Students can create their own neutralisation circles in this explorative practical using common chemistry classroom materials
Add drops of diluted acid and alkali to a sheet of paper, add universal indicator, and watch as neutralisation circles appear.
Drops of dilute acid and alkali are placed a few centimetres apart on a sheet of filter paper and allowed to spread out until they meet.
A few drops of universal indicator are then placed over the moist area of the filter paper and a band of colours showing the range of colours of the universal indicator is seen on the paper.
This experiment will take around 10 minutes.
- Eye protection
- Three dropping pipettes
- A pencil
- A white tile
- A sheet of filter paper, approximately 12.5 cm diameter
- Hydrochloric acid 0.1 mol dm–3
- Sodium hydroxide, 0.1 mol dm–3
- Universal indicator solution
Health, safety and technical notes
- Read our standard health and safety guidance.
- Wear eye protection throughout.
- Hydrochloric acid is low hazard (see CLEAPSS Hazcard HC047a).
- Sodium hydroxide is an irritant (see CLEAPSS Hazcard HC091a).
- Universal indicator solution is highly flammable (see CLEAPSS Hazcard HC032).
- Other acids and alkalis and other indicators (or mixtures of indicators) including ‘homemade’ ones (from red cabbage, for example) could be tried.
- Toilet roll and other white tissue may be used instead of filter paper, but they appear to dry less successfully.
- A hair drier or oven may be found useful to dry the filter papers quickly.
- Students should be given a piece of filter paper and asked to draw on it in pencil two circles about 1 cm in diameter and about 2–3 cm apart, which they label ‘acid’ and ‘alkali’ respectively.
- The filter paper should then be placed on a while tile and students use dropping pipettes to place a few drops of the appropriate solution in each circle.
- The concentrations of the acid and alkali are not critical, but they should be approximately the same.
- The solution will begin to spread out on the filter paper.
- The students should for a few minutes until the solutions have soaked through the filter paper towards each other and have met.
- Students should then place drops of Universal Indicator solution on the area of the filter paper where the acid and alkali have met and reacted.
- A ‘rainbow’ will be produced, showing the range of colours produced by Universal indicator.
- This experiment is quicker, simpler and safer than the traditional method of illustrating neutralisation by titrating acid with alkali using a burette. It also uses more familiar equipment (a dropping pipette rather than a burette), uses little of the reagents and has the advantage of producing a permanent record of the colour changes.
- The reaction is:
- HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
- Whatman paper no. 1 works well, but chromatography paper appears to be less successful.
- Experiment | PDF, Size 0.13 mb
This practical is part of our Chemistry for non-specialists collection. This experiment has been adapted from a version written by Ted Lister on behalf of the RSC.