Sciencemadness Discussion Board

Calcium Oxide not absorbing water in dessicator bag

CHRIS25 - 23-10-2012 at 04:04

I made copper nitrate over two weeks ago and placed the 200mLs solution in a dessicator bag with Sodium Hydroxide and Calcium Oxide. The NaOH absorbs the water from the copper nitrate solution, which I have changed three times, about 20mLs of water has been extracted. But to my surprise the CaO has remained 100% dry for two weeks. I am assuming that the CaO has been carbonized as seems to be the procedure when making it for household purposes? I have placed the CaO in a normal place in a room to see if it absorbs water, but can anyone in the meantime offer some insight here?

kristofvagyok - 23-10-2012 at 04:08

This is the reason why we don't use CaO in desiccators, it is way not enough hygroscopic than other salts.

weiming1998 - 23-10-2012 at 06:08

Quote: Originally posted by kristofvagyok  
This is the reason why we don't use CaO in desiccators, it is way not enough hygroscopic than other salts.


But CaO can heat water to a boil when water is poured on it. It releases a lot of energy when it is hydrated to Ca(OH)2. I wouldn't think it'll be much less hygroscopic than NaOH or KOH.

To the OP: Pour some water over the CaO. It should heat up considerably. If it doesn't heat up at all, it's not CaO, but something else.

[Edited on 23-10-2012 by weiming1998]

hissingnoise - 23-10-2012 at 06:33

One should be aware that hydrated lime is often referred to as calcium oxide . . .

CHRIS25 - 23-10-2012 at 09:36

What does OP mean? (old age pensioner)? Hope not. Long way to go yet.....

I poured water onto the guilty party, turns milky, slowly, dissolves very slowly, temp of water was 17.5c at start, after 10 minutes temp went to 19.5c and the 20grams has almost dissolved; I looked at the CaO that is in the room now for 5 hours almost, it indeed has started to hydrate, and another sample I have hydrated pretty quickly also. So it hydrates, and it solidifies into a concrete mass that is impossible to break except with a hammer. Now what? It has to be CaO but not chemical standard so to speak, some cheap ingredient with lots of added extras? I have some Calcium Chloride, I shall mix that with the NaOH in the bag sometime tomorrow. But at the rate that the Copper nitrate is losing water I think I shall give it the heat treatment otherwise I will still be waiting for it next year.

Hexavalent - 23-10-2012 at 09:46

Quote: Originally posted by CHRIS25  
What does OP mean? (old age pensioner)? Hope not. Long way to go yet.....

I poured water onto the guilty party, turns milky, slowly, dissolves very slowly, temp of water was 17.5c at start, after 10 minutes temp went to 19.5c and the 20grams has almost dissolved; I looked at the CaO that is in the room now for 5 hours almost, it indeed has started to hydrate, and another sample I have hydrated pretty quickly also. So it hydrates, and it solidifies into a concrete mass that is impossible to break except with a hammer. Now what? It has to be CaO but not chemical standard so to speak, some cheap ingredient with lots of added extras? I have some Calcium Chloride, I shall mix that with the NaOH in the bag sometime tomorrow. But at the rate that the Copper nitrate is losing water I think I shall give it the heat treatment otherwise I will still be waiting for it next year.


I don't think this is CaO. Are you sure that it wasn't calcium chloride?

OP means original poster, BTW, i.e. he who started the thread.

UnintentionalChaos - 23-10-2012 at 09:47

Quote: Originally posted by CHRIS25  
What does OP mean? (old age pensioner)? Hope not. Long way to go yet.....


Original Post(er)

CHRIS25 - 23-10-2012 at 10:31

The packet was part of a Moisture absorber piece of contraption that I picked up in a cheap store. It quite clearly says calcium oxide. I have another one of these but a different company, that one contains calcium chloride, unopened sice I wanted to use all the cheap stuff first, I mean it was very cheap. It works well in a normal environment, seems to do the job well. i mean I have one in a room that is half full of water after 2 months - Calcium hydroxide I presume now. So hence the mystery about remaining totally dry inside the air tight dessicator bag where the NaOH is absorbing the water. That's all I can say.

ElectroWin - 23-10-2012 at 11:29

Calcium chloride is very good at pulling the water from vapours in the air, however my Ca(OH)2 seems to do nothing...

by the way, CaCl2 should also be pretty cheap.. something like $2/kg.
and you can re-use it: just boil off the water.

hissingnoise - 23-10-2012 at 11:36

Quote:
. . . and you can re-use it: just boil off the water.

And when you've done that, get some nitro ─ to break up the white, super-hard concrete you'll get . . .

kristofvagyok - 23-10-2012 at 12:37

Quote: Originally posted by weiming1998  
But CaO can heat water to a boil when water is poured on it. It releases a lot of energy when it is hydrated to Ca(OH)2. I wouldn't think it'll be much less hygroscopic than NaOH or KOH.

KOH will also heat water to boil if it is added in enough large amount, and I also thing that same will happen with NaOH.

The hygroscopicy of these compounds are not calulated from the exothermic dissolvation of them, so I would say again that CaO is NOT good in an exsiccator.

weiming1998 - 23-10-2012 at 17:31

Quote: Originally posted by kristofvagyok  
Quote: Originally posted by weiming1998  
But CaO can heat water to a boil when water is poured on it. It releases a lot of energy when it is hydrated to Ca(OH)2. I wouldn't think it'll be much less hygroscopic than NaOH or KOH.

KOH will also heat water to boil if it is added in enough large amount, and I also thing that same will happen with NaOH.

The hygroscopicy of these compounds are not calulated from the exothermic dissolvation of them, so I would say again that CaO is NOT good in an exsiccator.


Ok. I don't have any CaO, so I can't test it out practically.