Salt Lowers Freezing Point of Water

bbc-snow-eventThis morning, my kids are listening eagerly to the local radio to hear if their school will be closed? Why? Because we’re in England, a few millimetres of snow has fallen, it’s a little chilly, and the nation is in chaos. Airports are shut, driving conditions are almost impossible, and schools are closing across the country. The radio meteorologists are telling us it’s the worst snow “event” since the early 1990s, although it was worse almost exactly twenty years ago as I seem to recall tramping to work in snow when I first moved to Cambridge.

Anyway, what has all this go to do with salt lowering the freezing point of water? Well, The Highways Agency, which looks after our main roads has had its fleet of gritter lorries spray pink grit across the roads. So, what is it they’re spreading? What is that grit? Well, it’s usually common salt, sodium chloride, but calcium chloride is also used.

Dissolving any compound in another will lower its freezing point slightly. So adding salt to water will lower its freezing point. Scattering salt on the roads therefore allows some salt to dissolve in the surface of snow and ice forming on the roads especially as the early cars put pressure on and briefly melt the very surface of any ice as they roll over it. The salt dissolved means that the temperature has to be that bit lower for water to remain frozen or to freeze further.

The same phenomenon, known to scientists as freezing point depression is the reason why the oceans, in all but the most extreme polar winter conditions, tend not to freeze. The salt content lowers the oceans’ freezing point so that conditions have to be a lot colder to make it freeze.

Freezing point depression

Freezing point depression, like boiling point elevation discussed previously on Sciencebase.com, is a colligative property. This means that the effect depends on the presence of dissolved particles and their number, but not their identity. Anything that dissolves in water will lower its freezing point. It does not have to be salt, indeed it does not have to be ionic like sodium chloride.

It is an effect of the dilution of the solvent in the presence of a solute. It is a phenomenon that happens for all solutes in all solutions, even in ideal solutions, and does not depend on any specific solute-solvent interactions. In other words, the explanation does not lie in some kind of interaction between the solute particles (the dissolved material, sodium chloride, usually used in the salt on roads case) and the solvent (the water ice on the roads) that prevents a solid from forming, it’s an entropy effect:

At the freezing (or melting if you’re heating a solid) point, the solid phase and the liquid phase have the same energy. The chemical potential of each is equivalent. Chemical potential depends on temperature, and at other temperatures the solid or liquid will be favoured over the other phase. At higher temperature, a liquid will exist, at lower a solid.

freezing-point-depression

Often, a solute will dissolve only in the liquid and not the solid solvent. If such a solute is added, the liquid is diluted, but its chemical potential does not change. To balance the energy books (you cannot create nor destroy energy), the equilibrium between liquid and solid shifts to a lower temperature, hence freezing point is depression.

Calcium chloride road gritting

So, why choose calcium chloride instead of sodium chloride for gritting roads? Well, both are readily extracted from brine, but CaCl2 has a subtle advantage over NaCl. Not only does calcium chloride shift that energy equilibrium when it dissolves in water so that the freezing point is depressed, it also releases heat when it dissolves in water. This has the distinct advantage of heating the surrounding ice, melting that and allowing more particles of solute to dissolve.

The dissolving process is highly exothermic and in the lab can produce temperatures of around 60 Celsius:

CaCl2 + 2 H2O –> CaCl2·2H2O

Aided by the heat evolved during its dissolution, calcium chloride is also used as an ice-melting compound. Unlike the more-common sodium chloride (rock salt or halite), it is relatively harmless to plants and soil; however, recent observations in Washington state suggest it may be particularly harsh on roadside evergreen trees. It is also more effective at lower temperatures than sodium chloride.

Regardless of the wonders of freezing point depression, at the time of writing, my kids were still huddled around the radio in the desperate hope that they are forced to stay away from school today and build snowmen in the garden and annoy each other with snowballs. And, whether they get to school or not you can rest assured that the “snow event” will continue to plunge Britain into chaos, at least that’s what the BBC will keep telling us.

Wrap up warm, now.

