It’s a hot summer out there, and cold beverages are a great way to beat the heat. But how many times have you been dying for a cold one, only to find it left out of the fridge?
To get those tasty beverages cold in a hurry, here’s a neat trick. Fill a bucket, basin, whatever, with cold water, and dump in your drinks and as much ice as you can find. Not a very clever trick, eh? Here’s the trick: add a bunch of salt. The salt lowers the melting temperature of the ice, allowing the water temperature to drop below zero, pulling the heat out of the drinks faster.
If you’re interested in the details of how this works, read on. If you just want to use the tip, go cool off some beer.
There’s a few concepts in play here, and we’ll go in order of scale. First, it’s obvious that the colder the water is, the faster we’ll cool down the drinks. That’s seen most simply in Newton’s law of cooling,
which tells us that the exchange rate of heat, , between the beverages and the water is proportional to the difference in temperature, , of the beverages and the temperature of the water.
But why does salt lower the melting temperature of the ice? That has to do with the molecular structure of both. Table-salt is a crystal of sodium and chlorine. During the chemical bonding of these atoms, they exchange an electron, making each atom in the crystal an ion, with a positive or negative charge. Water, while being electrically neutral, is a polar molecule because the electrons have a higher density on top of the oxygen atom, and low density around the two hydrogen atoms. This separation of the charge results in an electric field.
When water freezes into ice, it forms crystals where the positive poll (the hydrogen atoms) of one molecule are attracted to the negative poll (oxygen atom) of the next, and so on. The ions from the salt interfere with the interaction between the water molecules making it more difficult for the bonds to form making ice crystals. This has the effect of lowering the melting temperature of the water.