Explain why benzene is colourless whereas azo dyes (such as methyl yellow) are coloured.

Azo dyes are coloured because they contain alternating single and double bonds, known as a conjugated system. This creates and areas of delocalisation above and below the plane of the molecule. This area of delocalisation lowers the gap between the electron energy levels, meaing the energy of the light needed to excite the electrons to higher energy levels is less. From the equation E=hf, lower energy corresponds to lower frequency, meaning the light absorbed by the compound is in the visible part of the spectrum. The frequency of light absorbed corresponds the the complementray colour of the colour we see in azo dyes. Benzene only contains a small area of delocalisation, which is not suffecient to lower the energy gap to the visible part of the spectrum, meaning benzene absorbs UV light, and is colourless.