Demo Video: Photoreduction of Silver Chloride

Video Overview

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 tightly together.

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 atom.

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 obvious.

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.