Fryspunkter

Fryspunkter, or freezing points, are the temperatures at which a liquid begins to solidify into a solid phase. For a pure substance under a fixed external pressure, the freezing point equals its melting point, so the substance changes between liquid and solid at a characteristic temperature. The most familiar example is water, which freezes at 0°C at standard atmospheric pressure (1 atm).

In mixtures, solutions or alloys, the freezing point is typically depressed: adding solutes lowers the temperature at which the liquid freezes. This freezing-point depression is a colligative property and can be approximated by the relation ΔT_f = i K_f m, where i is the van’t Hoff factor, m is molality, and K_f is the cryoscopic constant. The effect becomes more pronounced with higher solute concentration.

Pressure can also influence fryspunkter, though for many substances the change near normal pressures is small. Some materials exhibit different solid phases, so multiple freezing transitions may occur with changes in conditions.

Practical considerations include the phenomena of supercooling, where a liquid is cooled below its freezing point without solidification if nuclei for crystallization are absent. Impurities, surfaces, and agitation often trigger freezing.

Applications span food preservation, cryopreservation, metallurgy, and chemistry. Common examples include water (0°C at 1 atm), ethanol (-114°C), and saline solutions with freezing points well below 0°C, depending on salt concentration. Measuring freezing points is done via differential scanning calorimetry, cryoscopy, or cooling curves, providing insight into purity, composition, and phase behavior.