Naked Science Forum
General Science => General Science => Topic started by: thedoc on 20/03/2013 18:07:36
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We were asked why councils don't use seawater to melt ice on the roads. Why not find out the answer with an experiment?
Read more about this kitchen science experiment. (http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/melting-ice-with-salt/)
[chapter podcast=2379 track=10.01.17/Naked_Scientists_Show_10.01.17_5420.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) Listen to the Experiment[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/10.01.17/Naked_Scientists_Show_10.01.17_5420.mp3)
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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.
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...We have a limitless supply and even though it would need transporting...
I think this transport is the problem.
Apart from the run-off problem, when you transport and apply rock-salt, that's nearly 100% salt. Seawater is only going to be something like 10% salt if that (someone correct me with a genuine figure, please!) so you're transporting an awful lot of deadweight.
Further, by adding reasonably pure salt to the road to relatively little water on the road, the salt concentration will be fairly high, and it will continue to remain effective even as it gets washed and swept away and the concentration diminishes. If you added seawater, in addition to existing water/ice on the road, the concentration would start out being weaker than that of seawater, and would only get worse as more precipitation or frost/condensation was added to it.
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This is a really interesting and thought-provoking question.
Broadly, I agree with the points raised above.
Seawater is 3-5% salt solution and, by virtue of being a liquid, would not necessarily remain where it was put - especially on hills and slopes where, arguably, grit and salt are most needed. It would also be subject to dilution. To work out whether the dilution effect would be significant we would need to know how much salt councils use per unit distance of road. Then we could calculate the relative proportions of salt to snow to calculate the strength of the resulting salt solution; does anyone know the salt quota per kilometre?
To summarise, while we wait for a contribution from someone who knows the above, there would be problems with the use of seawater as a road-salt replacement owing to issues of making the liquid stay where it was needed, making it work for more than just a limited time and, lastly, sourcing it and moving it around efficiently.
Chris
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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.
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Hi
I'm from the Isle of Wight and we're experiencing our first heavy snowfall for around 26 years, and basically we're not geared up for it and so the Island has come to a halt.
Would mixing sea water with grit help solve the run off problem, or would that again weaken the salt content. If this was to work it would be a lot cheaper than using rock salt.
I envision a cement mixer vehicle with a sprayer to apply the mix to the road.
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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.
In fact, at what temperature does seawater begin to freeze?
Chris
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The amount of salt they use should depend on the depth of snow and the ambient temperature.
Seawater freezes at about -2C (it depends what sea you get it from).
Since it's quite often colder than that the seawater simply wouldn't work (unless you heated it).
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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.
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I just happened to be chatting to a highway man a couple of hours ago.
Cumbria is using an average of 900 tonnes of rock salt per day on 1500 miles of "priority 1 & 2" roads.
However, as supplies of rock salt are running low, industrial salt is being mixed with quarry grit as a supplement which would be applied at slightly higher rates (although would have lower salt content).
The county varied the amounts of grit being applied as conditions and forecasts dictated. Such specialist forecasts are provided by a company called Vaisala whose forecasts are supplemented with fixed weather stations and direct temperature measurements from sensors embedded in the road.
Different application rates are used on different roads in different conditions
To further protect supplies of rock salt / salt mix priority 3 roads will be treated with sea washed stone chips from a local supplier.
Presumably seawater is not routinely used as the risk of compounding an icing problem is far greater & transporting the required tonnages of water around would be prohibitively expensive.
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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.
On salinity, while rock salt might be a more practical and proven solution on main roads and motorways and seawater being only around 3% salt, remember there are hundreds of locations which require ice cleaning. From car parks, pavements, footpaths etc, I see no reason why seawater might not be used for the less critical areas. Currently the only option we have is rocksalt to clear main routes - or nothing at all. The salt is being retained for these important routes, not used on side roads and estates.
Transport:- There are only 3 salt mines in the UK - Cleveland, north east England, County Antrim, Northern Ireland and Winsford, Cheshire (http://www.saltsense.co.uk/aboutsalt-prod03.htm). So all salt has to be transported hundreds of miles already. Most areas are much closer to the sea than they are to one of these mines!
Cost - rock salt seems to be around £50 / tonne + transport from the north of England. many councils seem to be using 250 tonnes / day, that's £12,500 just on salt, before transport. The cost of seawater is, er, nil and transport from origin to area of usage must be less than rock salt for most of the country.
In short, while rocksalt might be the most practical solution for our main roads, I see no reason why seawater can not be used as a cheap and quick 'solution' on less critical areas such as housing estates, pavements, car parks etc.
Even if it's not seen as a good idea, I would like to give it a go! I'm sure spraying a few hundred litres of finest North Sea water on my snowbound estate would improve things. Can you do a special kitchen science on this one??!
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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.
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Worse still, if it becomes really cold then it'll freeze, making a worse ice-rink!
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But wouldnt the water in the seawater freeze?
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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.
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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.
They do the same in eastern European countries where they also use studded tyres. However, they also leave them on well after the ice has gone causing huge amounts of damage to the road.
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Listen to this experiment on our podcast by clicking here (http://www.thenakedscientists.com/HTML/podcasts/show/2010.01.17/)
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... 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.
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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,
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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.
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lol! wow! its so kool.
i cant belive its that simple and so kool
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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
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The amount of salt they use should depend on the depth of snow and the ambient temperature.
Seawater freezes at about -2C (it depends what sea you get it from).
Since it's quite often colder than that the seawater simply wouldn't work (unless you heated it).
BC is this related to
Colligative property
http://www.chemistryexplained.com/Ce-Co/Colligative-Properties.html
From what I read from a different source, I do not know the reliability, I may be in error, that you can get the same effects from sugar but it is way to expensive.
Way I understand that it is not the chemical chemical reaction it is more like creating a molar solution.
The solid state of the salt molecules are racing around trying to desolve. This movement creates the heat that cascades the melt intill the quantity of salt is disolved.
Since salt water is already passed tha molar solution physical state, what is next for it to do?
Pre-storm maintenance crew use a calcium base brine solution to spray the roadway surface area permitting it to dry before the storm, it has to dry to be effective anytime of day or night. Old timers told me this.
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instead of mining rock salt could our abundant supply of seawater not be harvested and seperated ,for its salt content ,and then applied ?????
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best project
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this experiment was bad because samuel macleod said it was
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interesting...
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**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.