There are four states of matter in the universe: plasma, gas, liquid and solid. But, matter on Earth exists mostly in three distinct phases: gas, liquid and solid. A phase is a distinctive form of a substance, and matter can change among the phases. It may take extreme temperature, pressure or energy, but all matter can be changed.
There are six distinct changes of phase which happens to different substances at different temperatures. The six changes are:
Freezing: the substance changes from a liquid to a solid.
Melting: the substance changes back from the solid to the liquid.
Condensation: the substance changes from a gas to a liquid.
Vaporization: the substance changes from a liquid to a gas.
Sublimation: the substance changes directly from a solid to a gas without going through the liquid phase.
Deposition: the substance changes directly from a gas to a solid without going through the liquid phase.
For most substances, the melting and freezing points are the same temperature; however, certain substances possess different solid-liquid transition temperatures.
Most liquids freeze by crystallization, the formation of a crystalline solid from the uniform liquid.
Freezing is almost always an exothermic process, meaning that as liquid changes into solid, heat is released.
The energy released upon freezing, known as the enthalpy of fusion, is a latent heat, and is exactly the same as the energy required to melt the same amount of the solid.
Terms
NucleationIn the context of freezing, nucleation is the localized budding of a crystalline solid structure.
FreezingFreezing or solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.
Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered to or below its freezing point. All known liquids, except helium, freeze when the temperature is low enough. (Liquid helium remains a liquid at atmospheric pressure even at absolute zero, and can be solidified only under higher pressure.)
For most substances, the melting and freezing points are the same temperature; however, certain substances possess different solid-liquid transition temperatures.
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