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.
The demonstration itself takes only a few minutes. It provides a good attention-grabbing lesson starter or lesson endpoint.
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
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.
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.
A white background is useful to show the colours.
Health & Safety checked, 2016
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
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
The experiment is also part of the Royal Society of Chemistry’s Continuing Professional Development course: Chemistry for non-specialists