Final answer:
The broadness of the hydroxyl group peak on an IR spectrum is due to hydrogen bonding, which causes a variation in the energies of the O-H bond stretch, resulting in a broad absorption peak.
Step-by-step explanation:
The broadness of the hydroxyl group peak on an IR spectrum is due to hydrogen bonding between molecules. This broad 'mountain' centered at about 3400 cm³¹ in the spectrum of compounds like alcohols is indicative of the O-H stretching mode. Hydrogen bonds can vary in strength and can interact with various partners, leading to a range of possible energies for the O-H bond stretch, hence producing a broader peak compared to other functional group absorptions in the IR spectrum.