Different substances freeze at different temperatures because the molecules that make them up are different. Some kinds of molecules have stronger forces holding molecules to each other than other kinds of molecules. In water, for example, the positively charged hydrogen end of the molecule electrostatically attracts the negatively charged part of neighboring water molecules to form "hydrogen bonds". These are responsible for the cohesion of water molecules in the liquid and the arrangement of water molecules in ice.
Other kinds of substances have different freezing points. For example, nitrogen molecules have only very weak attractive forces between each other (although the bond between the two nitrogen atoms in an N
2 molecule is very very strong). Consequently, nitrogen freezes at a much lower temperature than water. It even liquifies at a much lower temperature than water: -195.8 °Celsius. It freezes solid at -209.86 °C. Water in comparison, freezes at zero degrees Celsius. We use liquified nitrogen here at the Physics van for lots of great demonstrations because it is so very cold.
Mercury is a poisonous liquid at room temperature and freezes at -38.87 °C. Benzene, a flammable, poisonous, carcinogenic hydrocarbon (C
6H
6), freezes at 5.5 °C. Butane liquifies at -0.5 °C and freezes at -138 °C (it liquifies at higher temperatures when it's under pressure). Most substances have solid, liquid, and gas phases under different temperature and pressure conditions. Some, like oil and glass, don't have clear demarcations between solid and liquid phases. They get hard when they get cold, but there's no real temperature you could call the "freezing point".
Water with some salt in it will have a lower freezing point than pure water. Check out some of the other answers on water and salt to find out how this works.
Tom
(published on 10/22/2007)
