Ice

Pure liquid water is transformed to its solid state, ice, at a temperature of 32°F (0°C) when the pressure is at one atmosphere. Interestingly, the density of liquid water at the freezing point is 62.418 lb/ft³ (0.99984 g/cm³) but decreases to 57.23 lb/ft³ (0.9168 g/cm³) when that water organizes itself into crystalline ice at 32°F (0°C). This density difference is due the large open spaces within the crystal lattice of ice. The increased volume of the solid lattice causes pure water to expand by approximately 9% upon freezing, resulting in ruptured pipes or damaged engines when the process occurs in a closed vessel. Ice is one of a very few solid substances that is lower in density than the corresponding liquid state. Surface ice floating on a lake or pond helps to insulate the water below, reduces mixing, and can prevent the water body from freezing solid. This fact has often been cited as an important factor in the development and evolution of life in freshwater.

The freezing point of water containing dissolved solids is proportionately reduced below 32°F (0°C) depending on the quantity of solutes. As the salinity of the water increases, the freezing temperature is lowered. This is the principle behind the practice of road salting. The salt causes the freezing point of the water to be lowered, hopefully below the ambient temperature, and the ice or snow is forced to melt.

When pressure is exerted on ice crystals at temperatures near the melting point, the edges of those crystals may melt. When that pressure is released, the water refreezes. This process, called regelation, may be familiar to those that have formed snowballs. The loose snowflakes are partially melted by the pressure of the hands. When the pressure is released, the refreezing water hardens and causes the cohesion of the flakes into a ball. On very cold days, however, the pressure that can be exerted by the hands is insufficient to cause melting, and the snowball is more difficult to form.