Which substance can neutralize hydrochloric acid produced by chlorine gas in pools?

The correct choice, caustic soda or soda ash, is effective in neutralizing hydrochloric acid because it is a strong base. When hydrochloric acid, which is acidic in nature, is introduced in a pool environment, it can lead to a decrease in pH, potentially causing corrosive conditions that can damage pool surfaces and equipment. Caustic soda (sodium hydroxide) or soda ash (sodium carbonate) effectively raise pH levels by reacting with the acid to produce water and other neutral compounds, thus helping to stabilize pool water chemistry.

This action is essential in maintaining balanced water conditions, ensuring swimmer safety, and prolonging the lifespan of the pool infrastructure. On the other hand, while substances like calcium carbonate and sodium bicarbonate can raise the alkalinity and pH of the pool water, they do not have the same immediate neutralizing effect on strong acids like hydrochloric acid. Citric acid would exacerbate the acidity instead of neutralizing it, highlighting why it is not a suitable choice for this requirement.