Most of us would have been wowed by the magic mug trick. The mug is usually black in color or with half done picture. But when any hot liquid is filled in, the complete picture is revealed. So how it is designed?
The mugs are actually painted with heat-sensitive paints.
These paints has an amazing property called "Thermochromism".
Thermochromism is a quality of the material to change its color according to change in the temperature.
This phenomenon is used in fabrics, baby bottles(changes to different color when the drink is cool enough for the baby), mood rings(changes color according to the skin temperature of the wearer).
It can be achieved using thermochromatic liquid crystals or leuco dyes. Among these two leuco dyes are used commonly as it allows wide range of colors to be used.
Heat sensitive paints uses thermochromatic dyes that are actually a mixture of leuco dyes and some chemicals.
Thus a chemical reaction takes place when the dyes experience a change in temperature to reveal the picture in a mug.