Changes of State Chapter 3 Section 3

Energy & Change of State The change of a substance from one physical form to another is a change of state or phase. Remember, in a physical change the identity of the substance does not change, only it’s physical form. Ice, liquid water, and steam are all made of the same molecules. These substances are different only in the arrangement and motion of the molecules.

Movement of Particles In different states, or phases, the molecules move differently because they have different amounts of energy. To change a substance from one state of matter to another you must add or remove energy. The five phase changes are melting, freezing, condensation, vaporization, and sublimation.

Melting When heat is added to a substance to cause it to go from a solid to a liquid, melting has occurred. Adding energy increases the temperature of the solid, causing the molecules to move faster. Melting will occur when a certain temperature has been reached, the melting point.

Melting Look at figure 2 page 75. Why wouldn’t gallium, a metal element, be useful as jewelry? Gallium melts at 30°C. Normal human body temperature is 37°C, so gallium will melt in your hand. Melting occurs when molecules move fast enough to overcome their attractions to each other. Melting is endothermic because energy is added.

Freezing When heat is removed from a substance and it changes from a liquid to a solid, freezing has occurred. The temperature at which a liquid changes to a solid is the freezing point. Freezing and melting will occur at the same temperature.

Freezing Look at figure 3 page 75. At a certain temperature, what will determine whether water will melt or freeze? The addition or removal of energy. For a liquid to freeze the attractions between the molecules most overcome their motion. Removing energy will cause molecules to lock into place. This is exothermic because energy is removed.

Vaporization A term not used in your textbook is vaporization. This is the phase change from liquid to gas. There are two types of vaporization: Evaporation Boiling Vaporization requires the addition of heat, so it is endothermic.

Evaporation When liquid changes to a gas at temperatures below the boiling point, evaporation has occurred. Evaporation occurs at the surface of the liquid. Sweating helps cool your body because the water evaporates from your skin, taking the heat with it.

Boiling When liquid changes to a gas throughout the liquid, not just at the surface, boiling is taking place. The temperature at which a substance boils is the boiling point. Pressure will effect the boiling point of a liquid. When pressure is less, so is the b.p.

Condensation The change of state from a gas to a liquid is condensation. The condensation point is the temperature at which the gas becomes a liquid and is the same as the boiling point of the substance (at a given pressure). Energy must be removed for condensation to occur, so it is exothermic.

Sublimation The change of state from a gas to a solid or a solid to a gas (where the liquid phase is bypassed) is sublimation. Sublimation can be endothermic or exothermic. Dry ice, iodine crystals, and ice can all sublime.

Change of Temp vs. Change of State When most substances lose or gain energy, one of two things will happen to the substance (not both): The substance will change temperature OR The substance will change state

Change of Temp vs. Change of State When temperature changes, the speed of the molecules changes. Temperature cannot change until the change of state is complete. The temperature of boiling water stays at 100°C until all of the water has vaporized, no matter how much more heat you add.

Figure 7 Page 79 Look at Figure 7 on page 79. This is a graph of the “Heat Curve” of water. At what temperature does water melt? At what temperature does water boil? Does the temperature change when the water is changing states? Why? Why not?

Additional Questions Why is antifreeze added to automobile engines during the summer as well as the winter? What is the purpose of adding salt to boiling water when making pasta, and putting salt on icy roadways in the winter? Answer these questions by referring to melting and boiling points.