Following the logic discussed on the temperature and Phase Change page, it is reasonable to expect that there is a temperature at which these transitions occur.
For example, we know that water has a freezing point of 0oC. The melting point of ice is also 0oC. This means that the temperature at which the attractions between water molecules are just barely able to lock the molecules in place is an infinitesimal amount colder than zero. Likewise if the temperature is any tiny amount above zero, the molecules are moving too fast to be locked in place as a solid, making it a liquid. The question then is, if water is at 0oC, will the water be melting or freezing? The answer depends on heat.
When particles pull apart (when a solid becomes a liquid or when a liquid becomes a gas) they end up with potential energy. In order to achieve this, energy must be added from somewhere.That's why you need to add energy to ice in order to melt it. That's also why ice makes you cold. As it melts, the ice takes heat from you, making you feel cold.
When particles come together (either when a gas becomes a liquid or when a liquid becomes a solid) they lose potential energy. When these phase changes occur, energy is given off. This is why a steam burn is so dangerous - not only is the water hot, but the process of becoming a liquid gives away a great deal of heat.
Thus, whether water freezes or melts at 0oC depends on whether heat is being added or taken away.
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