Can someone explain what it is meant for a solution to be supersaturated with respect to calcite? and then what is meant for a solution to be in equilibrium with calcite?

Question

Many Thanks

A supersaturated solution contains more solute than is normally soluble. For calcite, that means that the solubility has been increased somehow (often by increasing the temperature so that more calcite will dissolve), then the solution is returned to normal conditions (by cooling if heated previously) and the calcite does NOT come out of solution. Droping a crystal of calcite into the solution, vigorous stirring, a piece of dust, or some other foreign matter entering the mix, often is all that is necessary for the calcite to begin precipitating and return to a saturated solution. A solution in equilibrium with calcite means that some calcite crystals are dissolving while others are precipitating. A saturated solution of calcite with some solid calcite in the bottom of the beaker holding the water, will meet the equilibrium conditions after the solvent has dissolved all that it can dissolve at a specified temperature.


OK then...when I see a S.I. or a Saturation Index for calcite, say, the number is 0.39. First of all, it's a positive number and so that tells you it is supersaturated with respect to calcite, but what does the 0.39 mean...0.39 moles per liter? 0.30 mg? or is S.I. unitless?

It is a bunch of things. Here is a site explaining what S.I. components are.

Sorry. I didn't get the site. Here it is.
http://www.csgnetwork.com/langeliersicalc.html


Thanks. You have been very helpful. Last, if a water goes from being undersaturated to supersaturated with respect to calcite, would an increase in pH explain this transition?

An increase in pH make calcite more soluble but whether that makes it supersaturated or not I don't know.
CaCO3 + 2HCl ==> CaCl2 + H2O + CO2.