Freezing Point of Heavy Water
Most recent answer: 04/02/2011
(published on 04/02/2011)
Follow-Up #1: How to raise the Freezing point of water?
- Matthew (age 27)
For something to dissolve in water, the water must prefer being in the solution to staying apart as pure water. In technical terms, the solute lowers the chemical potential of the liquid water. Unless the solute lowered the chemical potential of the solid ice even more, adding some solute will always favor the liquid, lowering the freezing point. Since the solid ice is a regular pattern of molecules, small amounts of solute break up the pattern, and are excluded from the ice. A few examples are sea water (with salt), sugar water, and water with alcohol. In all three cases, adding the solute lowers the freezing point. Here is a link to a previous question that was asked on our site that gives a more thorough explanation.
The only exception to this argument would be some molecule that fit into the ice structure even better than ordinary water. Actually, water made with the deuterium isotope of hydrogen rather than ordinary hydrogen fits the bill. This "heavy water" actually freezes at 3.8°C (39°F) rather than 0°C.
Thanks for a very interesting question! -Zach (+mbw)
(published on 12/28/2009)
Follow-Up #2: enriching deuterium by freezing
- Jean (age 14)
Now you might wonder if, by holding the temperature at say 1°C, you could get some ice to form that only contains D2O or DHO, since H2O doesn't freeze at all at that T. The loss of entropy in separating the D from the H would prevent any ice from forming in ordinary low-D water at that point. It takes a lot of energy to unmix things.
(published on 04/03/2011)
Follow-Up #3: raising the freezing point
- Ted (age 40)
The true thermodynamic freezing and melting points are identical, so raising either one means raising the other.
Any solute that doesn't get incorporated into the ice lowers the freezing point, for basic thermodynamic reasons, as we discuss elsewhere. () So you have to ask what sort of molecule would become part of the ice without changing its properties enough to make us want to call it something besides ice. Heavy (deuterated) water is the obvious example, so I've put your question in this thread.
(published on 09/05/2013)
Follow-Up #4: freezing at 40?F ?
- Abe (age 37)
You're right that no additive will raise the freezing point of water, except for the unacceptably expensive deuterated water. (Deuterated water also is toxic to plants and animals.)
I'm sorry to say that I can't think of any good water substitute for your purposes. Perhaps some reader will think of one, but that seems unlikely to me.
Here's a possible idea, but I'm not sure it's practical. When the temperature is around 32°F, your rink is too warm. Your house is too cold until you heat it. Maybe you could get a large heat pump to pump heat from your rink to your house. This general type of heat pump is a very efficient way of heating houses in moderately cold weather. I don't know how well one could be adapted to your pond and how much it might extend your skating season. Maybe you could interest some local geothermal HVAC installer in this project.
Something similar, run in reverse, should work well for swimming pools during the period when the pool isn't hot enough yet but the house is too hot. The pool application would almost certainly be doable, since it's easier to exchange heat with circulating liquid water than with ice.
(published on 11/27/2013)
Follow-Up #5: separating deuterium from hydrogen
- Derrick (age 44)
Edmonton Alberta Canada
As we pointed out in the answers up-thread, the freezing/melting process only enriches the concentration slightly. It's not very easy to repeat often enough to get good separation of the deuterium. There's some discussion of the techniques used here: http://fas.org/nuke/intro/nuke/heavy.htm.
(published on 05/06/2016)