Apparatus

One 100cm beaker.

Three boiling tubes and rack.

Dropping pipette.



Chemicals

The quantities given are for one demonstration


Phenolphthalein indicator solution
(0.1g solid dissolved in 60cm ethanol and 40cm water)

A few pellets of solid sodium hydroxide.

About 100cm of 0.5 mol dm sodium hydroxide.

A few cm of approximately 2 mol dm hydrochloric acid.

The Demonstration

Add a few drops of phenolphthalein solution to about 100cm of 0.5 mol dm sodium hydroxide in a beaker until a deep pink colour is visible. Divide this solution between the three boiling tubes. Leaving one tube as a control, add hydrochloric acid dropwise to one of the other two until the colour disappears. This is the 'usual' behaviour of phenolphthalein. To the third tube, add two or three pellets of solid sodium hydroxide and swirl to dissolve. The pink colour will disappear in this tube too. The colour changes can be reversed by appropriate additions of acid or alkali.

A white background will help.

The demonstration could be presented by starting with the colourless solution of phenolphthalein in concentrated alkali and adding acid to it to give an unexpected colour change.