How Everything Works
How Everything Works How Everything Works

Question 520

What is the principle behind adding salt to water to keep the boiling temperature lower? Do other substances have the same effect?
Actually, it's the other way around! Adding salt or sugar (or anything else that dissolves in water and that doesn't boil easily itself) to water actually raises the water's boiling temperature! That's because the salt or sugar molecules interfere with the evaporation of water molecules and boiling is just a special type of evaporation.

Boiling occurs when the evaporation of water molecules becomes so rapid that bubbles of evaporating water molecules form inside the body of the water itself and are able to grow larger and larger, despite the crushing pressure of the surrounding atmosphere. Below water's boiling temperature, any bubble of water vapor that forms inside the body of the water will be smashed almost instantly. But at water's boiling temperature, the pressure of water vapor inside each bubble is high enough to keep the bubble from being crushed. However, adding sugar or salt to the water makes it harder for water molecules to enter one of these water vapor bubbles because the water molecules in the water cling to the salt or sugar molecules and thus don't evaporate as often. With fewer water molecules entering a water vapor bubble, that bubble can't sustain itself and is crushed. Only when you heat the salty or sugary water above the boiling temperature of pure water is there enough evaporation into each water vapor bubble to support it against atmospheric pressure.


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