685. Why does water boil at lower or higher temperatures under varying atmospheric pressures? Do changing vapor pressures above a liquid play a role in changing boiling points of liquids? — KC, East Greenwich, RI
A liquid boils when its vapor pressure reaches atmospheric pressure. While a liquid will evaporate at temperatures below the boiling temperature, that evaporation only occurs from the surface of the liquid. That's because atmospheric pressure crushes any bubbles that try to form within the body of the liquid. Every once in a while, a few molecules of the liquid break free inside the liquid and form a bubble of gas. The pressure inside such a bubble is the vapor pressure of the liquid at its present temperature. If the liquid's temperature is below its boiling temperature, atmospheric pressure is greater than the pressure inside one of these spontaneous vapor bubbles and it crushes the bubble. But once the temperature of the liquid reaches the boiling temperature, the bubbles will have enough pressure to remain stable against atmospheric pressure. Each bubble that forms begins to float upward toward the top of the liquid and more molecules evaporate into it as it rises, so that it grows larger and larger.
If you lower atmospheric pressure, the liquid will boil at a lower temperature because the vapor pressure reaches atmospheric pressure more easily. If you raise atmospheric pressure, the liquid will boil at a higher temperature because the vapor pressure must rise higher before it reaches atmospheric pressure.