Salt lowers the freezing point of water, which is why it melts ice on winter roads.

The longer version

Dissolved salt gets in the way of freezing. Water molecules normally line up into a neat crystal as they cool, but sodium and chloride ions drift among them and block that tidy arrangement. More cooling is therefore needed before ice can form, so the freezing temperature drops below zero.

The same principle explains why seawater stays liquid well below the point where fresh water turns solid. Dissolved particles lower what chemists call the freezing point in proportion to how many of them are present. Any soluble substance produces some effect, and salt works well because it splits into two separate ions.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.