Phase Changes

Core Concepts

Any substance of matter can exist in different phases (solid, liquid, or gas) depending on its temperature and pressure. Changing these conditions, notably temperature, will result in the substance undergoing phase changes from one to another, such as melting from a solid to a liquid.

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The Phases: Solids, Liquids, and Gases

The three phases of matter are solids, liquids, and gases. Every substance can exist in all three, and can change between them as its conditions change. 

  • Solids: atoms are very close together and cannot move around. Solids have definite shape and volume, meaning they can’t compress or expand and cannot change shape depending on surroundings/container. 
  • Liquids: atoms are close to each other but not tightly bound, so they can move with respect to each other. However, they are still close together so they cannot compress or expand, but do change shape based on the container. 
  • Gases: atoms are very far apart. They can move around each other, and the space in between can increase or decrease, allowing them to compress and expand by volume. They also form to their container like liquids. 
Each phase showing how atoms exist relative to each other.

Primarily, the temperature determines the phase of a substance. Solids are the lowest temperature, and gases are the highest temperature. Liquids are in the middle (because of the relationship between temperature and volume, see why temperature makes atoms spread out here).

Changes Between the Phases

Because phase is based on temperature, changing the temperature of a substance will cause it to change phase as well. These phase changes happen at specific temperatures, which results in ranges of temperatures where the substance will be in each phase. 

temperature vs. heat absorbed as a substance changes phase, showing how temperature remains constant at the heat of fusion and vaporization

As the diagram shows, there is a specific temperature for the change between solid and liquid, and another temperature for the change between liquid and gas. Respectively, we call these the melting point and the boiling point of the substance. Because the phase change happens in both directions, different names exist for each change that happens at that point (they are shown on the diagram).

Heat of Fusion and Vaporization

As mentioned above, because changes happen in both increasing and decreasing temperature directions, the temperature of phase changes is given a more general name (i.e. because the melting point would also be the freezing point, the value has a single, more general name). 

For the solid-liquid phase change, we call this the Heat of Fusion. When the solid phase heats to this temperature, it melts, and when the liquid phase cools to this temperature, it freezes. 

The point of the liquid-gas phase change is the Heat of Vaporization. When the liquid phase heats to this, it vaporizes (or boils), and when the gas phase cools to this, it condenses. 

Temperature During Phase Changes

Just as some reactions require heat energy in order to occur (learn why!), energy has to enter or leave in order to cause the phase to change. This explains why the phase diagram is shaped the way it is. 

In between phases, adding temperature to a substance will increase the temperature linearly. However, when the substance reaches the heat of fusion or vaporization, the temperature will stop rising and all energy will go toward changing the phase. That means that while the phase change occurs, the temperature of the substance will appear to stay the same until the phase change completes. It then will continue rising. This is why the temperature the diagram shows constant temperature while heat energy increases during phase changes.

Phase Change Relationship to Pressure

So far, we have only discussed phase changes in relation to the substance’s temperature. However, the pressure also plays a part in the phase (and phase changes) of a substance. Pressure and temperature are also closely related, so including pressure creates a new diagram for the conditions of each phase. 

phase diagram of pressure vs temperature, showing what phase a substance would be in depending on temperature and pressure
source: wikimedia commons

This new diagram shows that liquids must be at a relatively high pressure as well as temperature. This means that in very low pressure conditions, increasing temperature will cause a solid to change directly into a gas rather than first going through the liquid phase. 

Considering another condition also means that one point exists (the triple point) where all three are in equilibrium. At this point, changing either temperature or pressure would be required to make it a single phase.