Silver chloride is a white solid. Pass a laser over silver chloride powder, and it will turn
gray. This is an example of a photochemical reaction; the laser light, made up of packets of
energy called photons, causes a chemical reaction. Sunlight will do the same thing, but it
will be slower than using a laser.
The silver chloride powder is made of ions of silver ()
and chlorine (, known as chloride ions), which are packed
A teaspoon of silver chloride contains about 2 hundred billion trillion ions ( ions).
When laser light hits the powder, electrons are transferred from the chloride ions to the
silver ions, producing chlorine gas and solid silver.
The reaction is as follows. It can also be written
using the symbols for silver () and chlorine ().
Chlorine atoms are more stable if they pair up, forming chlorine molecules (). The in the equation means that
each reaction produces half a chlorine molecule.
The reaction can be broken down further into two steps.
In the first step, the chloride ion loses an electron ().
A photon excites the electron enough that it is free to move.
The loss of electrons is called oxidation; the chloride ion is oxidized to a chlorine
In the second step, the silver ion gains the electron.
The gain of electrons is called reduction; the silver ion is reduced to a silver atom.
So, the silver ions turn to gray silver atoms, and the chloride ions turn to chlorine atoms
that pair up to form chlorine molecules. Chlorine gas is very difficult to see as it is a very
pale yellow, but the change from the white of silver chloride to the gray of silver is
Since the reduction of the silver was caused by photons, the whole process with respect to
silver can be called a photoreduction.
Silver chloride is one of the substances used in photographic film, along with other silver
halides. Tiny crystals are mixed with gelatin, which helps to hold them in place, and they
change color when exposed to light. Photographic film can be used to capture the image
produced by the lens of a camera.
After the film is exposed (when a picture is taken) the film is developed and fixed with
other chemicals, meaning that the image is preserved and the film is no longer sensitive to
light. Different colors are achieved by using colored filters to expose different layers of
silver halide crystals.