Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 201 10.1 Solutions and Mixtures You Know—Water and Antifreeze In Chapter 4, you learned that a solution is defined as one substance thoroughly dissolved in another, while a mixture is a combination of two or more substances that remain unchanged individually and can be separated again. The Self-Discovery activity “Solutions, Phases, and Mixtures—Sugar in Water” illustrated those definitions and showed how mixtures can change to solutions and back again. Antifreeze mixed into water is a solution. This solution will freeze or not, depending on its composition and temperature. The “frozen” form is a mixture of solid ice in a water-antifreeze solution. 10.1.1 Wat er-Antifreeze Solution If you live in the United States in the temperate zone, then you know that an automobile is going to experience temperatures below 32°F (0°C) in winter. If the water in the cooling system freezes, it can crack the engine block and ruin the engine. When antifreeze, usually glycol, is added to the water, the solution of glycol in water will freeze at a lower temperature than water alone. On the side of a can of antifreeze is a table showing how much antifreeze should be added to safely keep the solution liquid at different temperatures. Figure 10-2 is a graph made from the table, showing how the safe temperature drops as the amount of glycol increases. 10.1.2 Compo sition and Temperature Determine Phases Present As discussed in Chapter 4, a phase is a volume of material whose properties, such as composition, hardness, and color, are uniform. You can see the boundary between two phases by a sharp change in one or more properties, such as a change in Goodheart-Willcox Publisher Figure 10-2. This graph is a plot of the minimum safe temperature versus antifreeze composition (% glycol) for an automobile cooling system. The points were calculated from table data on multiple antifreeze containers. The red line shows the division between complete liquid and liquid plus solid, or “safe” and “unsafe” regions. 20 5 qt glycol 5 0.33 = 33% glycol 33% glycol: safe to 0°F (–18°C) 15 = = –20 –40 –60 –80 20% X-axis, glycol concentration Y-axis, temperature 30% 40% 50% Percent glycol by volume 60% 70% –65 –51 –40 Liquid phase –29 Temperature, ° C Temperature, ° F –18 –7 0 15 qt system Solid phase + liquid phase Completely solid phase Copyright Goodheart-Willcox Co., Inc.