(a)     Explain why the nitrogen molecule, \({{\text{N}}_2}\), does not absorb infrared radiation.

(b)     Describe two vibrations in the water molecule that absorb infrared radiation.

(a)     no change in dipole moment (bond polarity);

as vibration/stretching occurs;

Ignore bending if included.

(b)     symmetrical stretching;

asymmetrical stretching;

bending;

Accept diagrams of the water molecules which illustrate the bending and stretching.

Allow [1] for stretching alone.

Generally well done, though in part (a) many candidates discussed the polarity of the molecule rather than whether the dipole moment would change when it vibrated; indeed many seemed to think that because it was non-polar it could not vibrate! The various bending and stretching modes of the water molecule appeared to be well known.