Try this demonstration to create a rainbow effect using universal indicator, hydrochloric acid and sodium hydroxide
In this experiment, students observe as hydrochloric acid and sodium hydroxide are added to opposit ends of a long glass tube filled with a neutral universal indicator solution. As the demonstrator inverts the tube a few times to mix the solutions, students can see a ‘rainbow’ of universal indicator appear.
The demonstration itself takes only a few minutes. It provides a good attention-grabbing lesson starter or lesson endpoint.
- Eye protection
- Glass tube (see note 6 below)
- Rubber bungs to fit the glass tube, x2
- Beaker, 100 cm3
- Dropper pipettes, x3
- 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)
Health, safety and technical notes
- Read our standard health and safety guidance.
- Wear eye protection throughout.
- Hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC047a. The concentration of the solution is not critical.
- Sodium hydroxide, NaOH(aq), (IRRITANT at concentration used) – see CLEAPSS Hazcard HC091a. The concentration of the solution is not critical.
- Universal indicator solution (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000.
- The glass tube needs to be about 60 cm long with an internal diameter of around 1 cm.
- 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.
- Ensure that one end of the glass tube is firmly stoppered with a rubber bung.
- 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 8.
- Add 3-4 drops of the hydrochloric acid solution. The top few centimetres of the liquid should turn red.
- Stopper the upper end of the tube, remove it from the clamp, carefully invert it and then clamp it vertically again.
- 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.
- Stopper the tube. Both ends of the tube should now be firmly stoppered.
- 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.
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.
© Nuffield Foundation and the Royal Society of Chemistry
Health and safety checked, 2016