What's the point of gritting the roads?

Have you ever wondered why we grit the roads? I know the obvious answer here is because it makes the ice melt, but why?  Why does gritting the road cause the ice to melt? Well, here is the answer to that question on a molecular level.

Grit (or Halite) is sodium chloride with all the impurities from mining left in. When it is added to ice It causes the freezing point of water to be lowered. So at, say -5C, pure water is solid but water with a touch of NaCl is a liquid.

This is all to do with equilibria, imagine a puddle that's freezing, solid water in the puddle is in equilibrium with the liquid water in the puddle and as freezing is an exothermic process at this low temperature freezing is favoured over melting and the water freezes.

Adding salt to the liquid water means that you are effectively "reducing the concentration of liquid water" because now it is not pure water it is water and salt. What difference does make to the equilibrium? Well, reducing concentration of a reactant favours the production of that reactant so liquid water is favoured over solid water and ice isn't formed.

Is this a special property of ice? Nope, any substance that dissolves in water will do the job, it's just that grit is cheap.

This also explains another weird effect. If you add salt to ice the temperature drops significantly, it can get down to -10C. This is because melting is favoured and melting is endothermic so the process of melting removes heat from the surroundings and drops the temperature.

Feel free to bore your friends with this explanation every time a gritter drives past over Easter.