Answer to Question #250170 in General Chemistry for Binnor

Question

When magnesium burns, it becomes heavier, while when matches burn, they become lighter. Does this observation violate the law of conservation of mass?

Answer 1

No. The weight difference is because some of the mass comes from or goes to gaseous form in the atmosphere. If you weighed the sample plus the air it uses to burn, that mass won’t change (measurably, that is; at a deep mass-energy equivalence level, there will be a very slight theoretical change due to the heat released).

The mass of magnesium plus mass of oxygen gives magnesium oxide. Here, mass of reactants is equal to the mass of the product.