Chemical reactions are the heart of chemistry. People have
             always known that they exist. The Ancient Greeks were the firsts
             to speculate on the composition of matter. They thought that it
             was possible that individual particles made up matter.
             Later, in the Seventeenth Century, a German chemist named
             Georg Ernst Stahl was the first to postulate on chemical
             reaction, specifically, combustion. He said that a substance
             called phlogiston escaped into the air from all substances during
             combustion. He explained that a burning candle would go out if a
             candle snuffer was put over it because the air inside the snuffer
             became saturated with phlogiston. According to his ideas, wood
             is made up of phlogiston and ash, because only ash is left after
             combustion. His ideas soon came upon some contradiction. When
             metal is burned, its ash has a greater mass than the original
             substance. Stahl tried to cover himself by saying that
             phlogiston will take away from a substance's mass or that it had
             a negative mass, which contradicted his original theories.
             In the Eighteenth Century Antoine-Laurent Lavoisier, in
             France, discovered an important detail in the understanding of
             the chemical reaction combustion, oxigine (oxygen). He said that
             combustion was a chemical reaction involving oxygen and another
             combustible substance, such as wood.
             John Dalton, in the early Nineteenth Century, discovered the
             atom. It gave way to the idea that a chemical reaction was
             actually the rearrangement of groups of atoms called molecules.
             Dalton also said that the appearance and disappearance of
             properties meant that the atomic composition dictated the
             appearance of different properties. He also came up with idea
             that a molecule of one substance is exactly the same as any other
             molecule of the same substance.
             People like Joseph-Lois Gay-Lussac added to Dalton's
             concepts with the postulate that the volumes of gass...