# What is the formula for calcium sulfate dihydrate?

$C a S {O}_{4} \cdot 2 {H}_{2} O$
Calcium sulfate dihydrate is an ionic substance composed of $C {a}^{2 +}$ and $S {O}_{4}^{2 -}$. The associated water molecules are known as waters of crystallization, which are tacked on to the substance when it crystallizes (from water). They could be removed on prolonged drying.