Understand acids and alkalis, indicators and neutralisation within this experiment where learners use their own sense of curiosity to discover more

This session should take 100 minutes. 

Your task

Using an orange, and a lemon, learners can find out which of the two contains the greater total amount of acid.

You have laboratory glassware and equipment available, but no chemicals other than those which you are able to extract from the plant material provided.

You are also provided with the ash from a charcoal barbecue or garden bonfire, which is rich in potassium carbonate. 


  • Eye protection
  • Beakers
  • Burettes
  • Pipettes (& safety fillers) 
  • Graduated volumetric flasks
  • Conical flasks
  • Measuring cylinders
  • Glass droppers
  • Plastic syringes
  • Spatulas
  • Glass stirring rods
  • Watch glasses
  • Test tubes
  • Filter funnels and papers
  • Glass juicers
  • Fine-mesh sieves
  • Knives
  • White tiles to cut fruit on
  • Bunsen burners
  • Tripods
  • Gauzes
  • Heatproof mats
  • Clamp stands
  • A centrifuge and balances should be available.
  • Pestle and mortar x1 per group
  • Orange
  • Lemon
  • Hot water

THE ‘ASHING’ PROCESS: (ash prepared before lesson by technician)

A large quantity of leaves, seaweed or other plant material can be ‘ashed’ to provide an alkali (largely sodium carbonate from sea plants, and potassium carbonate from land plants). [Some investigation may be needed to determine suitable plants & suitable quantities. See Annales de Chimie, xix, 157 and 194.]

Health, safety and technical notes

Read our standard health and safety guidance here.

Ashing must be done in the fume-cupboard.

Eye protection should be worn during the ‘ashing’ process or when using boiling water to make the indicator, but the rest of the activity is low risk.

It is the responsibility of the teacher to carry out a suitable risk assessment. 

This activity is open-ended and as such teachers should be extra cautious when overseeing learners.

Possible approaches

The ash is dissolved in water to make an alkaline solution, this is then filtered. The oranges & lemons are sliced up and all the juice squeezed out of them; the juice and chopped pieces are boiled in water and then filtered.

The red cabbage leaves are boiled in water (the dye goes into solution), the red cabbage solution is filtered. A titration is then performed with both the acids from the lemon and the acids from the orange. The cabbage dye is used as an indicator (end point: red to green). The alkaline solution from the ash is placed in the burette.

Quantitative result

Typically, lemons contain approximately 3.5 times more ‘acid’ than oranges. This result can be linked with taste.

Alternatively, an acid and a carbonate react to give off CO2. This emission of CO2 will result in a weight loss. By calculating this weight loss, the relative amount of CO2 given off can be calculated. As more acid will react to give off more CO2, the quantity of CO2 emitted can be used to determine which contains more acid.

Extension work

Does acidity depend on variety of fruit, age, ripeness?

Is peel different from flesh? Titrate separately.

Experiment could also be allied to some work on wine-making (the wine producer needing to know the acidity of the grape). 


This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources.