We want to know why throwing salt on an icy sidewalk melts the ice. The first thing to point out is that at any temperature close to the melting/freezing point, there is constantly water freezing and un-freezing at the ice surface. This is called an equilibrium. The trick is to do something that doesn’t allow the water to re-freeze anytime it assumes the liquid state. That’s where the salt comes in.
Anytime you dissolve something in a liquid, you cause two things to happen:
- the freezing point of the liquid decreases
- the boiling point of the liquid increases
Think of all the temperatures where the substance is a liquid as being a range—go below that range and you get a solid, go above and you get a gas. Adding another dissolved substance (i.e. a solute, making a solution) widens this range in both directions.
Let’s consider the boiling point because that might be easier to understand. The vapor pressure over a liquid is exactly what it sounds like: the pressure of the vapor trying to evaporate out of the liquid. It increases with temperature, and when it reaches 1 atmosphere, the liquid evaporates. Adding a solute, like salt or NaCl, lowers the vapor pressure (see the figure). So boiling requires a bit higher temperature.
