Question: How does gritting roads work?

Neil Keddie answered on 5 Mar 2018:

This seems like a very good question, given the weather we had last week!

When the council grits a road, they are putting down a mixture of rock salt (sodium chloride) and sand/grit. The salt is used to melt the ice that is on the road (or that might form), by lowering the freezing point of the salt/water mixture when compared to water. (Water)Ice forms at 0˚C and can make driving on the roads really hazardous (if you’re ever been in a car that slips on black ice, you’d remember how scary it is!). When we add salt, we can stop the ice forming in temperatures down as far as -18˚C, as a saturated ice/water solution (also known as brine) freezes at that point. The sand in the salt mixture helps to better spread out the salt on the road, and the grit helps provide a little more traction on the road surface.

Try the experiment for yourself. Take an ice-cube, put some salt on it and you’ll see it melts quicker than an ice-cube with nothing on it. Then take the water from those two melted cubes and put them back in your freezer and see which one forms ice again.

In some countries, like Canada, where the temperature can get a lot lower (I’ve felt -40˚C out there – that’s really, REALLY cold!), you only tend to put grit and gravel on the road to help get some grip on the icy roads. In these colder conditions you can sometimes use deicing fluids (instead of sodium chloride) that can stop ice forming until much colder temperatures, however, a lot of these fluids are not that environmentally friendly, so are used less often.

Some of the gritters you might see in Scotland (the big ones with 8 wheels) have blue tanks as well as the big bin for grit and salt. The tanks contain deicing fluid that can be put on the roads as well as salt if there is really solid bits of ice formed.