Which of the following actions would result in a sample's alkalinity measurement decreasing?

Adding strong acid to the sample directly impacts the alkalinity measurement by decreasing it. Alkalinity is a measure of the water's ability to neutralize acids, primarily influenced by bicarbonates, carbonates, and hydroxides present in the water. When a strong acid is introduced, it reacts with these alkaline substances, particularly the bicarbonate and carbonate ions, leading to their conversion into carbon dioxide and water. This reaction effectively reduces the concentration of ions that contribute to alkalinity, therefore resulting in a lower alkalinity measurement.

Other actions, such as adding more carbonate ions or raising the pH, would actually increase the alkalinity by providing more alkaline substances or by shifting the equilibria in favor of these substances. Additionally, a decrease in temperature generally has little direct effect on alkalinity but can influence solubility and reaction rates, not the alkalinity number itself.