Calcium ions reacting with sodium hydroxide yield what?

• A. White precipitate; insoluble.
• B. White precipitate; soluble in excess.
• C. No precipitate.
• D. Green precipitate.

Answer: (B)

Calcium ions with sodium hydroxide produce calcium hydroxide, Ca(OH)2, which forms a white solid because it is only sparingly soluble in water. When a large amount of hydroxide is present, the strong base can pull some calcium into a soluble hydroxo complex (calciumate), so the precipitate dissolves. That’s why you see a white precipitate that is soluble in excess NaOH. The color and behavior don’t match a green precipitate or no precipitate, and the key point is the initial white solid formed, which then dissolves in excess base.