The humidity or water vapor content of a gas depends on its ability to hold or retain water molecules. In the case of carbon dioxide (CO2) and hydrogen gas (H2), the difference in their ability to retain water vapor is primarily due to their physical and chemical properties.
Solubility: Carbon dioxide is more soluble in water compared to hydrogen gas. CO2 can dissolve in water to form carbonic acid (H2CO3), which contributes to its ability to retain water vapor. On the other hand, hydrogen gas is not very soluble in water and does not interact with water molecules to the same extent as CO2.
Polar Nature: Carbon dioxide is a polar molecule, meaning it has a partial positive charge on the carbon atom and partial negative charges on the oxygen atoms. This polarity allows CO2 to form weak interactions, such as hydrogen bonding, with water molecules. These interactions increase the capacity of CO2 to retain water vapor. Hydrogen gas, however, is a nonpolar molecule and lacks such interactions with water.
Molecular Weight: Carbon dioxide has a higher molecular weight (44 g/mol) compared to hydrogen gas (2 g/mol). Heavier molecules like CO2 tend to have more collisions with water molecules, enhancing their ability to hold water vapor.
These factors contribute to the higher humidity of carbon dioxide compared to hydrogen gas. When exposed to the same conditions, CO2 molecules will have a greater tendency to interact with and retain water vapor, while hydrogen gas molecules will not retain water vapor as effectively.