Here a reaction which is not balanced.
Zn+ HCl ---> ZnCl2+ H2.
IN this reaction reactants ( left side) are not equal to products( right side). According to law of conservation of mass the mass of the substance participating in the reaction must be same before and after the reaction.
Zn+2HCl---> ZNCl2 +H2 here we can balance the equation by observing no of molecules in reactant side and product side of a particular atom..
In the first equation which was not balanced we can observe that no of molecules in product side are not equal to reactants side...
The equations which are balanced called as stichiometric equations.