Universal indicator ‘rainbow’

In this experiment, students observe as hydrochloric acid and sodium hydroxide are added to opposite 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.

Equipment

Apparatus

• 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

Chemicals

• 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.

Procedure

1. 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.
2. Ensure that one end of the glass tube is firmly stoppered with a rubber bung.
3. 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.
4. Add 3-4 drops of the hydrochloric acid solution. The top few centimetres of the liquid should turn red.
5. Stopper the upper end of the tube, remove it from the clamp, carefully invert it and then clamp it vertically again.
6. 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.
7. Stopper the tube. Both ends of the tube should now be firmly stoppered.
8. 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.