Quote:
Originally Posted by Dave R
It's normal for it to freeze, the difference between RV/Marine "anti-freeze" and water is that the "anti-freeze" does not expand when it freezes, thus it does not do any damage.
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That is not really so. Addition of a salt or solvent to water depresses the freezing point of the mixture, and more stuff added depresses the freezing point further. Similarly, adding some water to another solvent depresses its freezing point. A plot of freezing point of the mixture vs concentration typically shows that an abrupt minimum is reached somewhere around 40-60% of the solvent. This point is called the eutectic point.
When a glycol-water mix having less glycol than the eutectic concentration hits its freezing point, some (theoretically) pure water crystallizes out as ice, and the remaining liquid mix becomes more concentrated in the glycol, with a corresponding further decrease in freezing point. This continues all the way down to the eutectic point, where anything left freezes.
This reference shows freezing point vs concentration for propylene glycol ("RV antifreeze") in water:
https://www.engineeringtoolbox.com/p...col-d_363.html