Which reaction forms an ionic precipitate from a solution?

• A. Ca2+ + CO3^2- → CaCO3
• B. HCO3- + H+ → H2CO3
• C. Fe + Cu2+ → Fe2+ + Cu
• D. CaCO3 + H2CO3 → Ca(HCO3)2

Answer: (A)

A solid formed from ions in solution is a precipitate, which happens when two dissolved ions combine to make an insoluble compound. In the first reaction, calcium ions and carbonate ions meet and pair to form calcium carbonate, CaCO3, which is sparingly soluble in water and thus comes out of solution as a solid. The other options don’t produce an insoluble solid from ions in solution: the bicarbonate and proton simply form carbonic acid (which may release CO2 gas) in solution; the redox pair produces copper metal rather than an ionic solid precipitate; and calcium carbonate reacting with carbonic acid forms calcium bicarbonate that stays dissolved. So the formation of the insoluble solid CaCO3 is the precipitation event.