Making a Soda Slush

So I've done a experiment with soda and the steps to turn this soda into slush is to ; shake it up, put it in a freezer, wait 2 hours when your freezer is at 27 degrees take it out, twist the cap, twist it back on , then turn it up side down. Before it was a liquid then once I did these steps it turned to slush automatically. I was wondering how and why did it do this?
- Rob (age 16)
Maine, USA

At 27°F, or -2.8°C, your soda can probably usually supercool. (See  , .) That means it will get that cold but won't start to freeze. Twisting the cap and turning it upside down is likely to trigger the freezing, because the sudden reduction in pressure will trigger a lot of bubble formation, and those bubbles may help nucleate ice formation. Once the water starts to freeze, the formation of the ice releases "latent heat" that warms everything up. Starting at -2.8°C, it doesn't take much warming to get to 0°C. The freezing will stop just before that, because the various solutes (sugar,..) in the soda reduce its freezing point. Each gram of ice that forms releases enough heat to warm almost 30 grams of soda from -2.8°C up to 0°C. So you probably end up with about 3% of the soda in the form of ice, enough to make the whole thing slush.

Without supercooling, at -2.8°C your soda would freeze more solidly. It wouldn't all turn solid, because the sugar etc. that gets left out of the ice would make some very concentrated soda liquid with a freezing point of -2.8°C. You can figure out how big that effect is by taking your slush and leaving it in the freezer another few hours.

Mike W.

(published on 01/30/2015)