Silver and lead halides

In this experiment, students add silver and lead salts to a variety of solutions containing halide ions, producing insoluble silver and lead halides as precipitates. The silver chloride, bromide and iodide can be distinguished by their colours and their solubility in ammonia solution, providing tests for these halide ions in solution.

By investigating the effect of light on the silver halides, students can explore their use in film photography, while the solubility of lead halides in hot, but not in cold, water provides a useful illustration of recrystallisation.

These reactions can be demonstrated or investigated as a class practical.

Reactions can be confined to the silver halides as part of an investigation of Group 7 chemistry. Using ammonia to distinguish between the silver halides is more appropriate at an advanced level.

Equipment

Apparatus

• Eye protection
• Access to a fume cupboard
• Test tube rack
• Test tubes, x6
• Boiling tubes, x3
• Test tube holder, to fit a boiling tube
• Teat pipettes, x2
• Beaker, 250 cm³ or 500 cm³
• Bunsen burner

For optional stage

• Test tubes, x3
• Teat pipettes, x2
• Corks or rubber bungs to fit test tubes, x3

For optional stage in a fume cupboard

• Teat pipettes
• Corks or rubber bungs to fit test tubes, x3

Chemicals

• Potassium chloride solution, 0.1 M, about 30 cm³
• Potassium bromide solution, 0.1 M, about 30 cm³
• Potassium iodide solution, 0.1 M, about 30 cm³
• Silver nitrate solution, 0.05 M (DANGEROUS FOR THE ENVIRONMENT), about 1 cm³
• Lead nitrate solution, 0.1 M (TOXIC, DANGEROUS FOR THE ENVIRONMENT), about 1 cm³

For optional stage in a fume cupboard

• Dilute ammonia solution ~0.1 M, about 10 cm³
• Concentrated ammonia solution (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), a few cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Potassium (or sodium) chloride solution, KCl(aq) – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB068 or RB082.
• Potassium (or sodium) bromide, KBr(aq) – see CLEAPSS Hazcard HC047b.  
• Potassium (or sodium) iodide solution, KI(aq) – see CLEAPSS Hazcard and HC047b CLEAPSS Recipe Book RB072. 
• Silver nitrate solution, AgNO₃ (aq) – see CLEAPSS Hazcard HC087 and CLEAPSS Recipe Book RB077. Caution – even dilute solutions can stain skin and clothing.
• Lead nitrate solution, Pb(NO₃)₂ (aq), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a and CLEAPSS Recipe Book RB053. 
• Dilute ammonia solution, NH₃ (aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006. 
• Concentrated ammonia solution, NH₃ (aq), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.

Procedure

Silver halides

1. Pour about 3 cm³ of each of the halide solutions into separate test tubes.
2. Add a few drops of silver nitrate solution to the test tube containing potassium chloride solution. A white precipitate of silver chloride forms.
3. Add a few drops of silver nitrate solution to potassium bromide solution. A cream or off-white coloured precipitate of silver bromide forms.
4. Add a few drops of silver nitrate solution to potassium iodide solution. A yellow precipitate of silver iodide forms.
5. Pour half the contents of the three test tubes into another three labelled test tubes.
6. Place one set of three test tubes in a cupboard and the other set in bright light, such as on a window sill, and leave for 5–10 mins. In bright light, the silver chloride darkens quickly, the silver bromide more slowly, and the silver iodide is not affected at all. Compare with the solutions kept in the dark.

Optional: use ammonia solution to distinguish between the silver halides

USE A FUME CUPBOARD.

7. Prepare fresh samples of the silver halide precipitates as above, steps 1–4.
8. Slowly add an equal volume of dilute ammonia solution to the test tube containing silver chloride using a teat pipette. Shake well after each addition to mix the contents. The precipitate dissolves, giving a colourless solution.
9. Add an equal volume of DILUTE ammonia solution to the test tube containing silver bromide. Shake to mix. The precipitate does not dissolve. Now add concentrated ammonia solution to almost fill the test tube, stopper the tube and invert to mix. Most of the precipitate dissolves.
10. Add an excess of CONCENTRATED ammonia solution to the test tube containing silver iodide, stopper and invert to mix. The precipitate does not dissolve.

Lead halides

1. Pour about 5 cm³ of the potassium halide solutions into separate boiling tubes.
2. Add five drops of lead nitrate (TOXIC) solution to the test tube containing potassium chloride solution. A white precipitate of lead(II) chloride forms.
3. Heat the mixture carefully over a gentle flame until it boils. Avoid using a yellow tipped flame as it will make the tube sooty. The precipitate dissolves.
4. Place the boiling tube in a beaker of cold water to cool. Fine crystals of lead chloride appear.
5. Repeat steps 2–4 with potassium bromide solution. A white precipitate of lead(II) bromide forms, which dissolves on heating and recrystallises on cooling.
6. Repeat steps 2–4 with potassium iodide solution. A yellow precipitate of lead(II) iodide forms which dissolves on heating to give a colourless solution. On cooling, fine shimmering yellow crystals of lead(II) iodide form.

Teaching notes

Any spillages of silver or lead nitrate on the skin should be washed off with plenty of water. Silver nitrate causes black stains on the skin which wear off slowly.

These precipitation reactions can be represented by the following equations, where X = Cl, Br or I:

KX(aq) (or Na) + AgNO₃(aq) → AgX(s) + KNO₃(aq) (or Na)

2KX(aq) + Pb(NO₃)₂(aq) → PbX₂(s) + 2KNO₃(aq)

or the general ionic equations:

X^–(aq) + Ag⁺(aq) → AgX(s)

2X^–(aq) + Pb²⁺(aq) → PbX₂(s)

The silver chloride experiment can be modified to produce a photographic paper on which an image can be recorded. Here a simple extension is to filter off the freshly prepared silver chloride precipitate (covering the funnel to exclude light), and then opening the filter paper out onto a white tile and placing it in bright light. The silver chloride darkens quickly. Partly covering the precipitate on the paper will emphasise the effect of light.

The decomposition of silver chloride is an example of a photochemical reaction. Silver metal and chlorine atoms are produced.

The optional experiments involving ammonia to distinguish between the silver halides should be tried beforehand. This is because the result depends on the relative amounts of the precipitate and ammonia. Silver chloride and iodide are not usually a problem – the silver iodide loses some of its yellow colour. But the extent to which the silver bromide dissolves depends on the actual concentration of ammonia in the test tube.

The recrystallisation of lead iodide is particularly eye-catching, producing a shower of fine yellow crystals.