I found this exercise:
Shellfish have a shell made of calcium carbonate, which is formed when calcium ions, secreted from shellfish cells, find sea water, which is rich in dissolved carbon dioxide.
One can say that
I - one of the reactions is Ca2+ + CO32- --> CaCO3
II - The reaction which involves the calcium ions while forming the shell is an acid-base reaction
III - the product is classified as an basic oxide
Which ones of these are true? Answer: only (I)
I think it should be (I) and (II), because CO32- has electrons to give, and Ca2+ misses electrons. So one donates to the other, making it a Lewis acid-base reaction.
Can you guys shed some light here?