Termo

3-1

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.The saturation 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 At 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.
3-10C 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 nodistinct 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 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 Aperfectly 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.

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3-15C The molar massof 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 Icecan 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 canvaporize, 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.
3-17C Yes. Otherwise we can create energy by alternately vaporizing and condensing asubstance.
3-18C No. Because in the thermodynamic 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 thelower the hfg value.
3-21C Quality is the fraction 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,...