Creating an effervescent universal indicator ‘rainbow’

With a little universal indicator, acid, and sodium carbonate, students will be amazed by the effervescent reaction that causes a ’rainbow’ to appear. 

This experiment should take five minutes. 

Equipment

Apparatus

• Eye protection
• Burette, 50 cm^–3 
• Retort stand with boss and clamp
• Cotton wool plug 

Chemicals

• Universal Indicator solution
• Hydrochloric acid solution, 10 cm³, 2 mol dm^–3
  
• Sodium carbonate solution, 20 cm³, 1 mol dm^–3
  

Health, safety and technical notes

• Read our standard health and safety guidance
• Always wear eye protection
• Hydrochloric acid is low hazard (see CLEAPSS Hazcard HC047a).
• Sodium carbonate is low hazard (see CLEAPSS Hazcard HC095a).

Procedure

1. Clamp the burette vertically.
2. Add 0.5 cm³ of the Universal indicator solution followed by 10 cm³ of the hydrochloric acid to give a clearly visible red colour.
3. Add 20 cm³ of the sodium carbonate solution.
4. Insert a loose plug of cotton wool in the top of the burette.
5. The sodium carbonate and hydrochloric acid react, with effervescence, and the burette will be filled with liquid showing a ‘rainbow’ of all the colours of Universal indicator from red through orange, yellow, green and blue to purple.

Notes

Place the burette against a white background to show the colours to best advantage.