What acid is more corrosive when it is diluted?

• A. Hydrochloric acid
• B. Sulfuric acid
• C. Nitric acid
• D. Acetic acid

Answer: (B)

The concept here is how acidity and corrosivity change with dilution for different types of acids. Sulfuric acid is special because it is diprotic: the first proton dissociates completely in all concentrations, producing a lot of H+. The second proton, which only partially dissociates in concentrated solutions, becomes more able to dissociate as the solution is diluted. This means that when sulfuric acid is diluted, the amount of free hydrogen ions can increase relative to what you’d expect from a simple dilution, making the solution more acidic and thus more corrosive.

In contrast, hydrochloric and nitric acids are strong monoprotic acids that dissociate completely regardless of concentration, so diluting them just lowers the overall hydrogen ion concentration and reduces corrosivity. Acetic acid is a weak acid; diluting it shifts its dissociation equilibrium toward fewer H+ ions, making it less corrosive.

So, sulfuric acid is the one that becomes more corrosive when diluted due to the enhanced second dissociation as dilution occurs.