Rock salt is the most effective de-icer, but… why?

Unless you have been living under a rock or, like bears, you practice some form of hibernation, you’ve probably seen fleets of gritter lorries up and down the country, spreading salt to de-ice our roads in winter.

Salt is by far the best and most common de-icing agent for road maintenance, mainly because:

  • It is easy to spread / use
  • It is cheap and cost-effective
  • It is available in large quantities (In the UK, rock salt comes from three main mines)
  • It can be stored easily and for very long periods of time without losing its melting capacity

So, what is it that rock salt does to be such an effective de-icer?

Due to its mineral properties salt has the power to melt ice and snow and keep it from refreezing. The way the melting process works is quite simple and it can be explained in a very brief chemistry lesson; are you ready?

Let’s start by remembering that the freezing point of water is 0° C; meaning that when the temperature of water reaches this point, ice forms.

Salt has the power to lower this freezing point so, by adding salt, foreign particles are introduced, dissolving into the liquid water in the ice and making its freezing temperature drop, forcing it to melt. This is usually referred to as ‘freezing point depression’.

Photo by Andrew Kelsall / CC BY 2.0

Rock salt is by far the most effective de-icer (Photo by Andrew Kelsall / CC BY 2.0)

If you have ever spread salt over ice or snow, you might have seen how as soon as the grains of salt touch it, the snow immediately starts melting, spreading out from that point. This is the dissolving process.

It is important to note that freezing point depression is a colligative property of water, which means that it depends on the numbers of particles in a substance therefore, when it comes to ice, the temperature will drop depending on the amount of rock salt introduced.

For example, if you add a:

  • 10% salt solution, the freezing point drops to -6° C
  • 20% salt solution, the freezing point drops to -16° C

Sadly, there is a limit to the power of salt and it is that it can only act as a stable de-icer in temperatures above -10° C. If the temperature of the road is lower than that, then the salt won’t really have any effect.

So, there you have it, next time you spot those lorries de-icing our roads and motorways, you will know what exactly is happening there and why rock salt is such an effective de-icer.

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