Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

On a hot day a person cools down as sweat evaporates from the skin. Which property of water explains why this evaporation lowers skin temperature?

A.Breaking many hydrogen bonds as molecules escape absorbs a large amount of heat from the skin
B.Forming new hydrogen bonds among sweat molecules releases stored heat onto the skin
C.Water dissolves salts from sweat glands and removes heat dissolved in those salts
D.Water freezes on the skin into crystals that absorb energy while melting

Explanation

Core Concept

The water molecules that evaporate are the most energetic ones at the surface, and escaping requires energy to break the hydrogen bonds holding them to their neighbors. That energy is drawn as heat from the skin, which therefore cools. This high heat of vaporization is a direct consequence of the same hydrogen bonding that gives water its high specific heat. Salt removal carries away negligible heat, bond formation would release energy rather than absorb it, and sweat does not freeze at skin temperature.

Correct Answer

ABreaking many hydrogen bonds as molecules escape absorbs a large amount of heat from the skin

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