What you Need
What to do
Mix up some seawater by adding about 9g of salt to 250ml of water (or if it is easier, by dipping some out of the sea).
Fill several containers with ice.
Add some seawater to one container, salt in another, and some screenwash in a third.
Measure the temperature change.
What may happen
You should find that all the ice that you have added salt or antifreeze to will reduce in temperature, but the stronger the solution, the greater the reduction in temperature.
Why does it happen?
Salt, and other soluble substances will reduce the melting point of water, so if you put salt on ice, it will now be above its melting point, and start to melt. The stronger the solution, the more it reduces the melting point, so strong solutions will melt ice to lower temperatures than weak ones.
Melting ice involves breaking bonds between the water molecules, so it requires a huge amount of heat energy: This means that it will cool down as it melts, until ether it all melts or it reaches the melting point of the salt solution you have applied. The temperature the ice gets to gives you a good idea of how cold your de-icer will work on the roads.
Why does ice melt when you add salt?
The surface where ice and water meet is a very dynamic; molecules are always melting off and others freezing onto the surface. When the ice is stable (i.e. neither freezing nor melting) these two processes are still happening, but they are in balance. If you reduce the temperature the probability of a molecule melting off will reduce, so the melting slows down, and the freezing speeds up, which means that the ice grows. If you heat it up then the opposite will happen, and melting will dominate.
If you add salt, or any dissolved substance, the water molecules will effectively get lost amongst the salt, and take longer to make it back to the ice, so the rate of freezing will slow down, but the rate of melting will be unchanged, so overall the ice will melt at 0°C.
At lower temperatures the rate of freezing is higher, so there is a new freezing point below 0°C. In fact, the lowest temperature it is possible to get water using salt is around -18°C, and this was roughly how 0°F was defined (although using Ammonium rather than Sodium Chloride).
I think one problem is it would not stay on the road surface for very long. It would wash down the drains. You'd need to have a fleet of tankers continually spraying the roads with seawater. Geezer, Sat, 9th Jan 2010
This is a really interesting and thought-provoking question.
I hate salt on the roads. It corrodes bicycle chains. In Nordic countries people put chains round their car tyres to get more grip on icy roads. FuzzyUK, Sat, 9th Jan 2010
Can someone with "highways connections" please tell us how much salt the gritters deploy per unit length of road? Then we can work out the relative strength of the solution and this will determine whether sea water would be of any use whatsoever.
The amount of salt they use should depend on the depth of snow and the ambient temperature.
In South Lanarkshire they were, report as of Xmas Eve, using 40g per square metre on main roads. That was a thousand tonnes a day. They intended to reduce the amount to 20g. Wight Portal, Mon, 11th Jan 2010
I just happened to be chatting to a highway man a couple of hours ago.
Thanks for the interesting comments, I posted the original question. You've raised some valid points on the salinity of seawater compared to rock salt and transport.