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When the ambient air pressure drops, the temperature required for a liquid to boil also falls. This is why cooking at high elevations takes longer – water boils at a lower temperature and therefore holds less heat, demanding longer cooking times to achieve the same level of doneness.
Boiling point rises with atmospheric pressure. As pressure decreases, the temperature needed for boiling falls, making evaporation easier and requiring less heat to reach the boiling state.
The vapor pressure of a substance is the pressure exerted by its vapor in equilibrium with its liquid (or solid) phase at a given temperature. For instance, in a sealed container with half a liter of water at room temperature, the water evaporates into the vacuum and establishes a vapor pressure of about 0.03 atm (0.441 psi). Raising the temperature increases the kinetic energy of the molecules, and consequently, the vapor pressure rises.
All molecules vibrate in random directions above absolute zero. As temperature climbs, these vibrations become faster. The distribution of speeds is not uniform—some molecules move slowly while others accelerate rapidly. The fastest ones, upon reaching the surface, possess enough kinetic energy to overcome intermolecular forces and escape into the gas phase. This selective escape is what drives evaporation and elevates the vapor pressure.
In a vacuum, evaporated molecules encounter no resistance and form a vapor freely. In the presence of air, however, the vapor pressure must exceed the surrounding atmospheric pressure for evaporation to proceed. If the vapor pressure is lower than the ambient pressure, the molecules that leave the liquid surface are bombarded by air molecules and are forced back into the liquid, suppressing evaporation.
A liquid begins to boil when its vapor pressure equals the external pressure, allowing vapor bubbles to form and grow. At high atmospheric pressures, a liquid can become extremely hot without boiling because the surrounding pressure keeps the vapor bubbles from expanding. As ambient pressure drops, fewer collisions from air molecules impede escaping vapor. Consequently, the liquid reaches the boiling state at a lower temperature, making boiling easier and often faster under reduced pressure.
