Indicators with natural food dye.

Identifying the chemistry of kitchen substances.
Are they an acid, base or neither?

A number of natural food dyes can tell a great deal about the many chemicals in our kitchen. Some food dyes actually change colour in the presence of corrosive chemicals known as acids or bases. Depending on the colour change of a food dye we can identify chemicals as either strong or weak acids or strong or weak bases. A strong acid or base is not to be messed with so we used special dyes to identify these types of chemicals and take the necessary precautions. These special dyes that change colour are called indicators, because they indicate the type of chemical that is present.
A strong acid or a strong base can quickly cause a chemical burn if not handled properly.


Strong acids and strong bases are said to be corrosive. That is, they chemically change the material they come into contact with.


The video on the right shows a strong acid, called nitric acid, dissolving copper metal.


The video on the right shows a strong household base, called "Drano", slowly dissolving aluminium metal.

As you can see both strong acids and strong bases are very powerful chemicals.
So we need to be able to identify them without handling them. This is where indicators are very helpful. Below are directions of how to make one.


Red cabbage has a purple pigment that changes colour in the presence of chemicals such acids or bases. Acids and bases react together to neutralise each other. Acids taste sour and include, vinegar, lemon juice while bases are usually detergents.

The range of colours that the red cabbage pigment can change into is shown on the right.

Red usually indicates a strong acid, a light pink is common for a weak acid while the dark green is a sign that a type of chemical called a base is present.



Lets see if we can isolate the pigment in red cabbage and use it to identify what foods are acids or bases.

Step 1 - We will need to purchase half of a very small red cabbage. Chop it up into small pieces.


Step 2 - Place the pieces of cabbage into a small pot with one cup of water and boil .for 15 minutes.


Observe the pot boiling. Use your knowledge of particle theory to explain where the bubbles and steam come from.


Step 3 - After boiling the cabbage leaves allow the pot mixture to cool for 5 minutes. Use a strainer to strain the liquid into a glass.

Explain how the strainer is used to separate the mixture of liquid and solid leaf matter.


Step 4 - To start our testing place a small volume of food dye into a glass and triple its volume by adding some water. We have just made the dye solution less concentrated. When we make a solution less concentrated we say we dilute the solution.