“To pour water on calcium oxide”. Which is another way to classify the reaction? Explain.

Hint: “To pour water on calcium oxide” leads to formation of slaked lime which is called \[\text{Ca(OH}{{\text{)}}_{2}}\].
It is used in whitewashing.

Complete step by step solution:
Before the classification of the above reaction, let us know about the definition of the Combination reaction.
Combination Reaction: A combination reaction is a reaction in which two or more substances combine to form a single new substance.
Combination reactions can also be called as Synthesis reactions. The general form of a combination reaction is:
           \[\text{A + B}\to \text{AB}\]
Now, coming back to the question, when water is added to calcium oxide (Quicklime), it will be converted to calcium hydroxide (slaked lime) with the liberation of an enormous amount of heat.
           \[\text{CaO + }{{\text{H}}_{2}}\text{O}\to \text{Ca(OH}{{\text{)}}_{2}}\]
It is an exothermic reaction and calcium hydroxide is an alkali.
Dissolution of calcium oxide in water is a compound-compound combination reaction.
Therefore, the dissolution of calcium oxide in water is a compound-compound combination reaction.

Note: Please do not confuse it with displacement reaction type. Displacement reaction leads to formation of two products whereas combination reaction leads to formation of only one product and some energy.