1 minute read
Phase Changes
by AudioLearn
PHASE CHANGES
Within the range of temperatures, a substance can be in gaseous, liquid, or solid form. Gases act very much like ideal gases at high temperatures but fail at lower temperatures. Molecules will interact with one another and condensation occurs. There will be a significant decrease in volume as the substance becomes liquid. At even lower temperatures, the substance becomes solid. The volume becomes smaller but does not reach zero due to the mass of the molecules. Gases will become liquid at high pressures as well.
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Pressure-volume diagrams will plot the behavior of a substance based on these parameters. When a substance is an ideal gas, there is a particular relationship between pressure and volume. The volume of a gas will decrease as the pressure increases. At fixed temperatures, this leads to a series of hyperbolas called isotherms. At some point, the temperature becomes too low and the gas no longer behaves ideally. There is a critical point above which a liquid cannot exist. The critical pressure is the minimum pressure needed for liquid to exist at a critical temperature. Figure 76 shows these values:
Figure 76.
In the figure, each line represents an isotherm of equal temperature along the line. At lower temperatures, a critical point is reached, where liquid first becomes possible. With regard to the terminology referring to a vapor, a vapor is a gaseous phase that exists at a temperature below the boiling temperature.
A more easily understood graph on phase changes is the phase diagram. This plots the pressure of a substance versus its temperature. If you know the pressure and the temperature, you can determine what phase the substance is in. The boiling point of water is, for example, 100 degrees at one atmosphere of pressure. However, as the pressure rises (as in a pressure cooker situation), water can exist as a liquid in order to cook food at much higher temperatures. The curve for water shown in figure 77 , reaches a critical point above which liquid cannot possibly exist because the temperature is too high and the kinetic energy of the system is too great.
