18.4 Changes of state The state of matter of an object depends on the amount of heat energy it has.
Add Heat Melting
Freezing Remove Heat Solid Liquid
Add Heat Evaporating
Condensing Remove Heat Gas Fig. 12 Matter changes state depending on whether it is heated or cooled.
Therefore, matter will change its state depending on how much it is heated or cooled. A solid will change to a liquid (melting), or a liquid to a gas (evaporation) when heat energy is given to the molecules. A gas will change to a liquid (condensation) or a liquid to a solid (freezing) when heat energy is taken from the molecules, i.e. when the substance is cooled.
Heat energy added Gas (Water vapour) Solid (Ice)
Melting Freezing
Heat energy removed Fig 13 Changes in state of matter.
Changing state with pressure Fig. 14 shows a carbon dioxide (CO2
) fire extinguisher. Carbon
dioxide is normally a gas at room temperature. The fire extinguisher is filled by putting the carbon dioxide under high pressure. The pressure is so great that it causes the carbon dioxide to change state from a gas to a liquid. It remains as a liquid in the extinguisher until it is released from the pressurised cylinder. Carbon dioxide cannot remain a liquid under normal atmospheric pressure – for example in your classroom – so once the pressure is released it turns back into a gas and smothers the fire.
Fig. 14 A CO2 fire extinguisher. 203 Liquid (Water)
