I have no Bunsen burner and limited facilities for practical work. How can I do this?

A Bunsen burner is not essential – a container of boiling water will retain sufficient heat to extract some indicator from the cabbage. The extraction can also be done in cold water by mashing the cabbage in the water until you see the purple colour of the water is sufficiently intense. Similarly, an improvised beaker when using cold or warm water could be a plastic cup or the bottom section cut off a plastic water bottle.

I plan to make a stock of indicator as a demonstration and the students will do their own testing as a class experiment. What can we do while the cabbage is heating?

You could engage the students in another activity, for example a card sorting activity. Provide cards with pictures of the substances that are to be tested and ask them to group the cards as acids, alkalis and neutral. Later they can see how their predictions compared with the testing.

Can the indicator be used to make pH paper?

Yes, you can soak a piece of filter paper (or domestic coffee filter paper) or other strong paper in the indicator solution, then allow the paper to dry. It can be cut into strips and used for testing acids and alkalis as before. Note that with other types of paper try to select one that's 'acid free'.

What is it about red cabbage and other plants that makes a good indicator?

It's because they contain water-soluble dyes called anthocyanins – these coloured pigments change colour when mixed with an alkali or an acid.