What do we know?
Every phase change involves the transfer of energy; in other words, a phase change will either absorb or release energy. This occurs because of the forces of attraction or intermolecular forces that exist between molecules.
First, consider only two water molecules. As discussed previously, water is a polar substance and water molecules are strongly attracted to one another, resulting in hydrogen bonds. To separate two water molecules, work must be performed on the particles to overcome the attractive force between them. Because work is performed on the particles, the particles gain energy; their internal energy increases. In this case, the particles gain potential energy, because the particles now have the capacity to do work by virtue of their positions apart from one another. However, if two water molecules that are far from each other are brought close together, the particles lose potential energy and energy is released. Therefore, in general, whenever an intermolecular force is strengthened, energy is released; whenever an intermolecular force is weakened, energy is absorbed.
Now, consider what happens when ice melts. Solid ice is composed of water molecules which are tightly held in a highly ordered arrangement by hydrogen bonds. During melting, the hydrogen bonds between the water molecules are weakened so that the molecules can slide past one another, resulting in liquid water. As we just learned, when an intermolecular force is overcome or weakened, energy is absorbed. Because many intermolecular forces are overcome during melting, a substance will absorb energy when it melts. In chemistry, we say that melting is an endothermic process.
On the other hand, when liquid water freezes, the opposite process occurs. In order to freeze water, we cool it down, effectively removing energy from the liquid. Therefore, during freezing, the water molecules lose energy; the hydrogen bonds between the molecules are strengthened, causing energy to be released. Since energy is released as heat, freezing is an exothermic process.
