Investigating the solubilities of lead halides
The lesson may begin with a discussion of general solubility patterns among common ionic compounds. From these patterns, students predict the pattern of solubilities among the lead halides, and what might happen when solutions containing lead ions and different halide ions are mixed. They can test their predictions experimentally using dilute solutions containing the relevant ions. They then follow this up by testing the solubilities of the lead halides when heated, and confirm the pattern by allowing the solutions to cool again. Throughout they use ionic equations when making predictions and explaining observations.
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Lesson organization
This is a straightforward class practical, based on predicting behaviour from known patterns of chemical properties. The known patterns of solubilities and of properties of Group 7 compounds will need to be familiar to students – if not, these will need to be discussed first.
The practical itself will take about 25 minutes to the point at which the hot solutions are ready to cool down, plus however long it then takes for the precipitates to re-form on cooling.
Apparatus and chemicals
Each working group will need:
Eye protection
Boiling tubes (large test-tubes), 3
Test-tube rack
Test-tube holder
Beaker (250 cm3)
Bunsen burner
Heat resistant mat
The following 0.2 mol dm-3 solutions should be available in dropper bottles (see note 3)
• lead nitrate, 0.2 mol dm-3 (Toxic, Dangerous for environment at this concentration), 20 cm3 (see note 1)
• potassium chloride, 0.2 mol dm-3 (Low hazard), 10 cm3 (see notes 1 & 2)
• potassium bromide, 0.2 mol dm-3 (Low hazard), 10 cm3 (see notes 1 & 2)
• potassium iodide, 0.2 mol dm-3 (Low hazard), 10 cm3 (see notes 1 & 2)
Technical notes
Lead nitrate (Toxic, Dangerous for environment both as solid and as 0.2 mol dm-3 solution) Refer to CLEAPSS Hazcard 57A and Recipe Card 42
Potassium chloride (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe Card 51
Potassium bromide (Low hazard) Refer to CLEAPSS Hazcard 47B
Potassium iodide (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe Card 55
1 Sodium halides can be used instead of potassium halides if preferred.
2 If dropper bottles are not available for dispensing solutions, students will also require dropping pipettes to dispense solutions.
Procedure
HEALTH & SAFETY: Wear eye protection.
Wash hands after using lead nitrate solution.
a Place 3 boiling tubes in a row in a test-tube rack.
b To each tube add about a 3 cm depth of 0.2 mol dm-3 lead nitrate solution.
c To the first tube add 5 drops of 0.2 mol dm-3 potassium chloride solution, and note what happens. Keep the mixture formed for g below.
d To the second tube add 5 drops of 0.2 mol dm-3 potassium bromide solution, and note what happens. Keep the mixture formed for g below.
e To the third tube add 5 drops of 0.2 mol dm-3 potassium iodide solution, and note what happens. Keep the mixture formed for g below.
f Heat each of the test tubes from b, c, d in turn in a low Bunsen flame until the mixtures are boiling. Allow them to boil very gently for a minute.
g Allow the three boiling tubes to cool down. If time is short, they may be cooled by standing in a beaker of cold water.
h Observe and record what happens in each of the three tubes as the mixtures in them cool.
Teaching Notes
This practical can be carried out either as an investigative experiment, as described above, or as a simple exploration without use of prior knowledge. In the latter case, students will not need to be familiar with solubility patterns of ions in solution and their reactions, or with ionic equations.
However, to make it an investigation, the points below need to be considered:
1 Students will need some generalizations about the solubilities of salts before they can be asked to predict whether or not a precipitate will form on mixing two solutions and what the precipitate will be. If these have not been taught previously, the lesson may need to start with a minimum of these generalizations:
• all nitrates are soluble
• all sodium and potassium salts are soluble
• all chlorides are soluble except for the chlorides of lead and silver
• all lead salts are insoluble in cold water except for the nitrate (and ethanoate).
2 Students also need to be able to think in terms of cations and anions ions behaving independently in solution. Diagrams such as those below can be helpful.
3 Students need to be able to translate these diagrams into the formalities of chemical equations; eg the ionic equation:
Pb2+(aq) + 2I-(aq) →PbI2(s)
and the full equation:
Pb(NO3)2(aq) + 2KI(aq) →PbI2(s) + 2KNO3(aq)
The solubilities of the lead halides increase markedly with temperature, so that the three halides under investigation are all effectively soluble in boiling water. This means that on cooling these solutions, the lead halides will crystallise out again. For lead chloride and lead bromide, the effect is rapid and the crystals small, so their appearance returns to that of a precipitate.
However for lead iodide, especially on slower cooling, the effect of recrystallisation can be spectacular, with thin golden flaky crystals of lead iodide shimmering in suspension, and falling as golden rain to the bottom of the tube. If the students do not observe this phenomenon, it is well worth the teacher repeating this stage as a demonstration.
See also on the Practical Chemistry website Silver and lead halides.
Health & Safety checked, August 2008
Web Links
The variation of crystal size for lead iodide with rate of cooling can be used as a laboratory model to demonstrate how rate of cooling of magmas affects crystal size in the resulting igneous rocks:
Updated 29 Oct 2008
