The solubility of calcium carbonate in water decreases as temperature increases.

• A. Decreases
• B. Increases
• C. Remains the same
• D. Fluctuates

Answer: (A)

When a sparingly soluble salt like calcium carbonate dissolves in water, the heat of dissolution matters. For calcium carbonate, dissolving is exothermic (it releases heat). According to Le Chatelier’s principle, increasing temperature shifts the equilibrium toward the side that absorbs less heat — which is the solid CaCO3. That means fewer ions, so dissolved calcium carbonate decreases as temperature rises. In practical terms, warmer water holds less CaCO3 in solution, so calcium carbonate is more likely to precipitate as scale when the water is heated. Conversely, cooler water can keep a bit more CaCO3 dissolved. This is why hotter water tends to form more scale.