Jenny Harris, of Balclutha Primary School, asks :-

I have an outdoor ornament whose paint changes colour with the weather, and also a coffee mug which changes image when it holds hot liquid. How do these paints work?

Dave Warren, a chemist at the University of Otago, and Roy Rose, a retired paint chemist in Australia, responded.

Usually the weather device involves crystals of cobalt chloride crystals which, when it is humid, picks up water of crystallisation and goes pink and when dried out goes blue. A bit like the blue material many moisture sensitive items arrive with a little bag of in the container, silica gel plus a dye, blue when dry, pink or clear when it has taken up moisture. These are called hydrochromic pigments i.e. pigments that change colour with humidity

Another product that you may have seen is Dulux Nevermiss. This is a ceiling paint containing an indicator so that when the paint is wet (and therefore alkaline) it is pink but on drying (and therefore neutral) it is white. This ensures that an even coating can be applied to an existing white ceiling without bare spots – which show up like the proverbial at grazing viewing angles as is the case with ceilings.

For thermochromic pigments i.e. pigments that change colour with temperature, you may remember toy rings that changed colour with temperature using this principle or indeed the coffee cup that you mention. A lot of work has been done in this area with these sort of paints being used for customizing cars. There is even a prospective “Chameleon Mini” - see http://www.coolthings.com/the-mini-chameleon/ .