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How does salt melt ice?

How does salt melt ice?

How does salt prevent snow and ice forming?

A key part of winter maintenance in the UK is the application of rock salt on roads. Rock salt is used to prevent and melt frost and ice and stop surfaces from refreezing. Rock Salt works by lowering the freezing point of water (the temperature at which a liquid becomes a solid) – a term that is known as ‘freezing point depression’ in chemistry. 

Rock salt spread on roads and pavements mixes with any available moisture and creates a saline solution. This liquid saline solution that freezes at a lower temperature than water. Therefore, any areas that have been ‘gritted’ with salt will not freeze when the outside air or road temperatures drop below freezing point. The actual freezing point of the gritted areas depends on the strength of the saline solutions, however for a normal gritting spread rate of around 30 - 40 g per m2, the salt will be effective to around -9⁰C. Most local authorities or winter maintenance companies decided to grit when temperatures are forecast to be at 1⁰C as ice is likely to form, and they know that advance application of salt is the most effective de-icing method.

Will salt melt snow?

Salt will not directly melt snow, however once it has been dissolved and forms a salty liquid solution it will be able to lower the freezing point. For winter maintenance, salt is spread in advance of snowfall. The saline solution that is created with the first snow fall will help reduce the accumulation of snow and prevent the formation of ice - making the surfaces safer. During prolonged periods of heavy snowfall, snow may need to be ploughed to clear the roads. The advance spreading of salt will this job easier.