The ___ vapor pressure at high altitudes causes a liquid to boil at a temperature.

The _HIGHER__ vapor pressure at high altitudes causes a liquid to boil at a _LOWER___ temperature.

Explanation:

Vapor pressure is the pressure applied by the molecule of a liquid at the surface of the liquid as it transitions into a gaseous phase. When the vapor pressure exceeds the atmospheric pressure, this is when the liquid begins to boil. Therefore when the atmospheric pressure is low, then the liquid will boil at lower temperatures, because the vapor pressure of the liquid will overcome the atmospheric pressure at lower temperatures.

The reduced vapor pressure at high altitudes causes a liquid to boil at a lower temperature.

Explanation:

A liquid will change its form when its vapor pressure higher than the atmospheric pressure. At higher altitudes, the air pressure will be lower since there will be less air above your head compared to the sea level. This means the atmospheric pressure will be lower, and the vapor pressure needed for the liquid will be reduced.  

The pressure is directly proportional to temperature. Since the vapor pressure required is lower, the liquid can start to boil at a lower temperatures.

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