Which term describes water's ability to neutralize acids?

• A. Hardness
• B. pH
• C. Buffering capacity
• D. Alkalinity

Answer: (D)

Alkalinity is the water's ability to neutralize acids. It comes from bases in the water that can accept protons, mainly carbonate, bicarbonate, and hydroxide ions. When acid is added, these bases react with the hydrogen ions to form carbonic acid and CO2, effectively consuming the acid and resisting a drop in pH. This acid-neutralizing capacity is what we quantify as alkalinity, typically measured by titrating with a strong acid to a defined endpoint. Hardness relates to calcium and magnesium minerals and doesn’t describe acid neutralization. pH measures how acidic or basic the water is at a moment, not how much acid it can still neutralize. Buffering capacity is a broader idea about resisting pH change, but the specific term for the amount of acid that can be neutralized is alkalinity.