Every physical matter has three distinct states—solid, liquid and vapor. At a particular temperature and pressure, a specific matter may exist in only one state. Change of state is associated with the change in heat content (either the matter itself or the surroundings). In general, to convert a solid matter to liquid or a liquid matter to vapor, heat must be supplied to the matter from external source. However, for the reverse case, heat should be extracted from the matter.

Change of state from solid to liquid due to external heating is called melting; while change of state from liquid to solid due to external cooling is called freezing. Thus for melting, heat must be supplied from an external source and for freezing heat should be extracted from the matter. Important similarities and difference between melting and freezing are elaborated in the following sections. Note that, solidification is a generalized term of converting liquid to solid, which does not necessarily require extraction of heat from a liquid for converting it to solid.

Similarities between melting and freezing

• Both melting and freezing are phase transition processes and cause change in state or phase of the substance.
• Both are physical change, so no new substance formation occurs.
• Both changes are reversible.
• Both greatly depend on external pressure on the substance.

Differences between melting and freezing

Melting	Freezing
Gradual change of state from solid to liquid by absorbing heat from surroundings is called melting.	Gradual change of state from liquid to solid by leaving heat to surroundings is termed as freezing.
The temperature at which solid transforms to liquid is called Melting Point of the concerned substance.	The temperature at which a liquid transforms to solid by leaving heat is termed as Freezing Point of the concerned substance.
Heat is required to supply from external source for such change of state.	Heat is required to take away from the substance for this change of state.
Volume of the substance usually increases due to melting.	Volume of the substance usually decreases due to freezing.
Internal energy of the substance also increases for melting.	In freezing, internal energy of the substance decreases.
Example of melting— Ice to water Solid wax to liquid wax Melting of iron ore in furnace	Examples of freezing— Water to ice Liquid wax to solid wax, etc.