If the pH of a water sample increases, the calcium carbonate level will ________, and as the temperature increases, the solubility of the calcium carbonate ________.

• A. Decrease, Increases, Decreases
• B. Increase, Increases, Decreases
• C. Increase, Decreases, Increases
• D. Decrease, Decreases, Increases

Answer: (A)

Calcium carbonate behavior in water is governed by both pH-driven carbonate speciation and the heat-dependent dissolution of the solid. When pH rises, the solution becomes more basic and more carbonate exists as CO3^2−. Calcium ions readily combine with carbonate to form calcium carbonate solid, which precipitates out of solution. This removes dissolved CaCO3, so its level in the water decreases.

As temperature increases, the dissolution of calcium carbonate is favored, so more CaCO3 can dissolve into solution, increasing its solubility.

So the calcium carbonate level decreases with higher pH, and its solubility increases with higher temperature.