Pour Point

The freezing point marks the temperature T_G at which a substance turns from liquid to solid state at a certain pressure. This point is a constant for a pure substance. By impurities the freezing point is always moved to lower temperatures (freezing point depression, cryoscopic constant). The depression depends only on the amount, not the nature of soluted substance (colligative property). The pressure dependence is small compared to the pressure dependence of the boiling point but it can be well observed at high pressures (phase diagrams of substances without anomaly).

Freezing is a form of crystallisation. The molecules change from a unsorted, chaotic liquid phase to a more or less sorted crystal structure. This phase transformation is bounded with releasing of energy, the crystallisation enthalphy which is emitted as heat. This heat energy is contrary to the external cooling. In an ideal process this leads to a constant temperature during the whole crystallisation. The temperature of the substance decreases not until this process is finished.

In multi phasic alloys only in rare cases (eutectic mixtures) all components crystallise at the same temperature. Normally only one component crystallises, the formed crystals exist as colloids in a solution. Due to the Tyndall effect (light scattering at the phase interface of colloids) the solution becomes cloudy (cloud point). When growing the crystals bond to a three-dimensional structure and due to this the flowability of the solution decreases, the viscosity increases.

In solutions of complex composition like crude oil or petroleum products often dozens of substances are combined. This gives multiple interfering solution and crystallisation equillibria. These alloys normally produce mixed crystal systems.

By forming three-dimensional structures the viscosity of the compound increases more than only by simple cooling. The flowability is increasingly reduced until it is finally evanescent, the fluid freezes.