ice (chemistry)

Solid formed by water when it freezes. It is colourless and its crystals are hexagonal. The water molecules are held together by hydrogen bonds.

The freezing point of water at 1.013 bar, used as a standard for measuring temperature, is 0° for the Celsius and Réaumur scales and 32° for the Fahrenheit scale. Ice expands in the act of freezing (hence burst pipes), becoming less dense than water (0.9175 at 5°C/41°F). There are at least six different modifications of ice, some of which can only exist at very high pressures. These forms are identified by Roman numerals, thus the ‘ordinary’ ice formed at atmospheric pressure is technically known as ice I.