Intermolecular Forces 11.3

• A phase diagram shows the most stable state of a substance at any given temperature and pressure.
• The solid lines and curves on a phase diagram show the temperatures and pressures at which two states can coexist in equilibrium.
• The upper left hand portion of a phase diagram corresponds to conditions of high pressure and low temperature and the most stable state in that region is the solid state.
• The line separating the solid and liquid phases is called the fusion curve and each point on that line corresponds to a temperature and pressure at which the solid and liquid phases coexist in equilibrium.
• The curve separating the solid and gas phases is the sublimation curve and the curve separating the liquid and gas phases is the vaporization curve.
• The vaporization curve, which describes the conditions under which the liquid and vapor are in equilibrium, is identical to the vapor pressure curve we encountered in the previous section.
• There is one point on the phase diagram where the three curves intersect, called the triple point, which represents the temperature and pressure at which the three phases of a substance - solid, liquid, and gas - coexist in equilibrium.
• The negative slope of the water fusion curve indicates that as the pressure increases the temperature at which the solid and liquid phases are in equilibrium decreases.
• The vaporization curve for both CO2 and H2O don't continue indefinitely but abruptly stop at the critical point.
• The temperature and pressure at the critical point are called the critical temperature and critical pressure.
• If the temperature is increased enough, the kinetic energy of gas molecules eventually becomes so high that the gas cannot be condensed into a liquid regardless of how high the pressure is, and this temperature is defined as the critical temperature.
• The critical pressure is the pressure required to condense gas at the critical temperature.
• When a substance is at temperatures and pressures beyond the critical point, it exists as a supercritical fluid.
• A supercritical fluid is neither a liquid nor a gas but possesses properties somewhat intermediate between the two.