Interactive Primary Newsletter 37 Put some colour into your life

The indications are good Many of the coloured solutions can be used as pH indicators and will change colour when exposed to acids or alkalis. The solutions can be tested by adding a drop of vinegar or sodium hydrogen carbonate (bicarbonate) to a drop of the test sample on a white tile (Fig.5). Once a plant extract can be found which produces a pronounced colour change in acid or alkali, the solution is ideal for making indicator strips and a wider range of dyed colours in fabrics. Making indicator strips These strips, once dried, allow a convenient and portable method of testing liquids for acidity or alkalinity. Cut small strips (approx 1 x 8 cm) of absorbent paper, (blotting paper, filter paper, kitchen roll). Label with a pencil before soaking them in the coloured samples and allowing them to dry out. Take one strip and touch one end against something acidic e.g. a cut lemon or vinegar. You may see a colour change. Try the other end against wet soap or a solution of sodium hydrogen carbonate and note any colour change. Is it a different colour to that obtained by the acid? Figure 5 - Top row - Plant extracts, Middle row – Drop of vinegar added, Bottom row - + drop of sodium bicarbonate Make a number of indicator strips and use them to test other liquids or the cut surfaces of fruits. Make a table showing evidence of acids and alkalis. Using acids & alkalis for more interesting effects Soak pieces of fabric in the coloured solution and allow them to dry. If the colour change is most pronounced with acid use a cotton bud dipped in vinegar to make patterns, or use a solution of bicarbonate or soap if alkali gives a better colour change. It would be ideal if you were able to show both colour changes on the one piece of fabric.                  Figure 6 (left) - Paper stained with red cabbage water. Drops of alkali added (left), drops of acid added (right).  Figure 7 (right) - Fabric stained with red cabbage vinegar. Drops of alkali added. If fresh plant material is difficult to acquire all is not lost. Boiling chopped up red cabbage can make very good indicator solutions (Fig. 6). The water will take on a strong colour and this can be used as an indicator solution. Fresh red cabbage is not always available but if you buy a jar of pickled red cabbage, you will find that the vinegar has developed a strong colour. You can soak absorbent paper or fabric in this and let it dry. If you then apply an alkali, such as soap or bicarbonate you can see a distinctive colour change (Fig. 7). Primary Summer School