The states of matter, solid, liquid, and gases are interconvertible.
A liquid changes into solid state when the temperature of the liquid is lowered to its freezing point. This process is termed freezing.
It is to be noted that, the change of state is a physical change. No change in the composition of the substance takes place in the solidification process.
Also, the solidification process is an example of the exothermic process.
Some examples of the solidification process are the formation of ice from liquid, solidification of melted candle wax, etc.