A single replacement reaction is one in which a more reactive element displaces a less reactive element from its salt solution.Thus zinc can easily lose electrons as compared to hydrogen and result in the formation of zinc chloride and hydrogen.
According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
