When quicklime (calcium oxide- CaO ) is added to water, it reacts vigorously with water to produce slaked lime (calcium hydroxide) and releases a large amount of heat.
The reaction that takes place is:
CaO+H2O→Ca(OH)2
Quick lime Slaked lime
The above reaction is an example of combination reaction in which calcium oxide and water combined together to form a single product, calcium hydroxide.