The state of a substance can be changed either by heating or by cooling it.
The Types of State Changes:
State Changes on Heating:
When a solid substance is heated, the molecules start to vibrate more and more strongly. Eventually, the molecules vibrate more violently and inter molecular forces become weak. As a result material becomes a liquid. If the process of heating is continued further, then molecules have sufficient energy to overcome all of the attractive forces as a result the substance becomes a gas.
State Changes on Cooling:
When a gas is cooled, the molecules loose energy and move more slowly and forces of attraction between molecules increase. If we keep cooling the gas, eventually all of the molecules come closer together and form a liquid. Further cooling will cause all the molecules to loose more energy and the forces to become stronger still and the liquid turns to solid.
