Back   Home   Next

The Law of Conservation of Mass
The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant. We can state this important law in another way. The total mass of the universe is constant within measurable limits; whenever matter undergoes a change, the total mass of the products of the change is, within measurable limits, the same as the total mass of the reactants.

The formulation of this law near the end of the eighteenth century marked the beginning of modern chemistry. By that time many elements had been isolated and identified, most notably oxygen, nitrogen, and hydrogen. It was also known that, when a pure metal was heated in air, it became what was then called a calx (which we now call an oxide) and that this change was accompanied by an increase in mass. The reverse of this reaction was also known: Many calxes on heating lost mass and returned to pure metals. Many imaginative explanations of these mass changes were proposed. Antoine Lavoisier (1743-1794), a French nobleman later guillotined in the revolution, was an amateur chemist with a remarkably analytical mind. He considered the properties of metals and then carried out a series of experiments designed to allow him to measure not just the mass of the metal and the calx but also the mass of the air surrounding the reaction. His results showed that the mass gained by the metal in forming the calx was equal to the mass lost by the surrounding air.

With this simple experiment, in which accurate measurement was critical to the correct interpretation of the results, Lavoisier established the Law of Conservation of Mass, and chemistry became an exact science, one based on careful measurement. For his pioneering work in the establishment of that law and his analytical approach to experimentation, Lavoisier has been called the father of modern chemistry.

Note that this step forward, like so many others in science, depended on technology - in this instance, on the development of an accurate and precise balance (see Figure 1.5).

FIGURE 1.5 Lavosier's apparatus for heating mercury in a confined volume of air (after a drawing by Mme. Lavoisier).

Back   Home   Next