The light sensitive silver halides, silver chloride, silver bromide and silver iodide, are used to make photographic film and photographic paper. In this experiment, a photographic print is produced.

Class practical or Demonstration

The light sensitive silver halides, silver chloride, silver bromide and silver iodide, are used to make photographic film and photographic paper. In this experiment photographic paper coated in silver chloride is produced by bringing solutions of silver nitrate and potassium chloride solutions into contact on the paper in the absence of light. A photographic image of an object placed on it can be obtained when the paper is dried and exposed to strong light.

Lesson organisation

Depending on the availability of a darkened room, this experiment can be done either as a demonstration or a class experiment. It may even be possible to do it successfully in an open laboratory if the paper can be protected from strong light once the silver nitrate solution has been applied to it, and during the final drying.

Before producing the photographic paper, the formation of the insoluble silver halides as precipitates on mixing silver nitrate solution with solutions of potassium chloride, bromide and iodide in test-tubes should be demonstrated or done as part of the class experiment.

On placing the test-tubes containing the precipitates in strong light – e.g. on a window sill – the silver chloride darkens rapidly. The change is much slower with the silver bromide and iodide produced in this way.

When done as a demonstration, these experiments should take about 10 minutes, excluding light exposure time.

Apparatus Chemicals

Eye protection

For one demonstration or one pair of students:

Protective gloves (preferably nitrile gloves)

A square of white paper, about 10 x 10 cm, or a filter paper of similar size

Small paint brushes, 2

Test-tubes, 3

Test-tube rack

Hairdryer (Note 1)

Ultraviolet light source (Optional) (Note 2)

Each demonstration (or pair of students) requires 0.1 M solutions of:

Potassium chloride, 10 cm3

Potassium bromide, 5 cm3

Potassium iodide, 5 cm3

Silver nitrate, 10 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 chloride, KCl(aq), potassium bromide, KBr(aq) and potassium iodide, KI(aq), solutions are all LOW HAZARD. Solutions of the sodium salts can be used in place of the potassium salts. See CLEAPSS Hazcard and CLEAPSS Recipe Book.

Silver nitrate solution, AgNO3(aq), is LOW HAZARD at this concentration but will still stain skin, clothing and some bench materials. The silver nitrate solution should be made up using distilled or deionised water as the chloride content of tap water gives a cloudy solution due to the formation of a small amount of silver chloride. See CLEAPSS Hazcard and CLEAPSS Recipe Book for further information and disposal instructions.

1 Ensure the hairdryer has had a portable electrical appliance test.

2 The UV light source should be a safe UVA, or so-called ‘blacklight’, lamp such as that used for locating spots in chromatography. Shield the lamp so that it cannot be viewed directly.

Procedure

Silver halide precipitates
a Place about 5 cm3 of potassium chloride, potassium bromide and potassium iodide solution in three separate test-tubes.

b To each solution add about 1 cm3 of silver nitrate solution. A silver halide precipitate forms – ranging in colour from white (silver chloride), to cream (silver bromide) and yellow (silver iodide).

c Place the test-tubes containing the precipitates in strong light – e.g. on a window sill. After several minutes, depending on the light level, the silver chloride darkens to a dark grey colour as metallic silver forms. The other two silver halides change much more slowly, if at all, under these conditions.

Photographic paper
a Paint one side of the piece of paper with some of the remaining potassium chloride solution. Dry the paper with a hair dryer.

b In a darkened room, or shielding the paper from as much bright light as possible, paint the same side of the paper with some of the remaining silver nitrate solution, using a different brush. Dry the paper with a hairdryer.

c Put your chosen object (preferably something flat, with a sharp outline eg a coin or a key) on the treated side of the paper and place it in bright sunlight or under UV light until the exposed part of the paper darkens. Do not look directly at the UV light.

d Remove the object and the light source. An image of your object should be visible on the paper.

Teaching notes

The precipitation reactions forming the silver halides are also used as tests for the presence of halide ions in solution. The general equation for these reactions is:

MX(aq) + AgNO3(aq) → AgX(s) + MNO3(aq)

where M = K or Na and X = Cl, Br or I

Or more simply in ionic form:

Ag+(aq) + X-(aq) → AgX(s)

The decomposition of the silver halides in light is a photochemical redox reaction in which an electron is transferred from the halide ion to the silver ion, forming silver atoms and chlorine atoms:

Light 
AgX → Ag + Cl

The formation of metallic silver causes the darkening of the areas exposed to light. In photography the exposed paper is then ‘fixed’ to remove the unexposed silver chloride.

Digital photography has of course now largely replaced silver-based film for home photography

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