[Gary Mason]

Gary Mason  asked the Naked Scientists:
   
Hey Chris, newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] i have all of the episode of naked everything lol.

My little girl has a spoon that changes colour if her food is too hot. it is red when it is cold, but when you run it under a hot tap, it changes yellow. how does this work? i have also had cars when i was a kid that did the same thing, it changed colour by the heat from your hands.

thanks
Gary Mason, Birmingham

What do you think?

[chemgeek]

I think this is due to a process called thermochromism. There are two types and I don't know which one would have been used in the situation you described but here is each one briefly:
- Thermochromatic liquid crystals where the colour change depends on the selective reflection of certain wavelengths by the crystal structure as it changes between crystallic phases with temperature.
- Leuco dyes mixed with other chemicals where the object changes from the usually colourless leuco to the coloured other form. The dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazole derivates, and 4-hydroxycoumarin and act as proton donors, changing the dye molecule between its leuco (colourless) form and its protonated colored form.