Kitchen Science Experiments

Melting Ice with Salt

Sun, 17th Jan 2010

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What you Need

Ice cubes

Ice

Salt

Salt and optionally screenwash (be very careful, this should definitely not be drunk)

A glass

Small containers

A thermometer

A thermometer

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.

If seawater is added to ice the temperature falls to just below 0 celcius.

Adding salt to ice, the temperature drops very significantly

It would appear that this screenwash is not very good, and would explain why it froze in my car.

 

 

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

Heat movement

When you add brine (salt solution) to ice the melting point of the ice will be reduced and the ice melts

This takes a huge amount of heat energy, which comes from the surroundings, cooling them down, until the ice melts or reaches the freezing point of the brine.

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.

 

Ice and water

Water and Brine

At 0C there can be the same number of water molecules joining and leaving the ice.

When salt is added there is a lower concentration of water molecules in the liquid to come back and form ice, so the freezing slows, whilst the melting rate is unchanged. So overall the ice melts

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 0C. 

At lower temperatures the rate of freezing is higher, so there is a new freezing point below 0C. In fact, the lowest temperature it is possible to get water using salt is around -18C, and this was roughly how 0F was defined (although using Ammonium rather than Sodium Chloride).

Dave Ansell

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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. Geezer, Sat, 9th Jan 2010



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. techmind, Sat, 9th Jan 2010

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 chris, Sat, 9th Jan 2010

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

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. Wight Portal, 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.

In fact, at what temperature does seawater begin to freeze?

Chris chris, Mon, 11th Jan 2010

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). Bored chemist, Mon, 11th Jan 2010

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.
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. Mazurka, Mon, 11th Jan 2010

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??! 54north, Mon, 11th Jan 2010

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



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.
phatboy, 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


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.


. tommya300, Fri, 13th Aug 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

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