[Barelover]

Hi

This may seem like an obvious question but can someone please help me.  I know that lake whitening it caused by the formation of calcium carbonate, but what combines with calcium to form calcium carbonate??

I think it it possibly HCO3-, or H2CO3??? 

Thank you.

[DeP]

Hey,

CO2 dissolves in water to form H2CO3.
H2CO3 is in equilibrium with H+ and HCO3- and this in turn is equilibrium with H+ and CO3²-.
 And CO3²- can precipitate with Ca²+ as CaCO3

This happens in neutral to basic (so that the equilibrium goes complete to the right) and cold waters (CO2 dissolves beter in cold water).