At 0°C the solid is in equilibrium with the liquid.
Solid particles that vibrate too fast break loose and join the liquid phase.
Liquid particles that hit the surface and lose their speed cause more vibrations for nearby solid particles but stick and join the solid phase.
These two types of events happen at the same rate at 0°C for water. That is the meaning of equilibrium: just as many molecules of the solid become molecules of the liquid and vice versa.
Above 0°C more solid molecules become liquid than vice versa.
Below 0°C more liquid molecules become solid than vice versa.
For solutions (homogeneous mixtures of a solute and a solvent, like salt dissolved in water) the melting point is less than 0°C.
The molecules in the solid become liquid at the same rate in a solution at 0°C as they do in pure water at 0°C.
The molecules in the liquid become solid at a lower rate in a solution at 0°C than they do in pure water at 0°C.
The same number of particles hit the surface of the ice per unit time but fewer of them are particles of water.
Particles of the solute do not stick to the ice.
As a result, the freezing rate (the sticking rate of the liquid molecules) goes down.
The temperature drops when salt is added to ice.
Temperature is a measure of the average speed of a collection of molecules.
The faster the average speed of the molecules is, the higher the temperature. The slower the average speed, the lower the temperature.
Heat is energy that can be moved from place to place. When heat enters a substance it makes the molecules move faster: this is why heating some increases its temperature. When heat leaves a substance it makes the molecules move more slowly. When the molecules move more slowly on average for the whole collection of molecules then the temperature decreases.
Making molecules in the solid vibrate fast enough to come loose requires energy (heat).
This heat comes from surrounding molecules of the solid and liquid. When fast-moving molecules strike the molecules in the solid they can give them enough energy to break free. A similar example is if you strike a billiard ball that is standing still with a fast-moving cue ball. The cue ball slows down and the other ball speeds away.
This sets up a situation in which feedback affects the process and complicates it:
The solid melts more.
The solid and surrounding liquid gets colder.
The solid melts less.
I wrote this page to help organize my thinking about a presentation I do for students before doing the Salt and Ice Lab.
Last updated: Jun 10, 2011