Reef Chemistry Question of the Day #96 Calcium Carbonate Solubility

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #96

The solubility product constant, K_sp, is a constant (that is, just a number, like 6.1) that reflects the solubility of a salt.

For sodium chloride, for example, it is the sodium concentration times the chloride concentration at exactly the maximum amount of sodium chloride that can dissolve:

K_sp (NaCl) = [Na⁺] x [Cl^-] at saturation

Likewise, for calcium carbonate, it is the calcium concentration times the carbonate concentration at exactly the maximum amount of calcium carbonate than can dissolve:

K_sp (CaCO₃) = [Ca⁺⁺] x [CO₃^--] at saturation


Which of the following is true, and why?

A. The K_sp (CaCO₃) is higher in seawater than in fresh water (implying that more calcium carbonate can be dissolved in seawater than in fresh water)
B. The K_sp (CaCO₃) is equal in seawater and in fresh water (implying that the same amount of calcium carbonate can be dissolved in seawater and in fresh water)
C. The K_sp (CaCO₃) is lower in seawater than in fresh water (implying that less calcium carbonate can be dissolved in seawater than in fresh water)

Good luck!


edit:
Just to clarify the sections in parentheses in the answers, I don't mean how much can dissolve in these solutions starting with regular seawater and fresh water. I mean how much can dissolve counting what is already there. So the total calcium and carbonate that can be present in seawater and can be present in fresh water.


























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[tonizzy22]

Answer is C the more calcium carbonate in the water the more likely it is to precipitate out.

 

[Randy Holmes-Farley]

FWIW, I added a clarification.

 

[tonizzy22]

I would like to change my answer to A there will be more total dissolved calcium carbonate in saltwater then fresh because the magnesium will prevent precipitation.

 

[DFW]

A??

 

[beaslbob]

depends on ph

(ie I have not idea LOL)

 

[Randy Holmes-Farley]

And the answer is...A. The K_sp (CaCO₃) is higher in seawater than in fresh water (implying that more calcium carbonate can be dissolved in seawater than in fresh water)

The total solubility of calcium carbonate in seawater is higher than in fresh water (when taking into account the calcium and carbonate is already present).

The reason is that the other ions in seawater (such as magnesium and sulfate especially, but also just sodium and chloride) help stabilize the calcium and carbonate ions by surrounding them, making it less likely they will interact and precipitate.

In general, most ionic salts are more soluble in seawater than in freshwater at the same pH for this reason (unless something else in the seawater is reacting with one of the ions to form an insoluble precipitate).

Happy Reefing!