What happens to the temperature during a state change?

An interesting property of matter is that its temperature will remain constant during a phase change, assuming its surrounding pressure is constant.

Table of Contents Show

Solid/liquid transition
Changing solid to liquid
Example with ice
Changing liquid to solid
Liquid/gas transition
Changing liquid to gas
Changing gas to liquid
Exotic transitions
Gas/plasma transition
Glass transition
Resources and references
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In other words, once the temperature of a liquid reaches its boiling point, the temperature will not increase until all the liquid has changed its phase to become a gas. Likewise, once the temperature of a liquid is lowered to its freezing point, the temperature will not decrease until all the liquid has changed its phase to become a solid. A similar effect occurs in the opposite direction.

Temperature is the average kinetic energy (KE) of the material's molecules. That means for a given temperature there will be some molecules moving faster and having a higher kinetic energy, while others move at a slower speed.

Temperature does not necessarily remain constant in transitions to exotic states.

Questions you may have include:

What happens in a solid/liquid transition?
What happens in a liquid/gas transition?
What happens in exotic transitions?

This lesson will answer those questions. Useful tool: Units Conversion

Solid/liquid transition

Sufficiently heating a solid can cause it to become a liquid. Likewise, cooling a liquid can turn it into a solid.

Changing solid to liquid

In a solid, molecules vibrate in place according to the temperature of the material. As the solid is heated, its molecules move faster and gain kinetic energy (KE) until some of them break lose and escape the solid. Those molecules are in the liquid phase and are not as confined as in a solid.

Since the temperature of the solid is the average KE of its molecules, the solid actually loses some energy when these molecules escape. This keeps the temperature from rising beyond the boiling point.

As heat energy is added, more molecules escape, and the temperature remains relatively constant. Finally, when all the molecules have escaped the solid and transitioned into the liquid phase, the temperature can start to rise above the freezing point.

Example with ice

With ice, some of the molecules will break lose from the solid state and turn into water molecules as the material is heated. But when they do change, they take energy from the ice, thus keeping its average temperature of the ice at or below freezing. The temperature of the ice will remain near the freezing point until all of the molecules have changed their state to become liquid. Then the temperature of the liquid will increase as heat is added.

Changing liquid to solid

The opposite effect occurs when you cool a liquid to change its phase to a solid. Collections of liquid molecules will gather to form solid particles. That transition slightly increases the temperature or average KE of the liquid, which will remain at a temperature near to the freezing point until all the liquid has turned to the solid phase.

Thus water will remain at or near the freezing point until all the material has crystallized into ice. Then the temperature of the ice can start to decrease.

Liquid/gas transition

Sufficiently heating a liquid can cause it to become a gas. Likewise, cooling a gas can turn it into a liquid.

Changing liquid to gas

When a liquid—such as water—is heated at a constant pressure, some of its molecules get enough KE to escape the liquid. However, when the high energy molecules escape, they slightly lower the average energy of the liquid.

You can experience the lowered temperature when water evaporates into gaseous molecules.

Thus, more heat energy is needed to cause other molecules to escape. This keeps the liquid at a temperature slightly below that of the boiling point. Once all of the molecules have enough energy to escape into the gaseous state, the temperature of the gas can increase.

Changing gas to liquid

The opposite effect occurs when you cool a gas to change its phase to liquid. It will remain at a temperature near to the boiling point until all the gas has turned to liquid.

Exotic transitions

Heating or cooling a substance that is not among the three classical states can also cause a change in state or phase. Since the transition is more complex, temperature may not be constant in the change of state. Some examples include plasmas and glass.

(See Exotic States of Matter for more information.)

Gas/plasma transition

Heating a gas to extremely high temperatures can result in a change to the plasma state of matter. Likewise cooling a plasma will cause it to revert to a gas.

The temperature probably remains the same during the transition. However, I haven't seen any experiments to prove that.