In thermodynamics, energy can take many forms. It can be added as heat and measured by a change in temperature. It can be work, where the pressure is changed. It can also be chemical in nature, but when we talk about changes in phase, we are talking about physical changes, not chemical changes. When physical energy is added or removed from a substance, the molecules are either excited or slowed down, but they are not changed into new molecules (chemistry). The substance remains physically the same, but in a new phase.

Let's start with ice - solid water. For ice to exist outside, the temperature, a measure of how hot or cold something is, must be cold. If energy is added in the form of heat, the temperature rises and the ice changes phase to water, a liquid. We say the ice melts or liquifies.

If we continue to add heat, the water evaporates, or turns into a gas (steam). Evaporation is where a liquid turns into a gas. In the first part of the demonstration, you heated water until it was boiling, and the water turned into steam.

If we start with steam and remove heat, water droplets will condense from the steam. Condensation is where a gas turns into a liquid. In the demonstration, we captured the steam with the plastic wrap. But because the room temperature is much cooler than the temperature of boiling water, the steam cooled at the surface of the plastic wrap and condensed into water droplets. Another example of condensation is the formation of water droplets on the outside of an ice cold drink on a warm day. The water vapor in the air is cooled by the side of the container, and water droplets are formed. If we continue to remove energy by decreasing the temperature further, the water freezes, or solidifies into ice.

Another way to cause a substance to change state is to add or remove energy in the form of pressure changes. Pressure is the measure of small pushes, or forces, all over a specific area. The air has a specific pressure at sea level. If you've ever traveled high on a mountain, you may have noticed it was harder to breathe. The air pressure is less at high altitudes. Conversely, people used to the mountains think that the air feels heavier at sea level! We can change the phase of a substance by changing the pressure. For example, we can put water into a vacuum pressure chamber and decrease the pressure. Even though the temperature of the room is comfortably warm, the water will turn into ice! If we increase the pressure, the water will boil, even though the room temperature isn't that hot. A pressure cooker works on this principle. By increasing the pressure and increasing the heat by turning the burner on, the food inside will cook faster because the water molecules inside the food will be turning to steam faster. Sometimes, when the conditions (pressure and temperature) are just right, a substance can go directly from a solid to a gas without changing into a liquid first. This is called sublimation. An example in nature is when snow at some altitudes disappears without melting first. It has sublimated directly to a gas!

Scientists and engineers must be able to evaluate the properties of a substance as it changes phase. We look at how a substance changes with respect to temperature, pressure, and volume. We create diagrams called phase diagrams to help us compute the properties of a substance. The diagram shown below is a pressure-temperature diagram at a constant volume for a substance like water that expands as it freezes. For a specific temperature and pressure, it is possible for the substance to exist in all three states; this point is called the triple point. It takes a lot of energy and effort to find the exact conditions to have a triple point!