Thermodynamics of Supercooled Water
Most recent answer: 04/02/2014
- John Hobson (age 61)
Devizes, Wiltshire, UK
That's a brilliant question!
(For those who want to follow our earlier discussion of the supercooling effect, see .)
I get ~23% ice for a first crude calculation of fraction that could freeze starting with -20°C supercooled water, just using the standard latent heat and the standard heat capacity. So that's a little higher even without getting into your idea about the increased heat capacity.
Fortunately, the thermodynamic properties of supercooled water have been measured, including the constant-pressure (Cp) and constant volume (Cv) heat capacities. (See e.g. Figure 5 in .) Cp does show an increase of the sort you were thinking about, but it isn't all that big- around 20% for supercooling to about -30°C, and of course less in the range up to 0°C. So that doesn't increse the fraction of ice that can form by a lot. Since any real experiment is done in a cold bottle, you can count some of the bottle heat capacity too, which helps also to increase the frozen fraction a little. When you're done allowing for all these factors, maybe about 30% of the water could turn to ice- and more if the water was a little more supercooled.
So I think the bottom line is that although that was a great idea and does describe a real effect, the main effect is just that slush can have a smaller fraction of ice than you would guess by eye.
Mike W.
(published on 04/02/2014)