As we now know, heat is a form of energy. Also , all substances are made up of atoms, which are in state of rapid motion. As the temperature of a substance increases , the atoms move more rapidly. The temperature drops, they slow down. Also, there are three states of matter, solids, liquids and gas. Water , for example , exists in solid state ad ice block, in liquid state as water, and in gaseous state as water vapour. Most substances change their physical state with the addition or removal of heat. Water, in the form of ice, is a solid (under atmospheric pressure and a temperature below 0°C). When heat is applied to ice, it melts and becomes water (a liquid). In this state, the water molecules move faster than in solid state. Further application of heat will cause the water to turn into vapour (steam). At this stage, the vapour molecules move much faster than in the liquid state. The reverse process of heat removal, however , leads to what we shall now see.
By removing heat from the hot water vapour or steam, I.e. cooling, the steam will change to liquid state, I.e. hot water. This is called condensation.
Further removal of heat will lead yo a change of stage from liquid to solid (I.e. water ice at temperature of 0°C). This is the basic principle of heat removal by a refrigerant in any refrigeration system.
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