All substances are made of extremely tiny particles called atoms.. The closeness of these particles in a substance and their level of interaction determines the physical state of such a substance. Solids are known to constitute atoms tightly packed in a dense system. Liquid substances constitute atoms with more freedom while gaseous substances constitute.
An increase in temperature tends to cause excitement of atoms of a substance while a decrease in temperature leads to energy loss in atoms. This excitement of atoms will in turn cause the change in state of such a substance. For example, ice changes to liquid water when heat is applied to the mix. This is because an increase in temperature leads to an increase in the energy gain of the atoms of the ice. They break loose from one another and this leads to the ice, solid, becoming water in liquid form.