teodor - 2-11-2020 at 10:58
I was not aware about existence of the clathrate SO2 * 5 3/4 H2O. I've just decided to prepare the solution of SO2 in water and put it into a freezer
(in PE bottles) for later usage because I didn't want to prepare this solution every time. I expected that the frozen point of SO2 solution in water
should be lower than that of the pure water - like for ammonia solution which is still liquid at -18C.
I checked the bottles next day and the solution was completely frozen but by very unusual way because the bottles were not deformed at all, and water
always somehow deforms PE bottles in a freezer.
So, I've made experiment: I took 2 transparent bottles and put SO2 solution in the 1st bottle and the same volume of pure (demineralised) water in the
2nd bottle.
During freezing I observed formation of an opaque white substance, like crystals, they were formed on the bottom and on walls. When the solution was
frozen I just compared the ice in 2 bottles. The first is not transparent.
Indeed, the formation of clathrate somehow relaxes the tension during the water freezing process, so the first bottle is also not deformed in this
experiment (the 2nd bottle - with pure water - is still deformed slightly in the middle).
DraconicAcid - 2-11-2020 at 11:11
Solutes will depress the freezing point of water, but you're not trying to freeze the water; you're forming a clathrate. I believe these tend to form
slightly above the freezing point of water.
teodor - 2-11-2020 at 11:17
Yes, indeed, for SO2 clathrate the melting point is 7C.
I found some interesting information which proofs somehow my observation about the tension:
"Water crystallizes in the cubic system in clathrates, rather than in the hexagonal structure of normal ice" - from http://condor.wesleyan.edu/ethomas/ees123/clathrate.htm