TeacherExpt:Universal indicator ‘rainbow’
From Learn Chemistry Wiki
This procedure is designed as a demonstration to be performed by a teacher or other qualified person.
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.
Apparatus and chemicals
For one demonstration, the teacher will need:
- Glass tube (see note 1)
- Bungs (rubber), 2, to fit the glass tube
- Beaker (100 cm3)
- Dropper pipettes, 3
- Clamp stand, boss and clamp
- Distilled or deionised water
- Hydrochloric acid, 0.1 mol dm-3 (Low hazard at this concentration)
- Sodium hydroxide solution, 0.1 mol dm-3 (Irritant)
- Universal indicator solution (Highly flammable) (see note 2)
- Hydrochloric acid (Low hazard) Refer to CLEAPSS® Hazcard 47A.
- Sodium hydroxide (Irritant) Refer to CLEAPSS® Hazcard 91.
- Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and 40A, and CLEAPSS® Recipe Card 36.
- The glass tube needs to be about 60 cm long with an internal diameter of around 1 cm.
- The concentrations of the solutions are not critical.
Health & safety
Wear eye protection throughout
A. Add sufficient Universal indicator to about 60 cm3 of distilled or deionised 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.
This experiment has been reproduced from Practical Chemistry:
- An attractive video by chem-toddler.com (part of the Serbian Science Ministry) shows a similar experiment performed in a stirred beaker, while this related video by the same authors shows the experiment in a U-tube. Accessed January 2011.
- http://www.bbc.co.uk/schools/ks3bitesize/science/chemical_material_behaviour/acids_bases_metals/revise4.shtml on the BBC Schools website gives a useful overview of indicators, with pictures. Accessed January 2011.
Health & Safety checked, June 2007 Updated 29 Oct 2008