How Everything Works
How Everything Works How Everything Works

Question 200

Does salt/sugar raise the boiling point or lower it? I thought you added salt to water so that it could form bubbles faster, but if the boiling point goes up isn't this kind of pointless?
When salt or sugar are dissolved in water, they raise the boiling temperature. It reduces the fraction of water molecules at the surface of the water, so that fewer leave each second. Because of this reduced leaving rate, the water has to get hotter before enough water molecules will leave each second to allow evaporation to occur inside the liquid (i.e. for the water to boil). As for the value of adding salt, sugar, or any other material to water to encourage boiling, that is a very different matter. Adding anything that can serve as a site for bubble formation will help the water to boil. Nucleating the tiny bubbles that eventually grow into the large bubbles we associate with boiling isn't easy. Often it occurs at a hot spot in the pot, or near a defect on the pot's inner surface. If there aren't any hot spots or defects, then adding sharp objects will aid bubble formation. That's why sprinkling sugar or salt into extremely hot water can help it boil. This boiling occurs before the sugar or salt dissolve. They are just acting as nucleation sites. You'd do just as well to add sand, which doesn't dissolve at all.

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