Before Lavoisier, people used to think _________ could appear and disappear?

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Before Lavoisier, people used to think that matter could appear and disappear. Antoine Lavoisier, a french chemist, deduced (through careful experimentation) that in a chemical reaction, the mass of reactants and mass of products remains constant (within reasonable limits). This led him to establish one of the fundamental laws of chemistry: the law of conservation of matter. This states that matter can neither be created nor destroyed, although it can change form. This changed our perception of matter and is also considered the beginning of modern chemistry. 

Interestingly, Lavoisier's experiments were designed to calculate the amount of products and reactants when metals are heated in air. This reaction causes the production of metal oxides, a reaction in which an increase in mass of products (as compared to reactants) is observed. Lavoisier showed an equivalent decrease in the mass of air around the metal and thus concluded that matter is conserved. 

Hope this helps. 

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