What is a chemical reaction?

Chemical Reactions are certain chemical processes that involve rearranging the molecular structure of a molecule. The atoms of the molecule do not change, but the arrangement of the atoms and the bonds holding the atoms together do change.

There are many kinds of chemical reactions like synthesis reaction: where two molecules join together to form one molecule. The atoms don’t change but the molecular arrangement changes when both molecules merge into one.

Here is an image of a simple chemical reaction that produces water.

Reference: https://www.quora.com/What-is-a-chemical-reaction-2