Crush a few blueberry skins in water and a dilute solution of the skin colouring material is produced. The coloured pigments in the blueberry skin are called anthocyanins and they make a good acid/base indicator.
The blueberry skins can also be smeared over filter paper producing a reddish/purple canvas on which to carry out ‘electrolysis art’.
First the filter paper is moistened with a few drops of brine (saturated sodium chloride solution) and placed on a thin sheet of aluminium foil.
Then using a 4.5v dc laboratory power pack the aluminium foil is connected to the positive terminal with an electrical lead and a crocodile clip.
A paper clip was held in a second crocodile clip and connected to the negative terminal of the power pack.
The supply was turned on and when the paper clip (cathode) was passed over the filter paper electrolysis of the brine took place producing bubbles of colourless gas and turning the blueberry juice blue, green and yellow.
We also experimented by adding drops of white vinegar to the filter paper and these areas turned pinky-red.
Can you explain the colours produced by the blueberry juice?
What other dark coloured fruits and vegetables have pigments in their skins that might be used to in the same way?
If you carry out an experiment such as the one described above beware of touching the filter paper with your fingers since sodium hydroxide is produced around the cathode during the electrolysis process.