Why does salt water boil at a higher temperature?

When salt dissolves in water, it splits into individual ions (Na⁺ and Cl⁻) that spread out among the water molecules. These extra particles get in the way of water molecules turning into gas.

For water to boil, its vapor pressure has to reach the surrounding air pressure. Because the salt ions block part of the water's surface, the vapor pressure at a given temperature is slightly lower – so the water needs to get a bit hotter before it boils again.

In the kitchen, though, the effect is tiny: even heavily salted cooking water boils only about 0.2 to 0.5 degrees higher than plain water. So salt isn't mainly there to make water boil faster – it's mostly there for flavor.