# Metodos

Páginas: 30 (7392 palabras) Publicado: 25 de febrero de 2012
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Chapter 3 PROPERTIES OF PURE SUBSTANCES
Pure Substances, Phase Change Processes, Property Diagrams 3-1C Yes. Because it has the same chemical composition throughout. 3-2C A liquid that is about to vaporize is saturated liquid; otherwise it is compressed liquid. 3-3C A vapor that is about to condense is saturated vapor; otherwise it is superheated vapor. 3-4C No. 3-5C No. 3-6C Yes. Thesaturation temperature of a pure substance depends on pressure. The higher the pressure, the higher the saturation or boiling temperature. 3-7C The temperature will also increase since the boiling or saturation temperature of a pure substance depends on pressure. 3-8C Because one cannot be varied while holding the other constant. In other words, when one changes, so does the other one. 3-9C Atcritical point the saturated liquid and the saturated vapor states are identical. At triple point the three phases of a pure substance coexist in equilibrium. 3-10C Yes. 3-11C Case (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. 3-12C At supercritical pressures, there is no distinct phase changeprocess. The liquid uniformly and gradually expands into a vapor. At subcritical pressures, there is always a distinct surface between the phases.

Property Tables 3-13C 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. 3-14C A perfectly fitting pot andits 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.

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3-15C The molar mass of gasoline (C8H18) is 114kg/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 theair 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 fromthe 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. 3-17C Yes. Otherwise we can create energy by alternately vaporizing and condensing a substance. 3-18C No. Because in thethermodynamic analysis we deal with the changes in properties; and the changes are independent of the selected reference state. 3-19C The term hfg represents the amount of energy needed to vaporize a unit mass of saturated liquid at a specified temperature or pressure. It can be determined from hfg = hg - hf . 3-20C Yes; the higher the temperature the lower the hfg value. 3-21C Quality is thefraction of vapor in a saturated liquid-vapor mixture. It has no meaning in the superheated vapor region. 3-22C Completely vaporizing 1 kg of saturated liquid at 1 atm pressure since the higher the pressure, the lower the hfg . 3-23C Yes. It decreases with increasing pressure and becomes zero at the critical pressure. 3-24C No. Quality is a mass ratio, and it is not identical to the volume ratio....

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