In the preceding paragraphs oxidation and burning have been used as equivalent phrases: this is in accordance with the teachings of chemistry. To the chemist a substance is burned when it is combined with oxygen, whether this combination take place slowly or rapidly. If the combination occur rapidly the burning or oxidizing mass becomes very hot and also gives off light: such a rapid and vigorous oxidation is called a combustion ; no combustions take place in our bodies.

It has, however, been proved that whether the combination of oxygen with an oxidizable, or burnable, substance takes place rapidly or slowly, at the end of the process exactly the same amount of energy will have been set free in each case. When the oxidation occurs in a few seconds the oxidizing mass becomes very hot: when it occurs slowly, in a few days or weeks, the mass will never be very hot, because the heat set free in the process is carried off nearly as fast as it appears.

Illustrations Of Oxidations At A Low Temperature

If a piece of magnesium wire be ignited in the air it will become white-hot, flame, and leave at the end of a few seconds only a certain amount of incombustible rust or magnesia, which consists of the metal combined with oxygen ; under these circumstances it has been burnt or oxidized quickly at a high temperature. The heat and light evolved in the process represent the energy which is set free by the metal and oxygen when they combine. We can, however, oxidize the metal in a different way, attended with no evolution of light and no very perceptible rise of temperature If, for instance, we leave it in wet air, it will become gradually turned into magnesia without having ever been hot to the touch or luminous to the eye. The process then, however, takes days or weeks; but in this slow oxidation just as much energy is liberated as in the former case, although now all takes the form of heat; and instead of being liberated in a short time is spread over a much longer one, as the gradual chemical combination takes place. The slowly oxidizing magnesium is, in consequence, at no moment noticeably hot, since it loses its heat to surrounding objects almost as fast as it generates it. The oxidations occurring in our bodies are of this slow kind. An ounce of arrowroot oxidized in a fire, and in the human body, would liberate exactly as much energy in one case as the other, but the oxidation would take place in a few minutes and at a high temperature in the former, and slowly, at a lower temperature, in the latter.

What does a chemist mean when he says a substance is burned ? What is a combustion ? Do combustions occur in our bodies ?

Does the quantity of energy liberated by the complete oxidation of any substance vary with the rate of oxidation ?

Why is a slowly oxidizing mass of matter not very hot ?

Give an instance of the oxidation of the same substance at high and low temperatures.