Identify the factors that speed up or slow down evaporation, and understand why evaporation causes a cooling effect on its surroundings.
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Core principle that evaporation cools things down.
Evaporation always causes a cooling effect. When water evaporates and changes from a liquid to a gas, it absorbs heat from its surroundings. As this water vapour escapes into the air, it takes the heat away with it, leaving the remaining surface significantly cooler.
Factors affecting evaporation speed.
Ever noticed how water spills dry up faster if you wipe them across a wide area? Evaporation speeds up when water is spread out because more of it is exposed to the air.
Similarly, heat plays a huge role. Water left in bright sunlight will evaporate much faster than water left in a cool, shady spot. This is because higher temperatures give water molecules the energy they need to escape into the air.
Illustration of Fig 8.6.
Higher temperatures provide more energy, causing water to evaporate much faster.
Examples of cooling: matka, sweat, sanitiser.
Why does water stay so cold in a clay pot (like a matka or surahi) but not in a stainless steel jug?
Clay is porous, meaning it has microscopic holes. Water constantly seeps through these tiny pores to the outer surface of the pot. As this outside water evaporates, it absorbs heat from the water inside the pot, dropping its temperature. Stainless steel has no pores, so this cooling evaporation cannot happen!
Diagram of a Surahi/Matka cooling.
Water seeps through tiny pores in the clay and evaporates, taking heat away from the stored water.
MCQ on evaporation factors.
In which condition will a wet towel dry the fastest?