop moving quickly in all directions, and can only move around each otherStop moving around each other, and only vibrate on the spot

Conservation of mass

The particles in a substance stay the same when it changes state - only their closeness, arrangement or motion change. This means that the mass of the substance stays the same. For example, 10 g of water boils to form 10 g of steam, or freezes to form 10 g of ice. This is called conservation of mass.