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- Describe evaporation in terms of the escape of more energetic molecules from the surface of a liquid.
Liquids absorb heat from their surroundings and the sun. This increases their kinetic energy. At the surface of the liquid, a few molecules eventually gain enough kinetic energy to overcome the intermolecular forces surrounding it and escape as a gas. This process is evaporation.
Note that evaporation is a superficial process – it only occurs on the surfaces of liquids, unlike vaporisation, which can occur anywhere within the liquid (that’s why bubbles of gas form when we boil water).
- Demonstrate understanding of how temperature, surface area and air flow over a surface influence evaporation.
The higher the temperature, the greater the rate of evaporation. As the temperature increases, more energy is supplied to the liquid molecules, allowing more surface molecules to gain enough energy to escape the liquid at a time.
As the surface area increases, so does the rate of evaporation. This is because more of the liquid is exposed to the atmosphere, so there are more surface molecules that can absorb the surrounding energy and eventually escape.
Increased air flow also increases evaporation. When wind blows across the surface of the liquid, the airborne liquid particles are swept away. This decreases the humidity in that region, allowing more molecules to dissipate into the air.
- Relate evaporation to the consequent cooling.
During evaporation, the most energetic molecules escape into the atmosphere, carrying away their energy with them. This means there is less total energy left in the liquid.
As temperature is a measure of the average kinetic energy of the liquid, and there is less kinetic energy, the temperature of the liquid also falls, and thus, the liquid cools.
Notes submitted by Sarah.
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