Changes of Phase (or State)

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The term 'change of phase' means the same thing as the term 'change of state'.

There are four states, or phases, of matter. They are:

We will not be discussing the plasma state here.

When a substance changes from one state, or phase, of matter to another we say that it has undergone a change of state, or we say that it has undergone a change of phase.

These changes of phase always occur with a change of heat. Heat, which is energy, either comes into the material during a change of phase or heat comes out of the material during this change. However, although the heat content of the material changes, the temperature does not.

We will list five changes of phase. They are diagrammed in the above animation and listed below.

Description of Phase Change Term for Phase Change Heat Movement During Phase Change Temperature Change During Phase Change
Solid to liquid Melting Heat goes into the solid as it melts. None
Liquid to solid Freezing Heat leaves the liquid as it freezes. None
Liquid to gas Vaporization, which includes boiling and evaporation Heat goes into the liquid as it vaporizes. None
Gas to liquid Condensation Heat leaves the gas as it condenses. None
Solid to gas Sublimation Heat goes into the solid as it sublimates. None

So, how could there be a change in heat during a phase change without a change in temperature? Remember that heat is energy, and remember that there are two types of energy - kinetic and potential. The heat exchanges, or energy exchanges, present during a change in phase are changes in potential energy. These energy exchanges are not changes in kinetic energy.

If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. After the molecular bonds in the ice are broken the molecules are at a higher potential energy state, however they are not on the average moving any faster, so their average kinetic energy remains the same, and thus, their Kelvin temperature remains the same.

Look at the following diagram and continue to read the text below it.

In the ice the molecules are strongly bonded to one another, thus forming a rigid solid. When heat is added to the ice, these bonds are broken and the molecules, now at a higher potential energy state, bond to one another with less strength. Water is formed.

Now, before the melting the molecules were actually moving when in the solid state. They were vibrating back and forth. So, they had an average speed, and thus, an average kinetic energy. So, they had a Kelvin temperature proportional to this average kinetic energy.

After the melting the water molecules are moving, also. And they have the same average speed and the same average kinetic energy as they had before the melting. So, the water is at the same temperature the moment after the melting that the ice was at the moment before the melting.

Heat came into the situation, but it was not used to change the speed of the molecules. It was used to change the bonding between the molecules. To break the bonds between the molecules of the ice required energy, and this is was the use for the heat.

In a similar way heat enters a liquid to change the molecular bonding when the liquid boils or evaporates into a gas, and heat enters a solid to change the molecular bonding when it sublimates into a gas.

In an inverse way heat leaves a gas to change the molecular bonding when the gas condenses into a liquid, and heat leaves a liquid to change the molecular bonding when it freezes into a solid.

In none of these changes of state is heat input used to speed up the molecules, nor is heat output ever occurring as a result of the molecules slowing down. The average speed of the molecules is the same before and after a phase change, so, the average kinetic energy is the same. And thus, again, we state that the temperature does not change during a change in phase.

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