KH not associated with GH?

[Die Putzfrau]

Now everyone's telling me that KH has absolutely no effect on GH. Now if this is true, why don't the carbonate and bicarbonate anions which determine KH form bonds with the magnesium and calcium cations which determine GH?

[RTR]

Because they are ions in solution. If you dry the water out, the concentrations will exceed the solubilities and the anions and cations will associate and "salt out" or precipitate.

It is not uncommon for GH and KH to be similar, but chemically there is no requirement for that. The sodium, potassium, etc. ions do not register on GH testing.

[Die Putzfrau]

Oh, I think I understand. So the ions will only precipitate if they reach a certain concentration? This concentration is obviously not one that you'd typically find in aquarium water unless a great deal of the water were to evaporate. Isn't the same as if you dry out salt water, the sodium and the chlorine will reform into a solid? Am I understanding it correctly?

[RTR]

You got it. With hard or high TDS water you will see some mineral crusting from splash and drying - called salt creep, but it is not just NaCl, lots of carbonates/bicarbonates there also.

[oztun]

Funny I just learned today in chemistry about strong and weak electrolytes. This is exactly what we were discussing, a substance dissolves in water and the components become aqueous. We even used NaCl as an example...
NaCl --H2O--> Na+1 (aq) + Cl+1 (aq).

[Die Putzfrau]

Thanks guys, if I got any difficult chem questions, I know who to ask.