'mass can neither be created nor destroyed' this law is know as law of conservation of mass .
Antoine Lavoisier carefully performed many combustion experiments, namely, burning of phosphorus and mercury, both in the presence of air. Both resulted in an increase in weight. After several experiments he found that the weight gained by the phosphorus was exactly the same as the weight lost by the air. He observed that, Total mass of reactants = Total mass of products When hydrogen gas burns and combines with oxygen to yield water, the mass of the water formed is equal to the mass of the hydrogen and oxygen consumed.