When alum is added to water, it causes the pH to _______, and when ferric salts are added to water, it causes the pH to ________.

When alum is added to water, it causes the pH to _______, and when ferric salts are added to water, it causes the pH to ________.

• A. Decrease; Decrease
• B. Increase; Decrease
• C. Decrease; Increase
• D. Increase; Increase

Answer: (A)

Metal ions from these salts make the water more acidic through hydrolysis, releasing hydrogen ions. When alum dissolves, Al3+ ions interact with water to form hydrolyzed species (for example, AlOH2+), which releases H+ into the solution, lowering the pH. Ferric salts behave similarly: Fe3+ hydrolyzes to species such as FeOH2+, releasing H+ as well, which also lowers the pH. Because both processes increase the concentration of hydrogen ions, the pH decreases in each case. The other patterns would require a net removal of H+ or formation of basic species, which doesn’t happen here, so pH does not increase.

Metal ions from these salts make the water more acidic through hydrolysis, releasing hydrogen ions. When alum dissolves, Al3+ ions interact with water to form hydrolyzed species (for example, AlOH2+), which releases H+ into the solution, lowering the pH. Ferric salts behave similarly: Fe3+ hydrolyzes to species such as FeOH2+, releasing H+ as well, which also lowers the pH. Because both processes increase the concentration of hydrogen ions, the pH decreases in each case. The other patterns would require a net removal of H+ or formation of basic species, which doesn’t happen here, so pH does not increase.