April 2006

Introduction

Mirrors have been reflecting the faces of the curious since ancient times. A mirror is a reflecting surface that forms an image of an object when light rays coming from that object fall upon it. The earliest mirrors were highly polished disks of rock or metals, such as bronze, tin, copper or silver.

Beginning in the Middle Ages, the mirror began to resemble its present form. Craftsmen created mirrors by depositing thin layers of a reflective mixture of tin and mercury on glass. Today's mirrors are also glass, but with a thin backing of silver or aluminum. Chlorine chemistry, in the form of the compound stannous chloride, plays a role in manufacturing mirrors during the process known as "silvering."

The Chemistry of Reflection

Stannous chloride, SnCl2, is a chemical compound of tin (Sn) and chlorine (Cl). (The term "stannous" comes from the Latin word for tin, stannum.) During the silvering process of mirror-making, plain glass is treated with stannous chloride before a shiny layer of aluminum or silver is produced.

Naming Ions
A positively charged ion-
one that has LOST
electrons-is called a
CATION. A negatively
charged ion-one that has
gained electrons-is called
an ANION.

Chemically, stannous chloride is a "reducing agent," meaning it can transfer some of its electrons to other chemicals. Recall that ions are atoms that have either lost or gained electrons, and are therefore electrically charged. Most salts, such as stannous chloride, form ions when they dissolve in water. This can be represented as:

When stannous chloride dissolves in water, tin and chlorine separate from their formerly bonded state: tin loses two electrons, taking on a "plus two (+2)" charge and chlorine gains one electron, taking on a "minus one (-1)" charge. The chemical formula SnCl2 indicates that stannous chloride contains two chlorine atoms for every tin atom. When the compound separates in water into its ions, the net electrical charge is zero because for every "plus two" ion of tin, there are two "negative one" ions of chlorine.

Stannous Chloride: Reducing Agent

In order to coat mirror glass with a thin layer of silver metal, silver is introduced in the form of another salt, such as silver (Ag) nitrate (NO3), written chemically as AgNO3. In water, silver nitrate breaks up:

Note that, unlike the other ions discussed, the nitrate ion is not composed of one chemical element, but two, nitrogen and oxygen. For that reason, nitrate is known as an ionic complex.

When a silver salt, such as AgNO3, is applied to a glass surface already treated with SnCl2, the tin ion Sn+2, already missing two electrons, gives up two more, taking on a final electrical charge of "plus four" (+4). Silver, which has a "plus one" (+1) charge in the water, takes up an electron from tin and "drops out of solution" or precipitates as silver metal, represented symbolically as Ag or Ag0 (the zero represents zero charge). When this happens, silver deposits as a shiny surface on the glass, and a mirror is created. Here's one way to represent what is happening among the various ions:

Mirrors in Space
NASA reports that the Hubble Space Telescope will be replaced by the James Webb Space Telescope sometime after June, 2013. The primary mirror of the new telescope will be 6.5 meters in diameter, more than 2.5 times the Hubble mirror diameter. (Learn more about space science at NASA's For Kids webpage!)

A Mirror: More than a Looking Glass

The simple object known as a mirror can be much more than a looking glass for checking your hair. Drivers rely on rear-and side-view mirrors to safely navigate roadways. Dentists use long-handled mirrors to examine your teeth.

Mirrors are components of very sophisticated optical instruments. The Hubble Space Telescope's "eyes" include two very smooth, precisely shaped mirrors that collect light. The reflecting surfaces of Hubble's mirrors are coated with a 0.000003 inch (three one-millionths of an inch!) layer of pure aluminum. Mirrors are also important components of microscopes and lasers.

One-way mirrors are used when witnesses of crimes are asked to identify suspects in police "line-ups". The one-way mirror appears to be a normal mirror from one side, but transmits images to the opposite side, like a window. The shiny, metal-coated surface of a one-way mirror is said to be "half-silvered," meaning that only about half the normal quantity of silver or aluminum atoms is applied to the glass. The resulting thin, sparse film of metal atoms reflects images from one side, but the uncoated spaces in the glass transmit images to the other side. The key to keeping "one-way" visibility is having brighter lighting on the "line-up" side of the glass than on the witness side. The well-lit side reflects light so the "line-up" sees only its reflection. The poorly-lit side transmits very little light through the sparsely-coated glass, thereby protecting the identity of the witness.

Follow-up Activities

1. Complete the following table to verify that the chemical reduction reaction that produces a silver coating on mirrors is "electrically" balanced:

2. Write chemical reactions to describe what happens when the following salts are dissolved in water and ions are formed:

  • KCl
  • BaCl2
  • SnCl4

3. Mirrors are used in everything from the simplest everyday items to the most advanced optical technology. Invent a useful item of your own using mirrors.

For a list of previous "Chlorine Compound of the Month" features, click here.

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