Pure water freezes at zero
At one atmosphere, pure water freezes at 0 °C. At that temperature water molecules join the ice crystal as fast as they leave it, so ice and water can coexist. Drop the temperature slightly and joining wins, so the ice grows; raise it slightly and leaving wins, so the ice melts. To freeze, molecules must settle into an orderly hexagonal arrangement, and that means giving off heat. This latent heat, absorbed when ice melts and released when water freezes, is the key to several things later in this guide.
Dissolved particles get in the way of ice
When salt or sugar is dissolved in water, other particles sit among the water molecules. Ice crystals are made almost entirely of pure water molecules, so these particles get in the way of water molecules joining the crystal. With fewer molecules able to join, the temperature must drop further for ice to grow. This is freezing point depression. Interestingly, how far the freezing point drops depends almost entirely on the number of dissolved particles, not on what they are. Salt, sugar or alcohol, the same number of particles in the same amount of water lowers it by about the same amount.
Why salt beats sugar
Because the count of particles matters, salt and sugar have very different effects at the same weight. Salt splits into two ions, sodium and chloride, when it dissolves, while each sugar molecule stays one particle. Sugar molecules are also much heavier. Salt's molar mass is about 58.4 grams and sugar's about 342.3 grams, so one gram of salt yields more than ten times as many particles as one gram of sugar. That is why quite sweet sugar water still freezes close to zero, while a little salt noticeably delays freezing.
- Salt: splits into two ions
- Sugar: one molecule stays one particle
- Gram for gram, salt gives over ten times the particles
How far it drops
In dilute solutions the freezing point falls about 1.86 degrees for every mole of particles dissolved in a kilogram of water. Dissolve 58.4 grams of salt, one mole, in a kilogram of water and you get two moles of particles, lowering the freezing point to around minus 3.7 °C. In concentrated solutions this calculation is no longer exact, but the trend holds. Seawater, at about 3.5 percent salinity, starts to freeze at about minus 1.9 °C. As seawater freezes, salt is pushed out of the crystal, so sea ice is far less salty than seawater. Adding more salt does not lower the point indefinitely; salt and ice can get down to about minus 21 °C at most.
De-icing winter roads
Salt-based de-icers are spread on winter roads to dissolve into the thin film of water on the ice and lower its freezing point. If the freezing point drops below the air temperature, the ice stays melted. In very cold weather, though, far less can dissolve and the effect falls off sharply. That is why very cold places sometimes use de-icers such as calcium chloride, which work to lower temperatures and give off heat as they dissolve. De-icers rust metal, can harm roadside plants and soil, and can irritate pets' paws, so wash your dog's feet after a winter walk.
Salt on ice makes it colder
Mix salt into ice and the ice begins to melt, drawing the heat it needs from its surroundings and from itself, so the temperature drops below zero. This is how ice cream was made before freezers. Put the mix in a container inside a tub of ice and salt, turn it, and the mix meets temperatures below zero and freezes. Conversely, the way sugar lowers the freezing point keeps sorbet and ice cream from freezing rock hard. Strong spirits don't freeze in a home freezer for the same reason: the dissolved alcohol lowers the freezing point a great deal.
Boiling point rises too, but only slightly
Dissolved particles also raise the boiling point. For water, though, the rise is smaller than the freezing point drop, only about 0.51 degrees per mole of particles in a kilogram of water. Adding 10 grams of salt to a litre of pasta water raises the boiling point by less than 0.2 degrees. Claims that salted water boils faster or cooks pasta faster have little basis; think of the salt as seasoning. The burst of bubbles when you add salt happens because the grains give bubbles places to form, not because the temperature rose.
Common misconceptions and cautions
Freezing point depression is simple, which makes misconceptions easy. Keeping these in mind helps with winter decisions. Do not eat de-icers or let them stay on your skin, and store them out of reach of children and pets. Washing your car, including the underside, reduces corrosion.
- Salt always melts ice: in severe cold it works far less well
- Sugar lowers the freezing point as much as salt: gram for gram it's much weaker
- Salt makes water boil faster: the boiling point barely changes
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