Microscale neutralisation and precipitation reactions

Small and microscale chemistry practicals provide plenty of variety and opportunity to refine observation skills. For more tips and ideas on how to hone your students’ observation skills, refer to the EiC article Teaching observation skills at 11–14.

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

Kit

Per student/group:

• 2 dropping pipettes
• 2 microspatulas
• practical sheet, laminated or in a plastic wallet
• paper towel for clearing up
• universal indicator solution
• water
• citric acid, a few crystals (irritant)
• anhydrous sodium carbonate, a few crystals (irritant)
• lead(II) nitrate(V), a few crystals (harmful and toxic)
• potassium iodide, a few crystals

Reaction 1: Precipitation of lead iodide

To create a lead nitrate precipitate, rather than mixing potassium iodide solution with lead nitrate solution in a test tube, students push crystals of the solids into either side of the drop of water. 

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 small circle.
3. Add 10 drops of water into the large central circle.
4. Carefully push the crystals into the edges of the drop of water.
5. Observe the dissolution of the crystals and the formation of lead iodide.

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 students 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.

Example reaction

Reaction 2: Neutralisation of citric acid

The second reaction involves the neutralisation of citric acid. Students push crystals of sodium carbonate and citric acid into a drop of water containing universal indicator.

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 small circle.
3. Add 10 drops of water into the large central circle.
4. Add 1 drop of universal indicator solution to the drop of water.
5. Carefully push the crystals into the edges of the drop of water.
6. Observe the dissolution of the crystals, the change in colour of the indicator, and the formation of carbon dioxide bubbles.

The sodium carbonate will dissolve into solution, turning the green indicator to blue/purple. The citric acid will dissolve into solution, turning the green indicator orange/red. Where the solutions meet, neutralisation will occur, and bubbles of carbon dioxide gas will form over time. The carbon dioxide bubbles take time to develop, and close observation and patience are again important.

Example reaction

Health and safety

• Wear eye protection.
• Ensure hands are washed after the activity.
• Lead(II) nitrate(V) is harmful and toxic. Sodium carbonate is irritant. Citric acid is irritant. Only very small quantities of the solids are being used, reducing the risk.
• Indicator solutions can be flammable depending on the solvent used. Ensure there are no naked flames in the laboratory.
• Check CLEAPSS/SSERC or another reputable source for full risk assessment guidance.

Disposal

Wipe down the laminated sheet/plastic wallet with a paper towel and dispose of the towel in normal refuse. Rinse and dry the laminated sheet/plastic wallet.