Nitrogen prepared in the laboratory is denser than the air collected from the atmosphere. This is because the laboratory-prepared nitrogen is pure, while the air we collect from the atmosphere is a mixture of different gases, including nitrogen, oxygen, argon, and carbon dioxide. The atmospheric air also contains water vapor and dust particles. Hence, due to the presence of heavier oxygen, argon and carbon dioxide in air, the density of air is greater than that of pure lab-prepared nitrogen.