Which reagent(s) produce a color change indicating the presence of water?

• A. Blue cobalt chloride paper turns pink
• B. Anhydrous copper sulfate turns blue
• C. Both A and B
• D. Neither indicates water

Answer: (C)

Water causes hydration of these indicators, producing visible color changes. Cobalt chloride paper is blue when dry and turns pink when it absorbs water, so it signals moisture. Anhydrous copper sulfate is pale, but when it takes up water it forms the blue copper sulfate pentahydrate, giving a clear blue color. Because both reagents change color in the presence of water, they both indicate water presence.