Preparing an insoluble salt in a precipitation reaction

Using the procedure below, the experiment should take no more than 30 minutes to the point at which the solution produced has been filtered. If reagent solutions are provided in ready-measured quantities in small labelled bottles, the experimental work can start immediately.

Most classes should be able to perform this experiment as a class experiment, but if there are real doubts about safe behaviour or adequate manipulative skills while solutions are being heated, or hot solutions being poured into the filter paper, then student-aided demonstrations of these may be more sensible.

Equipment

Apparatus

• Eye protection (goggles)
• Test tubes, 100 mm x 16 mm, x2
• Rubber stopper to fit test tube
• Beakers, 100 cm³, x2
• Teat pipette
• Glass rod, 15 cm
• Measuring cylinder, 25 cm³
• Wash bottle with purified (distilled or deionised) water
• Filter funnel, 65mm diameter
• Filter paper
• Bunsen burner
• Tripod
• Gauze
• Heat resistant mat
• Centrifuge (note 1)

Apparatus notes

1. If a centrifuge is not available the solid can be separated by filtration, however the crystals obtained will not be as pure using this method.

Chemicals

• Lead(II) nitrate solution, 1 M (TOXIC, DANGEROUS FOR THE ENVIRONMENT), 5 cm³ (in a small bottle)
• Sodium chloride solution, 2 M, 10 cm³ (in a small bottle)

Health & Safety and Technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• Lead(II) nitrate solution (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a and CLEAPSS Recipe Book RB053.
• Sodium chloride solution – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB082.
• Lead(II) chloride (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a.
  
  At the end of the experiment the lead chloride should NOT be discarded into the rubbish. Collect together and place in a labelled bottle for future use (if you think it is pure enough) or for eventual disposal as hazardous waste by a licensed contractor.

Procedure

Stage 1

1. Add 5 cm³ of 1 M lead(II) nitrate solution to a test tube. Then add an equal volume of 2 M sodium chloride solution, stopper the tube and shake gently to mix thoroughly.
2. Centrifuge the suspension, and pour away the clear liquid. Add a few cm³ of purified water to the solid in the test tube, then stopper and shake to wash the precipitate. Centrifuge again, and pour away the clear liquid.
   
   If a centrifuge is not available, filter the precipitate using a filter paper and filter funnel. Wash the precipitate by pouring a small amount of purified water over the residue and allowing it to filter through.

Stage 2

3. If a centrifuge was used, add a little more purified water to the test tube, then stopper and shake to mix. Quickly pour the mixture into a beaker, and use more purified water to rinse any remaining solid from the test tube to the beaker. Make up the volume of liquid in the beaker to about 30 cm³ with more purified water.
4. If the precipitate was obtained by filtration, transfer the solid residue to a beaker and add about 30 cm³ of purified water.
5. Heat the suspension in the beaker gently over a medium Bunsen flame, with stirring, until the suspension has all dissolved in the hot water. Remove the flame and allow to cool. Crystals should form on cooling. They can be collected by filtration and dried on a paper towel. At the end of this experiment, you MUST wash your hands to remove any toxic lead compounds that may have splashed onto them.

Teaching notes

There may be problems associated with younger students heating beakers perched on tripods, and with lifting hot glassware off a hot tripod after heating. They should not be sitting down while carrying out these operations. Using tongs of suitable size is a good solution for lifting the hot beakers, but some schools may not have these. If there is any doubt about the safety of this step, you should assist students when lifting each beaker down onto the heat resistant mat.

Students should be told to be careful when handling the lead nitrate solution and the solid lead chloride produced. Insist that they wash their hands after any splashes and at the end of the preparation.

Students who have not used a centrifuge before need to be instructed on how to use it, especially the need for a balancing tube with a similar volume of water.

The balanced equation for the reaction is only relevant for older students:

Pb(NO₃)₂ (aq) + 2NaCl(aq) → PbCl₂ (s) + 2NaNO₃ (aq)

Otherwise, a simple word equation is sufficient, but note that there is no easy way of demonstrating that sodium nitrate is the other product left in solution.