Conservation of Mass in Chemical Reactions

Democritus (460-370 BC) and somewhat later John Dalton (1766-1844) were the first to consider matter at its most microscopic form. They both came up with the concept of the "atom" as being the smallest unit of matter and thus being undivisible*. This observation has an important and fundamental consequence: mass is neither created nor destroyed during the course of a chemical reaction. How do we come to this conclusion? We know that chemical reactions take place at the atomic/molecular level. That is molecules and atoms interact with one onother during a chemical reaction. If atoms are indivisible then they cannot be destroyed during a chemical reaction. If atoms cannot be destroyed then the mass of reactants must equal the mass of the products in a chemical reaction. e.g.,

 Reactants -------> Products Mass of Reactants = Mass of Products

This can be visualized by considering the formation of water from oxygen and hydrogen molecules:

Note that the hydrogen and oxygen atoms simply rearrange themselves but are not destroyed. Thereofore mass is conserved. Mass conservation can be used in chemical calculations. For example iron rust by combining with oxygen to form rust (iron oxide). Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron?

 Iron + Oxygen -----> Rust 100 g + ?g ------> 143g mass reactants = mass products mass products = 143g = mass reactants = 100 + mass of oxygen mass oxygen = 43 g

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C101 Class Notes
Prof. N. De Leon