Why Put Salt on the Roads?
- Some Ice
- Some salt
- A piece of thread
- A bowl or mug
- Put a piece of ice into the bowl. This experiment works best if the ice is a bit wet, just starting to melt.
- Take some cotton and lay it over the top of your ice cube. If you have a particularly big ice cube you may need to fold it over a few times to make it stronger.
- Sprinkle a little bit of salt over the top of the ice cube, on top of the cotton.
- Leave it for about 10 seconds.
- Try to pick the cotton up!
The ice cube should stick to the thread and you should be able to pick the ice cube up using the thread lying on top of it.
If you measure the temperature of an icecube, it's about zero degrees centigrade. If you then put salt on it and measure the temperature, it plummets down to about -8 or -10 degrees. It's even possible to get down to around -18. In fact, the lowest temperature in the Farenheit scale is actually the lowest temperature you can get by adding salt to ice. If you imagine ice as a big grid or matrix structure, of all these little water molecules are all stuck together a bit like magnets. All the molecules are vibrating a little bit, some more than others. Some of them are vibrating so much they can just leap off the ice and melt into the liquid around it. This takes energy out of the ice cube. Normally another molecule from the water then jumps back in it's spot, giving the energy back to the ice and keeping everything at about the same temperature, about 0 degrees centigrade. With the salt in the way however, the water molecule that has melted gets lost in all the salt molecules, and before another water molecule can rejoin the ice, another one has escaped, and then another. So lots more water escapes, and the ice melts. Melting ice requires loads of energy. Normally when ice is melting, that energy comes from the air, the water that has already melted, whatever is around it. But if you forcibly melt it by adding salt, that energy has to come from the ice itself. This is why it gets so much colder!
What's happening with the thread then?
In the places where the salt is, the icecube is forcibly melted and also those places get very very cold. In the other places where salt didn't land, you now have melted water sitting on a much colder icecube. This causes those parts to quickly refreeze. This sticks the tread to the icecube, and you can pick it up!So why salt the roads?
The reason they do that is salt on the roads will melt the ice, making the roads less slippery. The icy water now freezes at perhaps -5 rather than 0. So it has to be a colder day to make the roads ice up. If you get down to -20 or -30, the ice will no longer work. At these temperature you now need to add a different chemical. Sodium Acetate is the vinegar flavour in salt and vinegar crisps, and it works beautifully and isn't too bad for the environment, either!