What happens if the reactants are put under a bright UV light?

• A. Orange-yellow colour goes to colourless.
• B. The mixture boils.
• C. A gas is released.
• D. No changes occur.

Answer: (A)

Bright UV light provides energy to drive photochemical changes in molecules. If the reactant is colored because it has a conjugated system that absorbs visible light, UV irradiation can break bonds or alter the conjugation, destroying the chromophore and converting the colored compound into a colorless product. This decolorization is a common outcome when a dye or photoactive compound is exposed to light.

Boiling would require heat input or a reaction that releases a lot of energy as vapor, which isn’t guaranteed by UV exposure alone. Releasing a gas would need a reaction pathway that produces gas, again not guaranteed by light alone. If the substance is not light-sensitive, there could be no change, but in this case the observed decolorization indicates a photochemical change driven by the UV energy.