Temperature is the most critical component in any phase equilibrium experiment. Raising the temperature will convert most substances to vapor if the temperature increase is high enough and remains constant while the element heats to that point, while lowering the temperature changes most liquids to a solid state. The key to equilibrium is reaching a certain temperature and maintaining it throughout the experiment, so all molecules will achieve the same state. For example, a student attempting to freeze saltwater must lower the surrounding temperature well below 32 degrees Fahrenheit and hold it there; otherwise, some water molecules that are far from a salt molecule may freeze but the others will not, and the substance will not achieve a state of equilibrium.
Entropy is an underlying principle in the area of phase equilibrium. Gases have the highest level of entropy, or disorder, because the molecules are so far apart from one another, and solid or liquid states may spontaneously convert to gases. The reverse is not true, however, because in order to reduce the disorder and entropy in the gaseous state, the non-spontaneous process of compression must be applied. Under pressure, any substance will always change to a denser form. This is the reason naturally occurring gases must be stored in tightly sealed metal canisters that maintain a pressurized environment to prevent entropy from converting the gas molecules back to their original form.
While each pure substance will undergo state transition or achieve equilibrium at a specific temperature or pressure, homogeneous mixtures, or solutions, also possess certain properties that impact the phase equilibrium of the mixture or induce separation so that each component will achieve equilibrium on its own. These colligative properties are based on the solvent in which the mixture is placed and the amount of the solution. Colligative properties include vapor pressure lowering, osmotic pressure rate, boiling point elevation and freezing point depression. Conduct experiments on various solutions to learn about the unique colligative properties of each one.
A simple experiment using a pan and a stove top is the perfect introduction of phase equilibrium discussions to students of any age. Prior to instruction, teachers must fill a plastic container about the size of a shoebox with water and freeze overnight. Place the ice block in a pan and set it on the stove at medium-high heat. Watch the pan to determine when the ice block's temperature exceeds 32 degrees Fahrenheit and increase the stove temperature to the highest setting. Observe the air directly above the pan and use a stopwatch to determine the time elapsed between when the ice begins to melt and steam begins to escape from the pan. Ask students to explain how H2O can be present in all three forms simultaneously.