Conservation of Mass The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. If we account for all reactants and products in a chemical reaction, the total mass will be the same at any point in time in any closed system. 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. The Law of Conservation of Mass holds true because naturally occurring elements are very stable at the conditions found on the surface of the Earth. Most elements come from fusion reactions found only in stars or supernovae. Therefore, in the everyday world of Earth, from the peak of the highest mountain to the depths of the deepest ocean, atoms are not converted to other elements during chemical reactions. Because of this, individual atoms that make up living and nonliving matter are very old and each atom has a history. An individual atom of a biologically important element, such as carbon, may have spent 65 million years buried as coal before being burned in a power plant, followed by two decades in Earth's atmosphere before being dissolved in the ocean, and then taken up by an algal cell that was consumed by a copepod before being respired and again entering Earth's atmosphere. The atom itself is neither created nor destroyed but cycles among chemical compounds. Ecologists can apply the law of conservation of mass to the analysis of elemental cycles by conducting a mass balance. These analyses are as important to the progress of ecology as Lavoisier's findings were to chemistry. Sources: The Nature Education Knowledge Project http://www.nature.com/scitable/knowledge/library/the-conservation- of-mass-17395478 Fundamentals of Chemistry http://chem.wisc.edu/deptfiles/genchem/sstutorial/Text1/Tx14/tx14.html ©Sacramento Area Science Project 2014 .