Class practical or demonstration

Insoluble silver and lead halides form as precipitates when solutions of silver or lead salts are added to solutions containing halide ions.

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.

The use of silver halides in film photography is illustrated by the effect of light on the precipitates.

The solubility of lead halides in hot, but not in cold, water provides a useful illustration of recrystallisation.

Lesson organisation

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.

Apparatus Chemicals

Eye protection

Access to a fume cupboard

Each demonstration or working group requires:

Test-tube rack

Test-tubes, 6

Boiling tubes, 3

Test-tube holder (to fit a boiling tube)

Teat pipettes, 2

Beaker (250 cm3 or 500 cm3)

Bunsen burner

For optional part:

Test-tubes, 3

Teat pipettes, 2

Corks or rubber bungs to fit test-tubes, 3

For optional part, in a fume cupboard:

Teat pipettes

Corks or rubber bungs to fit test-tubes, 3

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 part, in a fume cupboard:

Dilute ammonia solution ~0.1 M, about 10 cm3

Concentrated ammonia solution (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), a few cm3

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection

Potassium (or sodium) chloride solution, KCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Potassium (or sodium) bromide, KBr(aq) - see CLEAPSS Hazcard.  

Potassium (or sodium) iodide solution, KI(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book

Silver nitrate solution, AgNO3(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. Caution - even dilute solutions can stain skin and clothing.

Lead nitrate solution, Pb(NO3)2(aq),  (TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book

Dilute ammonia solution, NH3(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book

Concentrated ammonia solution, NH3(aq), (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Procedure

Silver halides
a Pour about 3 cm3 of each of the halide solutions into separate test-tubes.

b Add a few drops of silver nitrate solution to the test-tube containing potassium chloride solution. A white precipitate of silver chloride forms.

c Add a few drops of silver nitrate solution to potassium bromide solution. A cream or off-white coloured precipitate of silver bromide forms.

d Add a few drops of silver nitrate solution to potassium iodide solution. A yellow precipitate of silver iodide forms.

e Pour half the contents of the three test-tubes into another three labelled test-tubes.

f 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.

(g-j Optional: use ammonia solution to distinguish between the silver halides.)

g Prepare fresh samples of the silver halide precipitates as above, a–d.

h 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.

i 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.

j 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
a Pour about 5 cm3 of the potassium halide solutions into separate boiling tubes.

b Add five drops of lead nitrate (TOXIC) solution to the test-tube containing potassium chloride solution. A white precipitate of lead(II) chloride forms.

c 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.

d Place the boiling tube in a beaker of cold water to cool. Fine crystals of lead chloride appear.

e Repeat b–d with potassium bromide solution. A white precipitate of lead(II) bromide forms, which dissolves on heating and recrystallises on cooling.

f Repeat b–d 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.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015