2013: Advent Chemistry: Freezing Mixture

by on December 4, 2013

How do you make things cold?

Chemistry the way these folks like it

Chemistry the way these folks like it

This isn’t an academic question. Quite a lot of reactions, some of them very ordinary lab procedures, put out heat. You don’t want to burn yourself or your worktop on scalding hot glass because your mixture is hot, and you don’t want things to catch fire, and often you don’t want them to start producing lots of steam. Other reactions will actually happen differently at different temperatures, and if you let them get warm, you won’t make the product you wanted. So when you’re getting your sodium hydroxide to dissolve, or making your nitrating mixture, you need to keep them cold.

Working in a refrigerated cabinet would be awkward and uncomfortable, not to mention an inefficient use of energy. You don’t need to cool the whole environment, just your beaker of stuff, so you get a plastic bowl, fill it with ice, and nestle your beaker in that like a bottle of wine.

That works pretty well, but maybe you’d like to cool your mixture faster, or take it down below 0° Celcius. You’d think you’d need a freezer to do that, but you don’t. There’s a clever trick.

Take your bowl of ice, slosh in a bit of water, and add ordinary table salt. The salt immediately starts dissolving, but salt is the opposite of sodium hydroxide in one important respect. Sodium hydroxide gives out heat when it dissolves in water – sodium chloride takes heat in. In the process of dissolving salt, water gets slightly colder.

Ordinarily this effect isn’t noticeable, because the water is warm enough to handle it. When your water is already at 0°C, because it’s full of ice, the mixture gets noticeably colder than freezing. Surround a beaker of chemicals with below-freezing slush, and you’re going to have cold chemicals. Also, making liquid water be below freezing point while it’s sitting in the open on your workbench is fun.

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