Precipitation reactions of lead nitrate

Many lead compounds are insoluble and some of them are brightly coloured. In this experiment, students observe the colour changes of lead nitrate solutions when different anions are added to identify which compounds would make good pigments.

They then compare what happens when deionised water and tap water are added to lead nitrate solution, giving explanations for what they observe.

The experiment should take approximately 20 minutes.

Equipment

Apparatus

• Eye protection
• Student worksheet (available for download below)
• Clear plastic sheet (eg ohp sheet)

Chemicals

• Sodium hydroxide, 1 mol dm^–3
• Lead nitrate, 0.5 mol dm^–3
• Potassium iodide, 0.2 mol dm^–3
• Sodium chloride, 0.5 mol dm^–3
• Potassium bromide, 0.2 mol dm^–3
• Sodium carbonate, 0.5 mol dm^–3
• Sodium sulfate, 0.5 mol dm^–3
• Potassium chromate, 0.2 mol dm^–3

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
• Sodium hydroxide solution, NaOH(aq), 1 mol dm^–3 is CORROSIVE. See CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
• Lead nitrate, Pb(NO₃)₂(aq), 0.5 mol dm^–3 is a reproductive toxin and a specific target organ toxin. It also causes eye damage and is a probable carcinogen. See CLEAPSS Hazcard HC057a and CLEAPSS Recipe Book RB053.
• Potassium chromate, K₂CrO₄, 0.2 mol dm^–3 is a carcinogen, mutagen and skin sensitiser. It is also toxic to aquatic life. Wear splash-proof eye-protection if transferring large amounts. Avoid skin contact. See CLEAPSS Hazcard HC078a and CLEAPSS Recipe Book RB069.
• The following chemicals are low hazard:
  • Potassium bromide, KBr(aq), 0.2 mol dm^–3 – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB068.
  • Sodium sulfate, Na₂SO₃ (aq), 0.5 mol dm^–3 – see CLEAPSS Hazcard HC098B and CLEAPSS Recipe Book RB107.
  • Sodium carbonate, 0.5 mol dm^–3 – see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080.
  • Potassium iodide, KI(aq), 0.2 mol dm^–3 – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB072.

Procedure

Part 1: adding different anions to lead nitrate solution

1. Cover the worksheet with a clear plastic sheet.
2. Put one drop of lead nitrate solution in each box of table 1.
3. Add one drop of each of the solutions containing the anions indicated to the appropriate box.

Part 2: adding deionised water and tap water to lead nitrate solution

1. With the worksheet still covered, put one drop of lead nitrate solution into each box of table 2.
2. Add one drop of deionised water and one drop of tap water to the appropriate boxes.

Questions for students

Part 1

1. Which of the lead compounds observed appear to be good pigments?
2. What is the main disadvantage of using these compounds as pigments?

Part 2

1. What explanations can you give for your observations?

Teaching notes and expected observations

Part 1

The addition of solutions of each of the anions produces precipitates, which indicates that in general lead compounds are insoluble. The iodide is an intense yellow colour, the chromate(VI) is also yellow and both could be used as pigments except for the fact that lead compounds are toxic.

Part 2

The fact that lead forms insoluble compounds is used as a basis for indicating the presence of anions in water. The addition of deionised water to lead nitrate gives no cloudiness. However, with tap water a cloudiness gradually develops if the water is from a hard water area since carbonates, sulphates or hydrogen carbonates may be present. If you live in a soft water area there will probably be no cloudiness. (One solution is to simulate hard water conditions.)