The vapor pressure of a substance describes how readily molecules at the surface of the substance enter the gaseous phase. At the boiling point of a liquid, the liquid's vapor pressure is equal to or greater than the atmospheric pressure exerted on the surface of the liquid. Since the atmospheric pressure at higher elevations is lower than at sea level, the boiling point of water decreases as the elevation increases. The atmospheric pressure at sea level is 760 mmHg. This pressure decreases by 19.8 mmHg for every 1000-ft increase in elevation. Elevation Pressure0 m 760 mmHg1000 m 695 mmHg2000 m 630 mmHgThe boiling point of water decreases 0.05 ?C for every 1 mmHg drop in atmospheric pressure. A) What is the boiling point of water at an elevation of 7000m ?