Why Boiling Water Sometimes Erupts When You Add Salt

Anyone who has dropped a spoonful of salt into a pot of water sitting right at a rolling boil has probably seen it suddenly foam up and nearly boil over. The cause has almost nothing to do with the salt changing the water's chemistry and everything to do with tiny irregularities on the surface of the salt grains themselves. Boiling requires bubbles of water vapor to form, and those bubbles need a physical starting point, called a nucleation site, to begin growing -- perfectly smooth, still water can actually be heated slightly past its normal boiling point without forming a single bubble, a state called superheating. The rough, jagged surface of undissolved salt crystals provides exactly the kind of tiny pockets and edges where trapped air and water vapor can gather and form a bubble far more easily than in the open water. When a spoonful of salt is dropped into water that's already superheated or right at the boiling threshold, it introduces hundreds of these nucleation sites all at once, and bubbles form rapidly and simultaneously across every grain. This sudden burst of bubble formation releases a large volume of trapped vapor almost instantly, which is what pushes the surface of the water up and can cause it to spill over the sides of the pot. The same eruption can happen when other rough or porous objects, not just salt, are dropped into overheated water.