The first step toward a theory of chemical reactions was taken by Georg Ernst Stahl in 1697 when he proposed the phlogiston theory, which was based on the following observations.

• Metals have many properties in common.
• Metals often produce a "calx" when heated. (The term calx is defined as the crumbly residue left after a mineral or metal is roasted.)
• These calxes are not as dense as the metals from which they are produced.
• Some of these calxes form metals when heated with charcoal.
• With only a few exceptions, the calx is found in nature, not the metal.

These observations led Stahl to the following conclusions.

• Phlogiston (from the Greek phlogistos, "to burn") is given off whenever something burns.
• Wood and charcoal are particularly rich in phlogiston because they leave very little ash when they burn. (Candles must be almost pure phlogiston because they leave no ash.)
• Because they are found in nature, calxes must be simpler than metals.
• Metals form a calx by giving off phlogiston.

Metal calx + phlogiston

• Metals can be made by adding phlogiston to the calx.

Calx + phlogiston metal

• Because charcoal is rich in phlogiston, heating calxes in the presence of charcoal sometimes produces metals.

This model was remarkably successful. It explained why metals have similar properties they all contained phlogiston. It explained the relationship between metals and their calxesthey were related by the gain or loss of phlogiston. It even explained why a candle goes out when placed in a bell jar the air eventually becomes saturated with phlogiston.

There was only one problem with the phlogiston theory. As early as 1630, Jean Rey noted that tin gains weight when it forms a calx. (The calx is about 25% heavier than the metal.) From our point of view, this seems to be a fatal flaw: If phlogiston is given off when a metal forms a calx, why does the calx weigh more than the metal? This observation didn't bother proponents of the phlogiston theory. Stahl explained it by suggesting that the weight increased because air entered the metal to fill the vacuum left after the phlogiston escaped.

The phlogiston theory was the basis for research in chemistry for most of the 18th century. It was not until 1772 that Antoine Lavoisier noted that nonmetals gain large amounts of weight when burned in air. (The weight of phosphorus, for example, increases by a factor of about 2.3.) The magnitude of this change led Lavoisier to conclude that phosphorus must combine with something in air when it burns. This conclusion was reinforced by the observation that the volume of air decreases by a factor of 1/5th when phosphorus burns in a limited amount of air.

Lavoisier proposed the name oxygene (literally, "acid-former") for the substance absorbed from air when a compound burns. He chose this name because the products of the combustion of nonmetals such as phosphorus are acids when they dissolve in water.

P4(s) + 5 O2(g) P4O10(s)

P4O10(s) + 6 H2O(l) 4 H3PO4(aq)

Lavoisier's oxygen theory of combustion was eventually accepted and chemists began to describe any reaction between an element or compound and oxygen as oxidation. The reaction between magnesium metal and oxygen, for example, involves the oxidation of magnesium.

2 Mg(s) + O₂(g) 2 MgO(s)

By the turn of the 20th century, it seemed that all oxidation reactions had one thing in common oxidation always seemed to involve the loss of electrons. Chemists therefore developed a model for these reactions that focused on the transfer of electrons. Magnesium metal, for example, was thought to lose electrons to form Mg²⁺ ions when it reacted with oxygen. By convention, the element or compound that gained these electrons was said to undergo reduction. In this case, O₂ molecules were said to be reduced to form O^2- ions.

A classic demonstration of oxidation-reduction reactions involves placing a piece of copper wire into an aqueous solution of the Ag⁺ ion. The reaction involves the net transfer of electrons from copper metal to Ag⁺ ions to produce whiskers of silver metal that grow out from the copper wire and Cu²⁺ ions.

Cu(s) + 2 Ag⁺(aq) Cu²⁺(aq) + 2 Ag(s)

The Cu²⁺ ions formed in this reaction are responsible for the light-blue color of the solution. Their presence can be confirmed by adding ammonia to this solution to form the deep-blue Cu(NH₃)₄²⁺ complex ion.

Chemists eventually recognized that oxidation-reduction reactions don't always involve the transfer of electrons. There is no change in the number of valence electrons on any of the atoms when CO₂ reacts with H₂, for example,

CO₂(g) + H₂(g) CO(g) + H₂O(g)

as shown by the following Lewis structures:

Chemists therefore developed the concept of oxidation number to extend the idea of oxidation and reduction to reactions in which electrons are not really gained or lost. The most powerful model of oxidation-reduction reactions is based on the following definitions.

Oxidation involves an increase in the oxidation number of an atom.

Reduction occurs when the oxidation number of an atom decreases.

According to this model, CO₂ is reduced when it reacts with hydrogen because the oxidation number of the carbon decreases from +4 to +2. Hydrogen is oxidized in this reaction because its oxidation number increases from 0 to +1.

Information provided by: http://chemed.chem.purdue.edu