What happens during a chemical reaction?
The formation of new substances, and the making and breaking of strong chemical bonds.
This is a specific definition, and of course most of chemistry is based on such reactions...
For the generalized reaction...
...the chemical reaction CONSERVES two properties: (i) mass is conserved, i.e. the mass of the products is PRECISELY equal to the mass of the reactants; and (ii) charge is conserved, i.e. the charge of the reactants is precisely equal to the charge of the products. And so if you start with