Matter has three phases – solid, liquid, and gas, each with a unique arrangement of molecules which is responsible for their state. Molecules in solids are more compact and close to each other, in liquids, molecules are farther apart from each other while molecules in gases are so much apart from each other and move freely. In terms of energy, gas has the greatest energy and solid has the least amount of energy. If matter absorb or release energy as heat, the molecules may rearrange and behave differently and cause change in the amount of energy – either increase or decrease. In this manner, matter may change from one phase to another – this process is called phase change. Some examples of phase changes are discussed below:
When a solid is heated, it gains energy making its molecules move faster and farther away from each other. This process turns a solid into a liquid. This phase change is called as melting. Some substances, like the pure substances, have to reach a specific temperature in order for it to melt, this temperature is called as the melting point. For example, when ice is heated, the energy absorbed by the ice will break the intermolecular bonds between the ice molecules. After the bonds are broken, the molecules then move farther away from each other.
During the melting process, the temperature remains constant until all the solids change to liquid. The heat absorbed by solids in order to undergo melting, without change in temperature is called as Latent heat of fusion of a substance.
Freezing is the phase change in which liquid turns into solid. Some substances freeze at a fixed temperature to which is known as the freezing point. During freezing, the liquid substance releases heat, losing the energy of molecules then at some point or temperature making the molecules behave like that of a solid.
When gas turns into liquid, without change in temperature, it undergoes condensation. During this process, the gas molecules form intermolecular bonds, thus making the molecules closer to each other forming into a liquid state. When a gaseous undergo condensation, heat is constantly removed from the substance until it liquifies.
Boiling happens when a liquid which is heated continuously turns to gas. Boiling point is the temperature at which certain substances have to reach in order to boil.