Demonstration

A long glass tube is filled with a neutral solution of Universal indicator. Hydrochloric acid is added to one end and sodium hydroxide solution to the other. The tube is inverted a few times to mix the solutions and the ‘rainbow’ of Universal indicator colours appears.

Lesson organisation

The demonstration itself takes only a few minutes. It provides a good attention-grabbing lesson starter or lesson endpoint.

Apparatus Chemicals

Eye protection

For one demonstration, the teacher will need:

Glass tube (Note 1)

Bungs (rubber), 2, to fit the glass tube

Beaker (100 cm3)

Dropper pipettes, 3

Clamp stand, boss and clamp

Deionised or tap water

Hydrochloric acid, 0.1 M

Sodium hydroxide solution, 0.1 M (IRRITANT)

Universal indicator solution (HIGHLY FLAMMABLE)

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection throughout. 

Hydrochloric acid, HCl(aq) - see CLEAPSS Hazcard. The concentration of the solution is not critical.

Sodium hydroxide, NaOH(aq), (IRRITANT at concentration used) - see CLEAPSS Hazcard. The concentration of the solution is not critical.

Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

1 The glass tube needs to be about 60 cm long with an internal diameter of around 1 cm.

Procedure

a Add sufficient Universal indicator to about 60 cm3 of deionised or tap water in a beaker, to give a solution with a visible green colour.

b Ensure that one end of the glass tube is firmly stoppered with a rubber bung.

c Fill the tube to about 2 cm from the top with the Universal indicator solution. Then clamp the tube vertically. It is important to leave a space above the liquid in the tube so that there is an air bubble – this helps the mixing in step h.

d Add 3-4 drops of the hydrochloric acid solution. The top few centimetres of the liquid should turn red.

e Stopper the upper end of the tube, remove it from the clamp, carefully invert it and then clamp it vertically again.

f Remove what is now the top stopper. Add 3-4 drops of the sodium hydroxide solution. The top few centimetres of the liquid should turn purple.

g Stopper the tube. Both ends of the tube should now be firmly stoppered.

h Remove the tube from the clamp and carefully invert it 2 or 3 times. The movement of the air bubble will mix the contents and produce a ‘rainbow’ in the tube, showing all the colours of Universal indicator from red through orange, yellow, green, blue and purple.

Teaching notes

A white background is useful to show the colours.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015