Quicklime, CaO, is added to water to make slaked lime.

Combination: $\text{A} + \text{B} \rightarrow \text{AB}$. Decomposition: $\text{AB} \rightarrow \text{A} + \text{B}$, by heat, light or electricity. Displacement: $\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$, when A is more reactive. Double displacement: $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$, often with a precipitate.