In its most common usage, the term phase change indicates that a substance has changed among the three classical phases of matter: solid, liquid and gas. A change from solid to liquid is called melting, while the reverse is called freezing. A change from liquid to gas is called boiling, while the reverse is called condensation. And a change from solid to gas is called sublimation, while the reverse is called deposition. For pure compounds, phase changes occur at well defined temperature and pressure conditions, as shown in a phase diagram. For mixtures of materials, a phase change may occur over a range of conditions. As studied in the field of thermodynamics, melting, boiling and sublimation absorb heat, while freezing, condensation and deposition give off heat.

A phase changing material (from liquid to gas for example) is used in thermal energy (heat) transfer. This technology is used in refrigeration and air conditioning equipment. A heat pump uses an expansion valve and a gas compressor to change the refrigerant repeatidly from a liquid to a gas (absorbs the latent heat) and from a gas to a liquid (releases latent heat). The process is also used as a form of computer cooling where a freon compressor based cooler is used.

Phase change and optical disc technology

Phase change technology is also used to write to optical discs, such as CD-RW or DVD-RW discs. This is accomplished by including both a read laser and a more powerful write laser inside of the drive. The discs are made of a crystalline material that, when hit by a pulse of laser light from the write laser, changes to an amorphous state, thus changing its reflectivity. The read laser is not powerful enough to induce a phase change, but can be used by the drive to tell whether a bit is "on" or "off" based on an area of the disc's reflectivity.

History of phase change optical disc technology

• 1995: PD (Phasewriter Dual) by Panasonic, store 650 MB.
• 1996: CD-RW (Compact Disc ReWriteable) by Philips, Sony, Hewlett-Packard, Mitsubishi Chemical Corp. and Ricoh, store initially 650 MB and later 700 MB.
• 1998: DVD-RAM (DVD - Random Access Memory) by Panasonic, store initially 2,6 GB and later 9,4 GB.
• 199x: DVD±RW (DVD - ReWriteable) by supplier consortium, store 9,4 GB.
• 2004: PDD (Professional Disc for Data) by Sony, store 20,5 GB.
• 2004: UDO (Ultra Density Optical) by Plasmon, store 28 GB.