When does a single replacement reaction occur?

Hint: As we know that during the replacement reaction, the elements present in the compounds will be shifted. And the replacement reaction takes place in the ions. And here, the less reactive element will be replaced by using the more reactive elements. The replacement reaction is mainly in two types, replacement reaction and double replacement reaction.

Complete answer:
We have to remember that the single replacement reaction is also known as single displacement reaction or exchange reaction. This is a chemical reaction, where one element is displaced or replaced by using another element which is present in the same compound and there is a formation of new compound. Here, the more reactive element replaces the less reactive element in a compound. Always the metal will replace the metal only and non - metal will displace the non – metal only. The general formula of the single replacement reaction can be written as,
\[A + BC \to B + AC\]
Let’s see one example of single replacement reaction,
\[Z{n_{\left( s \right)}} + 2HC{l_{\left( {aq} \right)}} \to ZnC{l_2}_{\left( {aq} \right)} + {H_{2\left( g \right)}}\]
Here, zinc is reacted with hydrogen chloride, and the hydrogen is replaced by zinc present in the $HCl$.

Note:
We have to remember that the replacement reaction is mainly divided into two types and that is single replacement reaction and double replacement reaction. In the case of a single replacement reaction, only one element is replaced in a compound. But in the case of double displacement reaction, there is an exchange of ions, (it may be cations or anions) of two ionic compounds and there is a formation of two new ionic compounds.