Question

why water boils below 100 degree celcius at mountains

Answer 1

because of low temperature results in lowering the density of water

Answer 2

Merry Christmas,

It is to do with the saturation vapour pressure of water (or any liquid). The saturation vapour pressure of water is the partial pressure of water vapour in the air when the air is saturated (i.e. relative humidity is 100%). For example, at 20C, with the atmospheric pressure at 1013.25mb, if the air was saturated, the water vapour would be contributing about 23mb to the atmospheric pressure...