I had some really nice CuSO4x5H2O crystals that I allowed to sit out for about 2-3 months and now they have turned a chalky white. They look more
like gypsum that had some CuSO4 mixed in with it but I'm certain they are pure CuSO4.
I don't have any tests of weight before and after (when deep blue) and I haven't tried dissolving these in water to see if they dissolve more easily
than the pentahydrate.
The crystals have been in room temp with mildly dry air, so I would doubt that the humidity has been less than ~20% or so and temp about 68-72. hissingnoise - 10-12-2018 at 04:21
Now you know that hydration gives CuSO4 its blueness...
You'll likely find some blue within the crystals.
[Edited on 10-12-2018 by hissingnoise]Ubya - 10-12-2018 at 06:27
now you know why they suggest to apply a coating of nail varnish on the crystals if you are planning on storing themRogueRose - 10-12-2018 at 07:05
The odd thing is that the penta hydrate has a decomp point at about 230F. I didn't think it was possible to loose the water below this temp even if
the air is dry. Will the air pull the water from the crystal even at ~70F? That seems very odd from my experience.MrHomeScientist - 10-12-2018 at 07:24
It will indeed. It's happened to my crystals, too. It's an equilibrium thing; it doesn't need to be hot to lose water, just be in an environment that
has less water in it than the crystal does. It's slow but it does happen, as you observed.
I did an awesome experiment in one of my videos a while back: you can also suck the water out of the crystal by immersing it in concentrated sulfuric
acid. It almost immediately starts turning white, and after some time it disintegrates into powder. I waited a month or two before poking at it, but
it might happen significantly faster. I just never messed with it because the crystal never lost its shape, despite being reduced to powdery
consistency!woelen - 10-12-2018 at 07:43
The extraction of water from crystals indeed happens. Slowly by dry air, quickly with strong dehydrating agents.
I have done the same experiment as MrHomeScientist. With CuSO4.5H2O you see that the blue crystals quickly become much lighter when added to
concentrated H2SO4.
Even more impressive is adding solid CuCl2.2H2O (which has a nice bright cyan color) to conc. H2SO4. As soon as the crystals are added, they become
brown like chocolate. This is due to formation of anhydrous CuCl2, which is brown (yellow/brown like mustard, when ground to a fine powder).MrHomeScientist - 10-12-2018 at 08:23
Thanks, I'll have to try that one!DraconicAcid - 10-12-2018 at 09:07
The odd thing is that the penta hydrate has a decomp point at about 230F. I didn't think it was possible to loose the water below this temp even if
the air is dry. Will the air pull the water from the crystal even at ~70F? That seems very odd from my experience.
The decomposition point of a hydrate is like the boiling point of a liquid- it will happen very quickly at that temperature, but it will also happen
slowly at much lower temperatures.
At any temperature, you will have the equilibrium reaction:
CuSO4*5H2O(s) = CuSO4*H2O(s) + 4 H2O(g)
At the decomposition point, the eq'm constant is 1. At lower temperatures, you will have a smaller equilibrium constant, but if the partial pressure
of water is lower than the vapour pressure of the hydrate, it will dry out. Tsjerk - 10-12-2018 at 09:53
Apperently the vapour pressure of the pentahydrate is 7.3 mmHg at 25oC (Reddit), how that translates to a humidity in which it is stable I
don't know, but it is about 5.5x lower than water at that temperature. Would that mean a humidity of 100/5.5 = 18% at 25 degrees? (wild guess)
Edit: I remember a classmate wondering why his copper sulfate turned white and wouldn't dissolve in the 0.05 molar sulfuric acid he "prepared"... His
volumetric flask was very heavy... He didn't dilute the acid but just poured it from the bottle 96% sulfuric acid.
