The Effects of Pressure on Boiling Points

• As the pressure on a liquid is increased, the boiling point rises.
• As the pressure on a liquid is decreased, the boiling point drops.

At sea level, where the atmospheric pressure is 14.7 psi, the boiling point of water is 212°F (100°C). At any point higher than sea level, the atmospheric pressure is lower and so is the boiling point. In Denver, Colorado (elevation 5,300 feet), water boils at only 206°F (97°C).

Atmospheric pressure is approximately 14.7 psi (absolute) at sea level, and somewhat lower at higher elevations. At sea level, the entire weight of a "column" of air approximately 600 miles high, presses down on everything. At higher elevations, the column of air is shorter and the air is thinner, so the pressure is lower.

Of course, you don't notice the 14.7 psi pressing in on everything, and air pressure gages are calibrated to read 0 psi at atmospheric pressure. But this atmospheric pressure exists, and you can feel its effects, particularly at higher elevations; for example, if you exercise vigorously, at a high elevation, you become winded more quickly.

In an air conditioning system, the pressure in the evaporator is low, so that all the refrigerant vaporizes. The pressure in the condenser is high, so that all the refrigerant readily changes state to a liquid.

In an automotive cooling system, an overpressure condition is maintained to raise the boiling point. For example, a cooling system having a pressure cap rated at 22psi (1.5 bar) would raise the boiling point of pure water 268°F (131°), a 56°F (31°C) increase.