# How do calcium ion and phosphate ion react in water?

$3 C {a}^{2 +} + 2 P {O}_{4}^{3 -} \rightarrow C {a}_{3} {\left(P {O}_{4}\right)}_{2} \left(s\right) \downarrow$
Phosphates are usually as soluble as a brick, and all except the alkali metal salts come a crashing out of solution. In fact, the phosphate species in aqueous solution is probably, $H P {O}_{4}^{2 -}$, $\text{biphosphate}$, and the precipitate is thus possibly $C a H P {O}_{4} \left(s\right)$.