Brilliant bubbles and disappearing dyes

A question I often get asked when dealing with the bleaching of coloured compounds is: why does the colour disappear?

One of my go-to demonstrations to address this question uses a chemical curiosity that came about following an 11-year dream of inventor Tim Kehoe to create a coloured bubble-blowing mixture. The resulting product (currently sold as Zubbles) works using leuco dyes. For a long time, producing coloured bubbles seemed almost impossible, since bubble walls are so thin dyes tend to sink to the base of the bubble.

Leuco dye molecules can exist in at least one coloured and one colourless form. The structures of these forms can be used to show students how organic dyes and pigments rely on delocalisation of electrons over extended molecular orbitals. This reduces the energy needed to promote electrons between energy levels to those corresponding to visible light frequencies. Any disruption to these extended delocalised systems can prevent a compound absorbing visible light, rendering it colourless.

With an accompanying video, Declan Fleming takes you through the steps to produce the 'brilliant bubbles' experiment, from set up to teaching and discussing the demonstration.