Colour creation

Mix acids, alkalis and bases to make 6 distinct colours.

This experiment should take 60 minutes.

Task

Starting from Universal indicator solution, water, hydrochloric acid solution and sodium hydroxide solution only:

1. Produce six different coloured solutions (ie red, orange, yellow, green, blue and violet).
2. When you have completed Part 1 (or after 20 minutes), produce a reliable “recipe” for creating one colour decided by your teacher.

Equipment

• Rack of test tubes
• Glass droppers
• Beakers, 100 cm³
• Measuring cylinders, 10 cm³
• Universal indicator solution, 5 cm³
• Distilled water or tap water, 50 cm³
• Sodium hydroxide (0.1 mol dm-3), 50 cm³

Health, safety and technical notes

• Read our standard health and safety guidance here.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• At these concentrations hydrochloric acid and sodium hydroxide are of low hazard so eye protection is not essential. 
• Universal indicator may be made up in ethanol and so could be flammable. If so, keep away from sources of ignition.

Possible approaches

Part one

Students may need encouragement to tackle the experiment systematically. They may also need to be helped to see that further dilution of the acid and alkali will result in less abrupt colour changes (alternatively, students could be given more dilute acid and alkali).

Some students found it very difficult to produce the 6 colours. To avoid the possible difficulties associated with different shades of the same colour, a constant volume should be presented in the tubes for judging (e.g. half full tubes) and also a constant volume of indicator used each time.

Part two

Each team must show their recipe to the judge. They should then try to make their given colour, following their recipe, under the eyes of the person acting as judge.

Notes

Check that the water is neutral before starting.