An open vessel with carbon dioxide was balanced on a balance Why was the balance of the balance upset over time?

Carbon dioxide is heavier than air, therefore an open vessel with carbon dioxide is heavier than exactly the same vessel, but without carbon dioxide (i.e. when there is air in place of carbon dioxide). Some time after the vessel was equilibrated on the scales, due to the phenomenon of diffusion, all carbon dioxide from it evaporated and air took its place. At the same time, the vessel became lighter, the balance of the scales was disturbed.