The Power of Salt to Melt Ice – How Does Rock Salt Work?
When your local roads or the walkway in front of your house are covered in ice and snow in the winter, either you or the city council will place down salt in order to make them less slippery and dangerous. However, have you ever thought about this and wondered how it actually works? Why in the world would we place salt on our roads and how does the salt cause the ice to melt?
How Does it Work?
Let’s take a look at the simple scientific reason why salt will help to make your pavement less icy. The reason why this works is because the salt changes the freezing point of the water. Pure water freezes when it reaches zero degrees Celsius. However, it has to be -6 degrees for a 10 percent salt solution to freeze. A 20% salt solution will not freeze until it is -16 degrees below zero. This means that the more salt you add to the water on the roads, the lower the temperature will need to be until it freezes.
Of course, this means that by sprinkling salt over the ice, you will be able to melt it. The salt will dissolve into the ice and lower the freezing point, making the roads much less slippery. In fact, if you look closely at a road after it has had rock salt sprinkled on it, you can actually see this process happening. The dissolving will begin right around each grain of salt and the melting will spread out from that point.
Also, once there is a layer of salt on the roads, this will stop any more ice from forming on the road in the future. Over time, the salt is ground down by pedestrians and vehicles, which makes it even more effective. This is known as the salt being “trafficked” and as many people walk on it or cars drive over it, it will be ground into smaller particles until it becomes a salty brine.
Now, this method will only work if the temperature outside is higher than -15 degrees Celsius. This is why they give up on using salt in countries such as Canada where the winter temperatures can be as low as -30 or -40 degrees Celsius. The salt would simply have no effect. Instead, in these temperatures it is better to spread sand over the top of the ice because it provides more traction.
Where Does Rock Salt Come From?
The rock salt that is used on UK roads usually comes from a natural rock salt mine, of which there are several in the UK. These natural deposits of salt were formed millions of years ago, when Ireland and the UK used to be covered by a sea. As the saltwater evaporated over time, there were large deposits of salt left behind on the ground. These salty deposits were gradually covered over and they can be mined today and then distributed by rock salt suppliers.
A machine called a “continuous miner” extracts the rock salt with its rotating sharp steel cutting picks. These picks will grind the salt from the walls of the vast interior of the salt mind. An anti-caking agent is added to the salt and it is then transported to where it is needed.
The Gritting Machine
In order to spread the salt over the motorway in the most effective fashion, a gritting machine must be used. A “gritter” will spray out a thin shower of salt out the back, with a mechanism that allows the driver to alter the angle of delivery and the width according to the road and the conditions. For example, if there is only a light frost on the road then a gritter might only spread about 10g of salt every square metre. However, in the snow they might need as much as 40g in order to melt the increased amount of ice and snow.
A gritter might sometimes go out right before the first snowfall, if such temperatures have been forecast. This will help to stop the ice from forming at all, so that there will be no problem in the first place. In fact, in some councils in the UK there are sensory devices that are embedded in the road surfaces which will predict the weather conditions. However, most councils will simply check the Met Office, which gives a four-hourly forecast and has its own sensors.
So now you know the simple science behind how salt works to keep the roads of the UK free from ice.
About the Author: Laura Walkerton’s favorite season is winter because, although the snow can be a pain, she loves how pretty it looks in the countryside. She has a long driveway up to her house, so she needs a lot of rock salt in the winter to keep it from getting icy.