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Unformatted text preview: 3-1Chapter 3 PROPERTIES OF PURE SUBSTANCES Pure Substances, Phase Change Processes, Property Diagrams 3-1CYes. Because it has the same chemical composition throughout. 3-2CA liquid that is about to vaporize is saturated liquid; otherwise it is compressed liquid. 3-3CA vapor that is about to condense is saturated vapor; otherwise it is superheated vapor. 3-4CNo. 3-5CNo. 3-6CYes. The saturation temperature of a pure substance depends on pressure. The higher the pressure, the higher the saturation or boiling temperature. 3-7CThe temperature will also increase since the boiling or saturation temperature of a pure substance depends on pressure. 3-8CBecause one cannot be varied while holding the other constant. In other words, when one changes, so does the other one. 3-9CAt critical point the saturated liquid and the saturated vapor states are identical. At triple point the three phases of a pure substance coexist in equilibrium. 3-10CYes. 3-11CCase (c) when the pan is covered with a heavy lid. Because the heavier the lid, the greater the pressure in the pan, and thus the greater the cooking temperature. 3-12CAt supercritical pressures, there is no distinct phase change process. The liquid uniformly and gradually expands into a vapor. At subcritical pressures, there is always a distinct surface between the phases. Property Tables3-13C A given volume of water will boil at a higher temperature in a tall and narrow pot since the pressure at the bottom (and thus the corresponding saturation pressure) will be higher in that case. 3-14CA perfectly fitting pot and its lid often stick after cooking as a result of the vacuum created inside as the temperature and thus the corresponding saturation pressure inside the pan drops. An easy way of removing the lid is to reheat the food. When the temperature rises to boiling level, the pressure rises to atmospheric value and thus the lid will come right off. 3-23-15CThe molar mass of gasoline (C8H18) is 114 kg/kmol, which is much larger than the molar mass of air that is 29 kg/kmol. Therefore, the gasoline vapor will settle down instead of rising even if it is at a much higher temperature than the surrounding air. As a result, the warm mixture of air and gasoline on top of an open gasoline will most likely settle down instead of rising in a cooler environment 3-16C Ice can be made by evacuating the air in a water tank. During evacuation, vapor is also thrown out, and thus the vapor pressure in the tank drops, causing a difference between the vapor pressures at the water surface and in the tank. This pressure difference is the driving force of vaporization, and forces the liquid to evaporate. But the liquid must absorb the heat of vaporization before it can vaporize, and it absorbs it from the liquid and the air in the neighborhood, causing the temperature in the tank to drop. The process continues until water starts freezing. The process can be made more efficient by insulating the tank well so that the entire heat of vaporization comes essentially from the water. that the entire heat of vaporization comes essentially from the water....
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- Spring '08