7: pH Painting at Home

Under different conditions of pH the dyes have different structures and therefore absorb different wavelengths of light resulting in different colours.

What you will need

Written permission from your parent/guardian to carry out this experiment (see your teacher).

Safety Advice

The work must be carried out under adult supervision.

Work on a stainless sink surface near a tap.

What you do PREPARING THE PURPLE pH PAPER Use a cheese grater to grate about half of the red cabbage. Place it in a bowl. Add about 150cm3 of water and mix very thoroughly until the liquid has turned dark purple. Be very careful with this purple cabbage juice as it will stain wood, plastic, hands and clothes. Pour the juice through the sieve, collecting the purple liquid in a large bowl. Throw away the cabbage left in the sieve. This could be quite messy! Take a piece of blotting paper, dip it in the purple liquid for about 20 seconds and then leave it to dry. If it is a nice day you could leave it outside. Alternatively, you could dry it in a warm oven or gently with a hairdryer. Preparing the pH paints PREPARING THE pH ‘PAINTS’ The colour of the dye extracted from red cabbage varies according to the pH as shown in the following chart. You can use this chart to work out the pH of various household materials (in other words, whether it is acid, neutral or alkaline). Some things you might like to try are: lemon juice, vinegar, washing powder and shampoo. Simply place small drops on the dry purple paper and watch the colour change. The table below shows how to make up colourless ‘paints’ which will make the purple dye turn a specific colour. Colour (when dry) Ingredients RED concentrated lemon juice PINK 2 teaspoons (5cm3) concentrated lemon juice in 300cm 3 of water BLUE 1 teaspoon (0.5g) baking soda in 300cm3 of warm water GREEN 6 teaspoons (3g) baking soda in 300cm3 of warm water GREEN/YELLOW 1 dessert spoon of washing powder (5g) in 300cm 3 of warm water It is important to use a clean and dry container to make up each of the different ‘paints’. It is also important to use clean brushes for each ‘paint’ and not to use the same brush for different paints. This is because if two paint solutions are mixed together, the pH and therefore the colour produced will change. You will find that the paints often dry to a different colour. This is because pH is a measure of the concentration of acid or alkali. As the ‘paint’ dries the water evaporates, and the concentration of the acid or alkali effectively increases. This means that a ‘paint’ which is blue when wet may dry to a green colour. Similarly, a ‘paint’ which is green when wet may appear yellow when dry.

wear gloves to avoid staining of hands avoid skin or eye contact