Any change in a solid, liquid, or gas to another physical state is called a phase change. A phase change always involves a transfer of energy but not a change in chemical composition. Scientists who do weather research look closely at the phase changes of water. On Earth, water can exist as a solid, a liquid, or a gas. The three states of water have different physical properties.

 

In its solid state, water exists as ice. As ice absorbs heat energy, its particles begin to vibrate faster. If enough heat energy is added, they will vibrate so violently that they “break free” of their locked position. When this happens, individual particles begin to flow. We observe a phase change from solid to liquid. It’s a common transformation called melting.

 

In the liquid state, water particles flow. This freedom of movement gives water its ability to be poured and to take the shape of its container. Although the liquid state of water can change shape, its particles are restricted to a fixed volume. That condition, however, can change if additional heat energy is added to water.

When liquid water gains sufficient heat energy, its particles enter the gas state. This phase change is called evaporation. As a gas, the individual particles are not restricted to a mostly sideways flow. Instead, they can spread up and out to fill a container. They can also scatter among the mixture of gases that make up the atmosphere. If conditions are right, the water in solid ice can change directly into water vapor. This process is called sublimation.

 

Water can also undergo changes when heat energy is removed or released. If enough heat energy is removed from water vapor, it becomes a liquid. This phase change is called condensation. If enough heat energy is removed from liquid water, it becomes ice. This phase change is called freezing. Snow and frost form when water vapor changes directly to a solid state. This process is called deposition.