Question

Why is the weight of product greater than the reactants when the reactants are burnt ??

Answer 1

Burning is the reaction of that substance with oxygen and giving out energy as heat.
For example, if magnesium os burnt, it is the reaction of magnesium with oxygen to form MgO (magnesium oxide) whose mass is more than that of magnesium.that's why we feel that mass of reactant increses after burning.actually we we dont take care of oxygen's mass before burning, but after burning , when it forms compound with the burning substance , oxygen's mass is included as we weigh the compound.
law of conservation of mass is true if we consider the mass of oxygen before burning.
hope you unserstand.all the best.