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.
If the temperature is below about -2C then putting sea water on snow just makes it wet snow. It doesn't melt it like salt. Bored chemist, Tue, 12th Jan 2010
Worse still, if it becomes really cold then it'll freeze, making a worse ice-rink! chris, Wed, 13th Jan 2010
But wouldnt the water in the seawater freeze? ch3ls3a, Sun, 17th Jan 2010
Yes, if it was cold enough the water would freeze out of the saltwater. Since the point of the exercise is to get rid of ice, adding seawater wouldn't help. Bored chemist, Sun, 17th Jan 2010
... RUST! Come on, any place that salts its roads regularly also has a high turnover of cars - ever seen a car older than 15 years in Montreal? RUST EVERYWHERE. CarLover, Wed, 20th Jan 2010
I drive a gritter for fife council in Scotland. our standard spread(preventive for frost) used to be 20 grammes of salt per square metre of road. Since the "big freeze" we have reduced the amount to ten grammes,this is doing a far better job,the cars are running the salt in and you can see where the salts working,where as before with 20 the whole road was slick and wet with brine. On a 10 gramm spread my route is 92 kilometers and i use 2.2 tons. Not all is gritting,some is free running,and there is travel to and fro my route, The Gritterman, Tue, 2nd Mar 2010
I can't help thinking that seawater may be an instant melting solution if poured in quantity to an area, just as hot water melts ice, but the relative sality of the sea water wouldn't provide a strong enough concentration of salt if it was briefly sprayed on roads and paths by some sort of machine like a gritter surely? I think people are forgetting that overnight temperatures have regularly gone down to as much as -5 degrees, which is lower than the freezing point of seawater which is about -2 degrees, so the seawater would still freeze and add to the problem. I think we really need to look at non chemical methods to deal with snow and ice as they do in countries like Canada where in certain places it's the law that you have to clear the snow from the pavement outside your property daily so the paths are clear to walk on and the snow doesn't get compounded and freezes into ice in the first place and then they sweep the snow off all the roads and remaining paths with road sweeper type machines. Mechanical clearance instead of chemical. Not so fun I know, but cheap and simple. Loubielou, Wed, 3rd Mar 2010
lol! wow! its so kool. i cant belive its that simple and so kool kabina, Sat, 20th Mar 2010
The answer must be to use any surplus heat from the sun to evaporate the water from the seawater in the summer and then store the salt till the winter. It was quite common in the past to have brine pans on the coast where salt was collected from sea water JohnM, Fri, 7th May 2010
instead of mining rock salt could our abundant supply of seawater not be harvested and seperated ,for its salt content ,and then applied ????? graham fraser, Fri, 7th Jan 2011
best project halona, Tue, 25th Jan 2011
this experiment was bad because samuel macleod said it was pad, Tue, 4th Nov 2014
interesting... sot, Sun, 28th Jun 2015
**This means that it will cool down as it melts** This seems incorrect. The temperature of the ice will not decrease as it melts. The energy fro the phase transition is coming from the environment, not from the ice. Melting is an endothermic reaction. BopPunkGreg, Tue, 30th Jun 2015