Indicators

Indicators are very special chemicals; they will change colour if you change the pH (by adding acid or alkali). Perhaps we should call them pH indicators because there are other kinds of indicators. You may have noticed that red cabbage changes colour when you slice it up and cook it. When you buy the red cabbage it is actually blue. It will be quite blue when you slice it up and add some tap water. That is because your tap water is very slightly alkaline. If you add some vinegar it will turn red. This is because there is a pigment (coloured chemical) in the red cabbage which acts as a pH indicator.

If you are not sure about acids and alkalis have a look at the pages on acids or alkalis.

Make your own “litmus paper”

  1. Chop the red cabbage up with a sharp knife.
  2. Crush it with a “pestle and mortar” OR put it in a food mixer and grind it.
  3. Add a very small amount of water.
  4. Decant the blue liquid (pour the liquid into a clean glass).
  5. Soak some blotting paper in the blue liquid.
  6. Let the blotting paper dry out.
  7. Cut the blue papers into small squares.

If you add a little bit of vinegar instead of the water, you will end up with red litmus.

Now you can go around the house testing things to see if they are acids or alkalis. Is that soap really kind to your skin (pH 5) ?????

If you put a piece of blue litmus paper into an acid it will turn red.

If you put a piece of red litmus paper into an alkali it will turn blue.

 

Indicators change colour at different pH values. Litmus changes at pH 7 but Phenolphthalein changes from pink to colourless at pH 9.

Indicator pH Colour in Acid Colour in Alkali
Litmus 7.0 Red Blue
Phenolphthalein 9.7 Colourless Red // Pink
Methyl Orange 3.7 Red Yellow
Bromophenol Blue 4.0 Yellow Blue

 

Do NOT try to memorise this table.

You have probably used Universal Indicator Papers or Universal Indicator Solution. These have a mixture of indicators so that they have a whole range of colours. You can compare the colour you get with a colour pH chart to find out the exact pH of any solution between pH 1 and 14.

I hope that you will be able to remember most of the facts on this page, but you must memorise the definition of indicators. Write it on a small index card. Put the red words on one side of the card and the blue ones on the other. Add the card  to your revision pack.

pH Indicators are chemicals which change colour when they are put into acids or alkalis. e.g. Litmus, Phenolphthalein, Methyl Orange, Methyl Red.

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