When pumping out the vapors from a closed container half-filled with water at room temperature, at some point the water begins to boil. Why is that?
The boiling point of any liquid is defined as the temperature at which the vapor pressure of the liquid is equal to the environmental pressure surrounding the liquid.
At sea level, where the atmospheric pressure is 760 mm Hg, water boils at 100ºC. When we create an isolated environment (a closed container) and apply a partial vacuum, the environmental pressure begins to drop. When the pressure reaches the water vapor pressure, the water begins to boil at room temperature.
Were we to boil water for coffee near the Dead Sea (below sea level), the boiling temperature would be higher than 100ºC. In contrast, on top of Mount Everest the water would boil at around 70ºC.