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When a molecule of a liquid approaches the surface, it experiences a force barrier that tries to keep it in the liquid. It thus has to do work to escape and loses some of its kinetic energy when it leaves.

1. Assume that a water molecule can evaporate from the liquid if it hits the surface from the inside with a kinetic energy greater than the thermal energy corresponding to the temperature of boiling water, 100 ^oC. Use this to estimate the numerical value of the work 'W' required to remove a water molecule from the liquid.

2. Even though the average speed of a molecule in water below the boiling point corresponds to a kinetic energy less than 'W', some molecules leave anyway and the water evaporates. Explain why this happens.

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Page last modified October 30, 2002: H&T05