When talking about a substance, it is important to know whether the substance is pure or not. Pure substances are often referred to as homogeneous mixtures; these substances contain only one type of molecules or atom. An example of a homogeneous mixture would be a container of only nitrogen gas ($ N_2 $) or perhaps a solid aluminum block ($ Al $).
The counter to a homogeneous mixture is a heterogeneous mixture; these are substances that contain a mixture of different molecules or atoms. One example of a heterogeneous mixture is air! Air contains many different atoms and molecules such as oxygen ($ O_2 $), nitrogen ($ N_2 $), carbon dioxide ($ CO_2 $), argon ($ Ar $), water vapor ($ H_2O $), and other trace molecules.
For substances, there are 5 possible phases they could be. 3 of these phases are more common, solid, liquid, and gas, but there are 2 that may be less common. The remaining 2 phases are plasma and a Bose-Einstein condensate, both of which are fascinating and we will talk briefly about these, but they are not needed for the remainder of the guide.
Now, all substances will have a “natural” phase they are found in; oxygen is naturally found as a gas, aluminum is naturally found as a solid, and mercury is naturally found as a liquid. However, this does not mean they cannot be heated or cooled to change phase. What is interesting is the energy associated with each phase because phase depends on internal energy!
Between solids, liquids, and gases, solids tend to have the least amount of internal energy whereas gases have the largest amount. As a solid is heated, its molecules start to increase their internal energy until they get to a stall point. This stall is where latent energy comes in because there is a latent stall before solids change to liquids and as liquids change to gases. This latent energy is the energy required to break or separate the bonds between molecules.
I like to picture these states like below in Figure 1.
Fig. 1 - Visualization of Matter Energy States
As for the other 2 phases, I want to mention them because they are interesting physically, but they are not necessarily needed for further study in this guide. We will start with plasma. Let’s say we have a gas and pretend we are heating it with some device. As the gas heats hotter and hotter, the atoms will increase speed until they reach the point where electrons can be knocked loose from atoms colliding with one another. This creates a conductive medium combined roughly half of positive particles and half of negative particles. Plasma is often seen around high voltage sources as a purplish hue.
For the last phase, a Bose-Einstein condensate, this is on the opposite end of the internal energy spectrum. When a solid is cooled to near absolute zero (around 0 K, -273.15°C, or -459.67°F), the atoms within the solid start to coalesce into a single quantum mechanical entity. What this means is it would be able to be described by a wave function on a near-macroscopic scale!
Let’s now look at an example of water in a container that is being heated and undergoes phase change to a gas.
Fig. 2 - Water Phase Change Graphic
In the first image, there is water at room temperature and 1 atmosphere of pressure. A candle is then placed underneath the container and the heat from the candle is transferred to the water. Due to this heat transfer, the temperature of the water will increase and the volume will as well, hence, raising the lid on the container. We know this because pressure is held constant, so as temperature increases, so will volume to satisfy equilibrium. We also know this from the ideal gas law
[ 1 ]
$$ PV = mRT $$
where P is the pressure, V is the volume, m is the mass, R is the gas constant, and T is the temperature.
Now in the second image, the water has been heated to 100°C which is on the verge of changing phase to a gas. However, since the water has not yet changed, it is considered a saturated liquid, or a liquid on the verge of changing into a gaseous state. As the saturated water is heated further, it starts to vaporize and then looks like image 3 where the water is collected at the bottom of the container and the gas is at the top. The water is continually heated until all of it is changed to a saturated vapor, like in image 4. Saturated vapors are on the edge between being a gas or being a liquid again; all it takes is the slightly removal of heat to start changing the saturated vapor back into a liquid.
The saturated vapor is heated further until the temperature rises significantly, and as a result, the volume the gas increased and pushed the lid of the container upward. This is due to the pressure and mass of the vapor remaining constant.