Final answer:
Water is lighter than N₂, O₂, and CO₂ because it has a lower molecular mass, and as a liquid, its molecules are arranged differently compared to these gases.
Step-by-step explanation:
The question asks why water is lighter than N₂, O₂, and CO₂, which are gases. The density of a substance depends on its molecular mass and the arrangement of its molecules. Water, with a molecular mass of about 18 amu (atomic mass units), is lighter than N₂ and O₂ (each with a molecular mass of about 28 amu), and CO₂ (molecular mass about 44 amu). Furthermore, in the case of gases, the molar mass is directly related to the density under the same conditions of temperature and pressure. Therefore, water being a liquid at room temperature with lower molecular mass is lighter than these gases.