why does water boil at lower temp

[tapmadi]

why does water boil at lower temp at highe altitudes...detailed advance response appreciated

[bernardo]

All the molecules in a body of water are whizzing about in all directions all the time. The ones at the surface of the water whizz away upwards into the air, thereby making the water evaporate gradually. The molecules below the surface of the water want to whizz about, but they are prevented from escaping by the pressure of the water and air above.

When water is heated, the molecules get hotter so they get more excited so they whizz about faster and more vigorously. Eventually the water gets so hot that the molecules of water below the surface become so energetic and fast that they are able to overcome the pressure from above. This leads to the formation of bubbles at the bottom of the kettle. Therefore water molecules (steam) escape from the liquid from all levels, rather than just the top surface.

At higher altitudes, the air pressure is lower, so there is less pressure pressing down on the water from above, so it is easier for the molecules to reach the point at which they are strong enough to form bubbles below the surface.