melting point
Hold an ice cube in your warm hand and watch where it turns to water — that exact temperature where solid gives way to liquid is the melting point. It is the same temperature, run backwards, at which a liquid freezes into a solid; melting point and freezing point are two names for the same crossover.
At this temperature, the orderly grip of the crystal lattice finally loses to the jostling of heat. Below it, the molecules are locked into a rigid pattern; above it, they have enough energy to slide past one another and flow. Right at the melting point, solid and liquid coexist in balance, and any heat you add goes into breaking the lattice apart rather than warming things up — that energy is the latent heat of fusion.
Melting point is one of the most useful fingerprints a chemist has: a pure substance melts at a sharp, reproducible temperature, while impurities smear and lower it, so a clean, narrow melting range is a classic sign of purity. Melting points barely budge with pressure (unlike boiling points), because solids and liquids take up nearly the same volume — squeezing them changes little.
Pure aspirin melts crisply at about 135 °C; a contaminated batch starts softening several degrees lower and over a wider range — a quick bench test for purity.
A sharp melting point signals purity; a smeared one signals impurity.
For water, higher pressure actually lowers the melting point slightly, because ice is less dense than liquid water — an exception to the usual rule, and the reason this melting line tilts the 'wrong' way on water's phase diagram.