The process of changing a solid substance into liquid by heating is called melting. Melting or fusion is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increases the temperature of the substance to the melting point. Example: The conversion of ice (solid) into water (liquid) is a melting process.

The process of changing a liquid into solid by cooling is called freezing. Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. Example: when water is cooled, it freezes to form ice.

The process of changing the state of a substance directly from solid to gas on heating is called sublimation. Sublimation involves the direct vaporisation of a solid without the formation of an intermediate liquid phase. The substances such as iodine, camphor, naphthalene, etc., which undergo a sublimation process are called sublime compounds.