In talking about the heats involved in changing temperatures, one thing we’ve neglected is what happens when a material changes phase. We talked about the three phases – gas, liquid, and water. When a material changes from one phase to another, we call this a phase transition or change of phase. Changes from the solid to the liquid or from the liquid to the gaseous state and vice versa are called phase changes.
They always involve a transfer of heat, even though the temperature of the substance undergoing the phase change stays constant. The heat flow from one object to another can change either the average kinetic energy of the random motion of the molecules, which changes the temperature of the object or can change the average potential energy of the molecules, which causes the phase of the object to change. Consider what happens when, for instance, a pot of water is heated on a stove.
At first, the temperature rises. Upon reaching 100°C (212°F) the temperature stops increasing, even though the flame keeps supplying heat at the same rate as before. We know that the supplied goes into breaking the bonds between the molecules, while the kinetic energy of the molecules remains unchanged. Gradually, more and more molecules gain sufficient energy to overcome the intermolecular forces binding the molecules one to the other. A similar phenomenon occurs when the ice melts.
We call the heat required to produce a phase change the latent heat (L’Heat). We can define it as:
Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process – usually a first-order phase transition.
Two examples of latent heat are:
The heat of freezing is the amount of thermal energy given off as a liquid freezes, and the heat of vaporization is the amount of thermal energy that must be added to change a liquid to a gas.
Heat added or subtracted for a phase change = Latent heat × Mass
|Q = L heat M|
For example, the latent heat of vaporization of water is 540 cal/g and the latent heat of freezing of water is 80 cal/g. Therefore, changing a given quantity of water to steam requires 5.4 times as much heat as warming it from 0°C (+32°F) to 100°C (212°F), and melting ice requires as much heat as warming water from 20°C (68°F) to 100°C.