Learning Objectives Determine the heat associated with a phase change. Matter can exist in one of several different states, including a gas, liquid, or solid state. The amount of energy in molecules of matter determines the state of matter . A gas is a state of matter in which atoms or molecules have enough energy to move freely. The molecules come into contact with one another only when they randomly collide. A liquid is a state of matter in which atoms or molecules are constantly in contact but have enough energy to keep changing positions relative to one another. A solid is a state of matter in which atoms or molecules do not have enough energy to move. They are constantly in contact and in fixed positions relative to one another. The following are the changes of state: 0 1 Solid → Liquid Melting or fusion Liquid → Gas Vaporization Liquid → Solid Freezing Gas → Liquid Condensation Solid → Gas Sublimation If heat is added to a substance, such as in melting, vaporization, and sublimation, the process is endothermic . In this instance, heat is increasing the speed of the molecules causing them move faster (examples: solid to liquid; liquid to gas; solid to gas). If heat is removed from a substance, such as in freezing and condensation, then the process is exothermic . In this instance, heat is decreasing the speed of the molecules causing them move slower (examples: liquid to solid; gas to liquid). These changes release heat to the surroundings. The amount of heat needed to change a sample from solid to liquid would be the same to reverse from liquid to solid. The only difference is the direction of heat transfer. Example \(\PageIndex{1}\) Label each of the following processes as endothermic or exothermic. water boiling ice forming on a pond Solution endothermic - you must put a pan of water on the stove and give it heat in order to get water to boil. Because you are adding heat/energy, the reaction is endothermic. exothermic - think of ice forming in your freezer instead. You put water into the freezer, which takes heat out of the water, to get it to freeze. Because heat is being pulled out of the water, it is exothermic. Heat is leaving. Exercise \(\PageIndex{1}\) Label each of the following processes as endothermic or exothermic. water vapor condensing gold melting Answer a. exothermic b. endothermic A phase change is a physical process in which a substance goes from one phase to another. Usually the change occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance. The melting point is the temperature at which the substance goes from a solid to a liquid (or from a liquid to a solid). The boiling point is the temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid). The nature of the phase change depends on the direction of the heat transfer. Heat going into a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat from a substance changes a gas to a liquid or a liquid to a solid. Two key points are worth emphasizing. First, at a substance’s melting point or boiling point, two phases can exist simultaneously. Take water (H 2 O) as an example. On the Celsius scale, H 2 O has a melting point of 0°C and a boiling point of 100°C. At 0°C, both the solid and liquid phases of H 2 O can coexist. However, if heat is added, some of the solid H 2 O will melt and turn into liquid H 2 O. If heat is removed, the opposite happens: some of the liquid H 2 O turns into solid H 2 O. A similar process can occur at 100°C: adding heat increases the amount of gaseous H 2 O, while removing heat increases the amount of liquid H 2 O (Figure \(\PageIndex{1}\)). Water is a good substance to use as an example because many people are already familiar with it. Other substances have melting points and boiling points as well. Second, as shown in Figure \(\PageIndex{1}\), the temperature of a substance does not change as the substance goes from one phase to another . In other words, phase changes are isothermal (isothermal means “constant temperature”). Again, consider H 2 O as an example. Solid water (ice) can exist at 0°C. If heat is added to ice at 0°C, some of the solid changes phase to make liquid, which is also at 0°C. Remember, the solid and liquid phases of H 2 O can coexist at 0°C. Only after all of the solid has melted into liquid does the addition of heat change the temperature of the substance. What is the direction of heat transfer in boiling water? What is the direction of heat transfer in freezing water? What is the direction of heat transfer in sweating? Answers 1. Boiling. Heat is being added to the water to get it from the liquid state to the gas state. 2. Freezing. Heat is exiting the system in order to go from liquid to solid. Another way to look at it is to consider the opposite process of melting. Energy is consumed (endothermic) to melt ice (solid to liquid) so the opposite process (liquid to solid) must be exothermic. 3. Sweating. Heat is consumed to evaporate the moisture on your skin which lowers your temperature. Key Takeaway There is an energy change associated with any phase change.