Question

What is the combination reaction? Write a combination reaction of quick lime with water.

Hint: A chemical reaction occurs between two compounds to give a product when they are allowed to come in contact. Such reactions are instant and do not require any activation. Quick lime is an oxide of calcium represented as $CaO$.

Combination reaction is a class of chemical reaction which two or more reactants combine with each other to generate a single product. No side products are formed in such a class of reaction. Such reactions are normally exothermic in nature which means heat is released.
Combination reaction can be further classified into three types.
Reaction involving a metal and a nonmetal.
In this class of reaction a metal for example sodium metal reacts with a non metal like chlorine gas to produce an ionic compound i.e. sodium chloride.
$2Na(s) + C{l_2}(g) \to 2NaCl(g)$
Reaction in which a compound and an element is involved.
In this class of reaction an element such as oxygen reacts with a compound like carbon monoxide to generate carbon dioxide
${O_2}(g) + CO(g) \to C{O_2}(g)$
Reaction between two compounds.
In this class of reaction a compound like lithium oxide reacts with another compound such as carbon dioxide to yield lithium carbonate.
$LiO(s) + C{O_2}(g) \to LiC{O_3}(g)$
The combination reaction of quick lime and water gives calcium hydroxide. The reaction is vigorous and is highly exothermic in nature. The reaction of quicklime $CaO$ and water $H_2O$ can be shown as:
$CaO(s) + {H_2}O(l) \to Ca{(OH)_2}(s)$

Note: In reactions where two products are formed from two or more reactants, they are not referred to as combination reactions. Normal combustion reactions can also be referred to as combination reactions. For example, if carbon is burnt in presence of oxygen to give carbon dioxide on full combustion.