Microscale precipitation and neutralisation reactions

Following integrated instructions, learners will observe the dissolution of the crystals, colour change of the indicator and formation of the product. 

This resource supports Teaching observation skills at 11–14 and Explicitly teach learners how to make observations, part of the Teaching science skills series from Education in Chemistry. Both articles offer ideas and tips for supporting your learners to develop their observation skills.

This microscale practical involves mixing chemicals in small drops of water on the instruction sheet and observing the reactions.

The potassium iodide and lead(II) nitrate(V) crystals will dissolve into the water. Yellow lead(II) iodide crystals will form in the centre of the drop over time as the ions diffuse towards each other. 

From their observations, encourage your learners to make inferences on what is happening at the sub-microscopic level as the solid dissolves, the ions diffuse, and a new insoluble substance is produced. 

Intended outcomes

1. Make detailed and accurate observations of chemical reactions.
2. Use your observations to infer what is happening as these reactions take place.

Equipment

• Integrated instruction sheets, laminated or in a plastic wallet
• 2 x dropping pipettes
• 2 x micro spatulas
• Paper towel for clearing up
• Universal indicator solution
• Water
• Citric acid, a few crystals (WARNING: irritant)
• Anhydrous sodium carbonate, a few crystals (WARNING: irritant)
• Lead(II) nitrate(V), a few crystals (DANGER: harmful and toxic)
• Potassium iodide, a few crystals
• Eye protection: safety glasses to EN166 F 

Health and safety

• Read our standard health and safety guidance and carry out a risk assessment before running any live practical. If in doubt, contact your school health and safety advisor. 
• Check CLEAPSS/SSERC or another reputable source for full risk assessment guidance. Hazard classification may vary depending on the supplier. 
• Wear eye protection (safety glasses to EN166 F).
• Ensure hands are washed after the activity.
• See the downloadable technician notes for further health and safety advice about the chemicals used in this practical. 

Method

Reaction 1: Precipitation of lead iodide

1. Add a few crystals of potassium iodide to the left-hand small circle.
2. Add a few crystals of lead(II) nitrate(V) to the right-hand small circle.
3. Add 10 drops of water to the large central circle.
4. Carefully push the crystals into the edges of the water. Observe the dissolution of the crystals and the formation of lead iodide.

Example reaction

Reaction 2: Neutralisation of citric acid

1. Add a few crystals of anhydrous sodium carbonate to the left-hand small circle.
2. Add a few crystals of citric acid to the right-hand small circle.
3. Add 10 drops of water to the large central circle. Add 1 drop of universal indicator solution to the water.
4. Carefully push the crystals into the edges of the water. Observe the dissolution of the crystals, the change in colour of the indicator and the formation of carbon dioxide bubbles.

This resource is part of our Teaching science skills series, bringing together strategies and classroom activities to help your learners develop essential scientific skills, from evaluation to ethics.

Example reaction

Disposal

Wipe down the laminated sheet/plastic wallet with a paper towel and dispose of the towel in laboratory waste.