The ice point is a temperature at normal atmospheric pressure at which it is possible for solid and liquid water to coexist in a state of thermal equilibrium. Many temperature scales have historically utilized the ice point of water for calibration. In Fahrenheit, this temperature is 32 degrees, while the Celsius scale places it at zero degrees. The point of icing is not the same thing as the freezing point, an important distinction, especially among quibbling and precise scientists.
When discussing the freezing point of water, people often refer to the temperatures of the ice point, because water does in fact freeze at this point. However, several factors can influence the freezing point of water, including salinity. Saltwater can actually become supercooled, meaning that it passes below the point at which it would normally freeze before it freezes solid. Many substances possess the capacity to become supercooled under certain conditions.
A closely related concept is the triple point, the temperature at which solid, liquid, and gaseous water can all exist together. This measurement is only a fraction of a degree higher than the ice point, but it is the preferred method of calibration for precise temperature scales and scientific instruments in most regions of the world. Triple points can also be found for many other substances, but since water is an abundant and well-known chemical compound, it is the preferred standard to use.
This temperature also reflects a phase change barrier, at which water passes from a primarily liquid state to a mostly solid state. Phase changes also occur as temperatures rise and water begins to turn into steam because it cannot maintain a liquid state. The phase change barriers for many substances can be profoundly influenced by the amount of atmospheric pressure, which is why things like water behave differently at high altitude than they do at low altitude.
Knowing the ice point is critical from a number of perspectives. It's an important benchmark in the determination of temperatures, and it can also play a role in scientific research and experiments. Meteorologists, navigators, and public safety officials also have an interest in the ice point, as it can influence environmental conditions in cold regions of the world. People may also use this temperature at home in the calibration of thermometers, ensuring that the instruments measure the temperature correctly by chilling water to the ice point, at which point a known temperature will be reached.