SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 7

Liquids, Solids, and Phase Changes

BOILING POINT

The vapor pressure-temperature relation can be plotted on a graph for a closed system. (See Figure 24.) When a liquid is heated in an open container, the liquid and vapor are not in equilibrium and the vapor pressure increases until it becomes equal to the pressure above the liquid. At this point the average kinetic energy of the molecules is such that they are rapidly converted from the liquid to the vapor phase within the liquid as well as at the surface. The temperature at which this occurs is known as the boiling point. Notice that in this graph, water’s normal boiling point is at 760 mm Hg pressure and 100°C temperature.

Figure 24. Vapor Pressure-Temperature Relationship for Carbon Tetrachloride and Water

TIP 

Boiling point is defined as the point at which the liquid’s vapor pressure equals the atmospheric pressure.