Red Cabbage Indicator

We made an acid-base indicator using red cabbage.

Red Cabbage

Indicators are chemical compounds that can be added to a solution to determine whether it is acidic or alkaline.
The indicator will change colour depending on whether an acid or an alkali is added.
The colour in red cabbage (it is a pigment called an anthocyanin) makes a very good indicator.

Acids and alkalis

Acids have a sour taste, like vinegar (which contains ethanoic acid) and lemons (which contain citric acid).

Alkalis are substances that react with acids and neutralise them. Soap and washing powder are alkaline.

Acids and alkalis are found in a surprising number of places. Some are edible and are found in foods. Others are very strong and can be harmful, such as the acid in car batteries and the alkali in oven cleaners.


Chopping the cabbage

Our experiment

First we had to extract the purple dye. To do this we tore our red cabbage leaves into very tiny pieces.

You could also chop up the leaves using a sharp knife or a blender, or grind them in a pestle and mortar.

 


We put the chopped cabbage into a jug, poured hot water onto it and left it for a few minutes. The water turned a bright purplish-blue colour. Then we used a fine sieve over a funnel to strain the pieces of cabbage out.

straining
pipetting

Each of us had a pipette and a small clear beaker. We pipetted a small amount of indicator solution into our beakers.

To begin with the indicator was purple. This is because water alone is not acidic or alkaline - we say it is neutral.

Then we added a different test compound to each small beaker.

We started by testing vinegar, which is weakly acidic, and sodium bicarbonate, which makes a weak alkaline solution in water.

The acid turned the indicator pink, and the alkali turned it blue.

 

Miss Mackie and Mrs deBoeck tested some limescale remover and some oven cleaner. (We had to be careful with these, as they are corrosive or caustic, which means they can burn if you get them on your skin.)

indicator colours

The limescale remover turned the indicator red and the oven cleaner turned it green.

Limescale remover contains a strong acid called hydrochloric acid and oven cleaner contains a strong alkali called ammonia.

So we could use our indicator to tell the difference between strong and weak acids and alkalis.

We also tested some bleach, which contains the strong alkali sodium hydroxide. The sodium hydroxide in the bleach turned the indicator pale green at first, but then (as the bleach bleached the colour out of the indicator) it went pale yellow!

 

test substances

 

Now we tested some other substances, these are some of the other things we tried:

• orange juice
• washing powder
• cream cleaner
• soap
• lemon juice
• cream cleaner

 


We put a few drops of each substance into our little beakers of indicator and stirred them carefully, then watched for the colour changes. Here are some photos of us doing the experiment.

children
children

We found that the acids turned the indicator pink or red, and alkalis turned it blue, green or yellow depending on how strong the alkali was.

We also tried testing sugar and salt. Neither of them made the indicator change colour - so they were not acid or alkaline. Sugar and salt solution are neutral (like water).

Here is a photo of all the beautiful colours our red cabbage indicator made with all our different test solutions!

indicator colour rainbow

 



More science to try at home

You can do this experiment at home - chop up about a quarter of a red cabbage, and add about 400ml of hot water. Strain off the cabbage, and put a little of your cabbage juice into clean yoghurt pots or plastic cups to test.

How about testing lemonade or coke, water softener powder, apple juice, crushed indigestion tablets? If you want to test any cleaning chemicals make sure you get an adult to help you!

Now try getting two matching cups/pots/jars and putting a small amount of cabbage juice indicator in each. Get a drinking straw and blow bubbles (carefully!) through the liquid in just one of your cups. Keep blowing for two or three minutes. What happens? Can you see a colour change? What do you think it means?


If you make your cabbage juice with as little water as possible, so that the juice is really concentrated, you can try dipping a coffee filter (if you don't have coffee filters, ask at Science Club for a couple of pieces of filter paper) into your cabbage juice. Dry your purple paper out in a warm place, then cut it into strips. You can use these paper test strips to test common household chemicals - they will change colour just like the solutions.

Find out more about acids and alkalis