Liquefaction, in the context of the thermal properties of matter, refers to the phase transition or change of state from a gaseous state to a liquid state. This process occurs when a substance is cooled or subjected to a decrease in temperature while maintaining constant pressure. Liquefaction is one of the common phase transitions that matter can undergo, along with processes like evaporation, freezing, and sublimation.

When a substance is in a gaseous state, its particles have enough kinetic energy to move freely and are relatively far apart. During liquefaction, as the substance is cooled, its particles lose kinetic energy and start to move more slowly. This reduction in kinetic energy causes the particles to come closer together, leading to the formation of liquid droplets or a liquid phase. In the liquid state, the particles are more densely packed and have more ordered and cohesive arrangements compared to the more disordered and spaced-out configuration in the gas phase.

An everyday example of liquefaction is the condensation of water vapor in the air to form liquid water when you breathe on a cold surface or when moist air cools down and forms dew. In industrial processes, liquefaction can also refer to the conversion of gases into a liquid state, which is important in applications such as liquefied natural gas (LNG) production.

Liquefaction is governed by the principles of thermodynamics, and it is an important concept in the study of the thermal properties of matter and phase transitions. The conditions at which liquefaction occurs for a particular substance depend on its vapor pressure, temperature, and pressure, which are often represented on phase diagrams specific to that substance.