Why is the boiling point of a liquid lowered at high altitudes?
A liquid boils when the (upward) pressure exerted by its vapours and the (downward) pressure exerted by the atmospheric pressure becomes equal. At high altitudes, the atmospheric pressure becomes very low. It takes less energy for the vapour pressure to reach the atmospheric pressure and that is why the boiling point of the liquid is lowered at high altitudes. For example, water boils at 212°F at the sea level but on top of a high mountain, it boils at 176°F.
