Which of the following gases will absorb in the IR spectrum?

• A. CO_2
• B. N_2
• C. H_2O
• D. O_2

Answer: (C)

Verified step by step guidance

1. Understand that for a molecule to absorb infrared (IR) radiation, it must have a changing dipole moment during its vibrational motions. This means the molecule must be IR active.
2. Recognize that diatomic molecules like N_2 and O_2 are homonuclear (consist of two identical atoms) and have no permanent dipole moment. Their vibrations do not change the dipole moment, so they are IR inactive.
3. Consider CO_2, which is a linear triatomic molecule. Although it is overall nonpolar, some vibrational modes (like the asymmetric stretch) cause a change in dipole moment, making CO_2 IR active.
4. Look at H_2O, a bent molecule with a permanent dipole moment. Its vibrational modes cause changes in dipole moment, so it strongly absorbs IR radiation.
5. Conclude that among the given gases, CO_2 and H_2O absorb in the IR spectrum due to their molecular structure and vibrational properties, while N_2 and O_2 do not.