The law being demonstrated in this experiment is the Law of Conservation of Mass, which states that the total mass of a closed system remains constant, regardless of changes in state or composition of the system. In this case, the tightly capped bottle of hydrogen peroxide can be considered a closed system, and since the cap is tightly closed, no mass can enter or leave the system. As a result, despite the chemical reactions occurring inside the bottle due to exposure to light, the total mass of the system remains constant, and hence the reading on the mass scale balance does not change.