Water boils at a lower temperature when heated at the top of a mountain than at sea-level because at the top of the mountain the
Answer Details
Water boils when its vapor pressure equals the external pressure acting on its surface. At sea level, the atmospheric pressure is high, and water requires a higher temperature to achieve the vapor pressure necessary to boil.
However, at the top of a mountain, the atmospheric pressure is lower due to the decrease in the weight of the air column above it. As a result, the vapor pressure required for water to boil is lower, and water boils at a lower temperature. This is because the atmospheric pressure is not sufficient to hold the liquid water molecules together, and they escape into the atmosphere as water vapor, which is the gaseous state of water.
Therefore, the correct option that explains why water boils at a lower temperature when heated at the top of a mountain than at sea-level is: "pressure of the atmosphere is lower".