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Sunday, 12 February 2012

Why do we spread salt on snowy roads?

A very seasonal post as London is presently experiencing some nice freezing cold days and nights and the occasional snow!! Hope you like it :)

As we say, a picture is worth a thousands words, so for you I adapted a figure that I found here:
Let's go through it and you'll see it's not that complicated:

In yellow, we have the frame through which we see trends. We are going to follow the horizontal line which corresponds to the Temperature (going up from left 0C to right 100C, as written in pink).

The pink curve represents the behaviour of water depending on the atmospheric pressure (up and down) and the temperature (left and right).

In red, we are going to follow the normal atmospheric pressure which is 1Atm. That is the pressure that we are feeling at sea level.


At the intersections between red and pink is the change of behaviour of water at normal atmospheric pressure (not in altitude!): So the first intersection on the left means that at 1Atm, water freezes at 0  degrees Celcius, and at the second intersection, water boils at 100 degrees Celcius! That is why I wrote Solid before 0C, Vapour after 100, and Liquid in the middle


When we add salt or any other component to water, it's called a Solution!
So now, we are not going to follow the pink curve which is the water by itself, but the black one which corresponds to the Solution!
We see that the black curve intersects with the red line more on the left for its freezing point and further on the right for its boiling point!

And that is why we put salt on snowy roads, because it changes its freezing point to a much lower temperature, so the snow will melt!!

It is also by falling into this logic that we add salt to water when we want to cook our potatoes, the temperature will be higher than 100C and will cook them faster- although admittedly as you'd have to add loadssss of salt to see a big change, it is mainly because our tastebuds just love it so we keep adding it even though chemically there's not much point!


The reason for this phenomenom is that by adding a solute (salt or other) to water molecules, we generate more interactions between foreign molecules and that makes them vibrating at a different speed than normal ( no vibration being the freezing point, the fastest vibration being the boiling point).
Because again, an animation is better than explanation, please do visit this website where you can play with water molecules and solute, and temperature, I loved it!!!

Thanks for stopping by,


  1. What a fascinating post. Very clear and easy to understand. I also loved the link to the animation on that website. So fun to play around with!


  2. Thank you so much! I was getting worried I hadn't any comments yet!! I was afraid I had made it too complicated!! So Million thanks to you :)


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