26 thoughts on “Salt Lowers Freezing Point of Water

  1. This website was really helpful! I’m doing a science fair project on the ways to affect the freezing point of water, and I used this sight as my #1 referance! :)

  2. Here’s an interesting twist for teachers to ask their students:

    “Why would adding 10 grams of NaCl to water depress the freezing point more than it would elevate the boiling point? There are no wrong answers but keep in mind you’re talking about the motion energy of the molecules.

  3. Thank you,

    Not sure what you ment, way above my relm of understanding but still thank you it was verry informetive. I belive what it boils down to (pun?) is that 5:1 = -16F for every 5parts water equal 1 part salt.

  4. One can calculate the change in freezing point relative to the pure solvent using the equation:

    Change = i * Kf * m

    where Kf is the freezing point depression constant for the solvent (1.86 degrees Celsius per kg per mole for water), m is the number of moles of solute in solution per kilogram of solvent, and i is the number of ions present per formula unit (e.g., i = 2 for NaCl). This formula is approximate, but it works well for low solute concentrations. 10 grams of sodium chloride in 100 grams of water lowers freezing point to −5.9 Celsius. 20 g in 100 g of water would lower it to -16 Celsius, I believe. Seawater is about 3.5 g per 100 g so doesn’t freeze till about -2, but your suggested concentration is definitely going to kill your grass.

  5. Question-s’
    Do you know how what percent of salt is needed per measurable amount of water to keep it in liquid form at temperatures colder than 32° F?

    Is there a percentage increase per decreasing degree, trying to keep water from freezing outdoors this winter (in 2 qt. containers) might get down to maybe 15° F below~ with out killing the grass?

  6. Good question Aijaz, it’s all to do with energy.

    You’re not adding salt to the icecream, you add it to the ice used to freeze the icecream ingredients. Ice has to absorb energy to melt. When you use ice to cool the ingredients for icecream, energy is absorbed from the ingredients. When you add salt to the ice, it lowers its freezing point so more energy must be absorbed to allow the ice to melt, which means the icecream ingredients get colder and themselves freeze.

    Here’s a recipe for icecream that offers the same explanation – http://chemistry.about.com/cs/howtos/a/aa020404a.htm

  7. ist of all i will thank u sir for giving such a attractive answer with attractive examples
    sir as a collagativa property is the reason for it
    but i don,t understand why is salt used in by the icecream sellers to keep the icecream freez in the summers {sorry for having gramatical errors as it was my first day on broadband net i was some how confused}

  8. thanxxxx! my science project on “why we put salt on ice” makes sooooo much sense now!

  9. its really very interesting thing to know.and to know the reasons of different phenomeno brings great plaesure to naturelover person

  10. This is a really clear explanation of something I personally find very difficult to understand. I had a bet with a friend of mine about this but I can’t remember what I claimed. I now see that he was right and I was just not clever enough to understand his cogent and, I realise now, correct argument. Thank you David Bradley

  11. Both my kids get a day off school because of the “extreme” weather conditions here in Cambridge (couple of inches of #uksnow). They’re both planning independent, mass village snowball fights, I wonder if they’ll overlap and go for all out snow-war.

  12. This is great. Although I have a science degree I was really confused by this issue and didn’t understand it at all. I had a bet with a friend, who explained it exactly as you have and unfortunatley I have lost. Oh well, he is better than me at most things!

  13. Someone was asking me in the pub the other day what grit is supposed to do with ice, and I couldn’t really explain it properly. Cheers, you’ll help me win a bet!

  14. Thanks for verifying, Robert. Someone suggested that the heat from the exothermic hydration of calcium chloride would make little difference to road ice, but I have sources that say it would, and mention of 60 Celsius temperatures observed in a lab.

    Oh, it’s definitely worth jetting your vehicles frequently during gritting season.

  15. Looks entropically accurate to me. Here’s a practical question, though- Will CaCl2 make your car rust out in a few years like NaCl does? I’ll say yes, because it’s the chloride that catalyzes the oxidation of iron.

  16. Thanks Sciencebase. That answers the question I wanted to ask and the ones my son will ask when he gets back from his (sadly for him, open) school.

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