(published on 04/02/2011)
(published on 04/03/2011)
(published on 12/28/2009)
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.
Mike W.
(published on 09/05/2013)
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.
Mike W.
(published on 11/27/2013)
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.
Mike W.
(published on 05/06/2016)