Antifreeze thought experiment

Let’s say you have a pool of standing water and you pour some standard automobile antifreeze in it. Later on you notice a nontrivial layer of ice on top of the pool. My question is, is the antifreeze concentration in the remaining liquid now more or less the same as it was before, or is it greater (and thus providing greater antifreeze protection)?

If the antifreeze is anything like alcohol (in fact, sometimes it is alcohol), then the remaining liquid will have a higher antifreeze concentration. This is in fact one method for fortifying alcoholic drinks: You leave some weak apple cider out on the back porch in winter, remove the ice in the morning, and end up with strong applejack.

I wasn’t aware of that trick—nifty! It’s reverse distillation!

If the antifreeze (ethylene glycol + water mixture) forms an eutectic then the concentration of antifreeze would be the same in the liquid and the solid.

I found this presentation where it says on page 14 that the mixture has an eutectic point at “just under” 60% ethylene glycol concentration and -50 degC (-58 degF).

There exist places in Canada where you could actually see this in practice.

Please don’t try this experiment in real life, as antifreeze is poisonous. If a dog can along and drank it, it would not be good.

Thank you Chronos. Yes, I didn’t think about it that way. That is the same way they make so-called ice beers.

Ice crystallization does a remarkable job at displacing contaminants. I have a system for creating clear ice blocks, which works similarly to a lake freezing from the top down, and stopping before the entire thing is frozen. I wanted to make colored ice using food coloring. The result was that the top half froze into perfectly clear ice, and the bottom half was liquid water with twice the concentration of coloring.

From memory:

The eutectic point for ethanol/water is somewhere around 95%. Water will freeze out, (with the ice gradually becoming more alcoholic), until the remaining liquid is around 90% ethanol. Then you’ll start to get ethanol/water ice mixed with the water\ethanol ice. The last remaining liquid will freeze out as a 5% water solid.

For Antifreeze, ethylene glycol, a 50% solution will freeze out water until it gets to 60%. A 75% solution will freeze out glycol until it gets to 60%. I haven’t seen the curve so I don’t know how ‘glcolic’ the frozen water will be, or how wet the frozen glycol will be, or if there will be a mixture of the two kinds of ice or only one kind of ice.