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I am interested in why water tends to cool to 0 deg C, form ice and then cool further once the ice has formed while a solution of 30% propylene glycol and water will form a slush of increasing density over a wide range of decreasing temperature.
An additional question, as water releases heat during formation does the propylene glycol release heat over the range of temperatures as the slush hardens? Is the reverse true on melting?
- Ken (age 55)
Berkeley, CA, USA
There's a basic reason why that solution freezes to slush over a range of temperatures. The propylene glycol (PG) is almost completely excluded from the ice. As a result, it lowers the freezing point of the water. We've discussed the reason that solutes do this in a number of other answers, e.g.:http://van.physics.illinois.edu/qa/listing.php?id=1601http://van.physics.illinois.edu/qa/listing.php?id=1640
The key point is that as the ice freezes, it reduces the volume in which the solute molecules roam around. Things spontaneously go toward conditions with the most available states, just like how a gas expands to fill the available volume. So this effect favors keeping the water liquid, and lowers the freezing point.
What happens when some ice forms? There's a higher proportion of PG left in the remaining liquid. That means its freezing point is lowered even further. And so on- the sharp freezing point of water is converted to a broad coexistence (slush) range between ice and a range of concentrations of PG-water solutions. The result is very general for solutes which are excluded from ice, as most are.The latent heat released by the formation of ice or taken up by its melting isn't changed much in this solution. The temperature at which happens is just spread out over that whole range, as you surmised.
(published on 02/11/11)
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