Why does an evaporating liquid cool down according to the Kinetic Molecular Theory?
Question
Why does an evaporating liquid cool down according to the Kinetic Molecular Theory?
Solution
According to the Kinetic Molecular Theory, particles of matter are always in motion. In the case of liquids, this motion is more random and faster than in solids, but slower and less random than in gases.
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When a liquid evaporates, it is the highest energy particles, those moving the fastest and with the most kinetic energy, that are most likely to break away and become a gas.
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When these high energy particles leave, the average energy of the remaining particles decreases.
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Temperature is a measure of the average kinetic energy of particles. So, when the average energy decreases, the temperature decreases.
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Therefore, the liquid cools down as it evaporates. This is why we feel cooler when sweat evaporates from our skin, or why water evaporates more quickly from a surface when that surface is heated.
This process is also known as evaporative cooling.
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