How Everything Works
How Everything Works How Everything Works

Question 972

I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come to a boil. But does it take longer? As a cook I've always been told to add a little salt to the water to bring it to a boil faster. It seems to work or is that just the power of suggestion? If it does boil faster, why does it? — ND, Ashland, OR
I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure water will reach its boiling temperature first.

However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that are boiling while others aren't. The edges and corners of crystals are particularly good at nucleating bubbles, so that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil when it really isn't.


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