[Edited on 10-12-2018 by Tsjerk]fusso - 10-12-2018 at 11:11
Will storing hydrated crystals in a humid environment (eg living in a humid area/put a small cup of water beside the crystal in the same airtight
container) keep the crystals hydrated?MrHomeScientist - 10-12-2018 at 11:23
Well I live in Florida and mine dry out, so "choking humidity" doesn't seem to be humid enough.unionised - 10-12-2018 at 13:57
Would that mean a humidity of 100/5.5 = 18% at 25 degrees? (wild guess)
[Edited on 10-12-2018 by Tsjerk]
Yes.RogueRose - 10-12-2018 at 13:59
I guess why it is so odd is b/c I had a large container of CuSO4 crystals (about 7-8lbs) that had recrystalized in to one large block of pentahydrate
and it sat in the same house for about 2 years, open to the air. After it sat out for that long, I recrystalized them using distilled water and a
very fine filter (and activated carbon), and I ended up with these crystals, of which I left about 200g out in the air. This time they turned chalky
white.
Now I do think the previous CuSO4 that sat out for 2+ years and stayed the same dark/bright blue, there might have been a little H2SO4 on the surface
of them where the crystals that turned white still had about 200g CuSO4 liquid left after the crystals formed, so these were more pure and didn't have
any H2SO4 (even in small amounts) left on them. I wonder if that could have been the difference.
On another note, I did find that my FeSO4 (heptahydrate??) has also turned a chalky color like a very light/whitish green w/ a little yellow. These
were left out at the same time pretty close to each other. When I did this before with FeSO4 I ended up with brownish/orange coating on all the
beautiful green crystals.
I'm wondering if my air isn't much dryer this year even though both sat open over last winter when it was dry as well.
[Edited on 12-10-2018 by RogueRose]unionised - 10-12-2018 at 14:01
I'm pretty certain they will be more or less the monohydrate.
Removing the last mole of water is much more difficult than the first 4.RogueRose - 10-12-2018 at 14:16
So here are some pics of the copper and iron sulfates that have air dried. I squeezed both between my fingers and both crumbled like chalk and they
weigh significantly less, like 1/2 the weight I would guess, especially for the iron sulfate
The red circle is a crystal I squeezed with my fingers and it shows that it has dried completely through the whole thing. It has turned completely
white. The iron sulfate looks kind of like popcorn!
[Edited on 12-10-2018 by RogueRose]walruslover69 - 10-12-2018 at 14:46
At 25C the partial pressure of water in air at 100% humidity is 30mmHg. Going off the vapor pressure of CuSo4(H2O)5 at 7.5mmHg that would lead me to
believe that dehydration to the monohydrate wouldn't occur at relative humidities greater than 25%. The dehydration clearly takes place at higher
humidties than 25%, I wonder what accounts for the discrepancy?
[Edited on 10-12-2018 by walruslover69]AJKOER - 11-12-2018 at 14:44
The structures loaded with copper ions and exposed to air, water vapor, dust particles rich in other metals, with residual acid are not likely inert
in my speculation.
So, in time, they literally breakdown.
[Edited on 11-12-2018 by AJKOER]pneumatician - 11-2-2019 at 18:27
if you tried to read the posts you would understand how the sun has nothing to do in this caseQuibbler - 12-2-2019 at 05:48
I've had the opposite happen. I have some anhydrous CuSO4 in a sealed bottle which (over about 10 years) has turned blue.
It's a glass bottle with a plastic top so I suppose the plastic is pourous to water vapour. At the moment we are in the middle of a severe dry season
and I'm measuring the humidity at 60% (26 °C).
The decahydrate can be dehydrated very easily using methanol. The decahydrate inintially dissolves in the methanol (to give a blue solution) then over
about an hour a white solid precipitates out and can be filtered off. MrHomeScientist - 12-2-2019 at 06:06
I'm sure copper sulfate has an equilibrium hydration level that it wants to be at relative to its environment. So the pentahydrate loses water and the
anhydrous gains it until a state somewhere in the middle is reached.
That's very interesting about using methanol to dry it. I'll have to try that!wg48temp9 - 12-2-2019 at 06:18
Wondering how hydrates can dry below their composition point is like being surprised that water can evaporate from a cup at room temperature.
Apparently America had a very cold spell recently as did the UK. When its cold outside even at 100% humidity there is not much water in that cold air.
When that outside air gets inside and warmed the humidity drops. That inside air has the same water contents as the outside air but its humidity is
less and it can dry damp cloths and some hydrated salts too.
fusso - 12-2-2019 at 09:24
Why would any decent chemists think that CuSO4 would be decomposed by sunlight?Abromination - 12-2-2019 at 15:29
