|MLA Citation:||Bloomfield, Louis A. "Question 972"|
How Everything Works 17 Jan 2018. 17 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=972>.
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.