Reef Chemistry Question of the Day # 178 Limewater (Kalkwasser) Cloudiness

[Randy Holmes-Farley]

Reef Chemistry Question of the Day # 178

Suppose you make a batch of limewater (kalkwasser) by dissolving the solids into pure fresh water in an old but clean salt bucket. It is clear a few minutes after mixing. But you come back the next day to find it very cloudy.

What is the suspended solid precipitate (the cloudiness) most likely to be?

A. Calcium carbonate
B. Magnesium hydroxide
C. Calcium hydroxide
D. Calcium oxide
E. Magnesium carbonate

Good luck!

























.

 

[Eckolancer]

A calcium carbonate

 

[Cory]

Magnesium carbonate

 

[beaslbob]

wild guess d

 

[JimWelsh]

Calcium is combining with absorbed atmospheric carbon dioxide via CO2 + H2O => H2CO3 => HCO3- => CO3-- to form calcium carbonate.

 

[GainesvilleReef]

Well it has to come from the salt left in the bucket. Also the calcium carbonate settles out within a few minutes and forms a crust on the surface preventing more co2. So it has to be B or E. The hydroxide will becomes carbonate so it has to be E. That's my guess anyway.

 

[jason2459]

Needs a better lid.


Edit: needs a lid. No lid mentioned.

 

[Randy Holmes-Farley]

Needs a better lid.


Edit: needs a lid. No lid mentioned.

Then there'd be no question to ask.

 

[Slm222]

A

 

[Randy Holmes-Farley]

And the answer is...A. Calcium carbonate (or maybe C. calcium hydroxide, see below)

Magnesium hydroxide will also preciptiate from limewater, but it happens very rapidly (almost instantly), and since the limewater was clear for 10 minutes, it cannot be magnesium hydroxide.

If magnesium hydroxide did not precipitate initially, there isn't enough magnesium to precipitate magnesium carbonate later.

Calcium hydroxide (what is used to make limewater/kalkwasser) won't precipitate unless the water is warmed since it is slightly less soluble in warm water. Still, if the water warmed significantly, this might be a correct answer.

Calcium oxide is the dehydrated form of calcium hydroxide. It cannot form in contact with water. Calcium hydroxide needs high temp dry heating to make calcium oxide.

Here's an article where I showed the precipitation I mentioned in this question:

The Degradation of Limewater in Air - Reefkeeping.com
http://reefkeeping.com/issues/2003-05/rhf/feature/index.htm

from it:

In a related experiment, a limewater solution with excess solids was allowed to settle for 24 hours and the liquid was decanted from the solids. This liquid was then monitored by conductivity while stored in an open container. In this case, the probe was generally not left in the solution, but was added for each measurement, breaking the solid crust and permitting much of it to settle to the bottom. Figure 6 shows that the conductivity does decline slightly over a period of several days. The drop in potency here is likely due to both the fact that there is no excess solid calcium hydroxide on the bottom that dissolves as potency drops, and because the crust was protecting the solution from penetration of carbon dioxide.

What I apparently forgot to mention in that article was that this solution was also cloudy, which was, I think, showing that CO2 was steadily dissolving into the water and precipitating calcium carbonate not just at the surface crust, but into the limewater.