<P> Joseph Louis Gay - Lussac recognized in 1808 that gases always react in a certain relationship with each other . Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which later resulted in the concepts of stoichiometry and chemical equations . </P> <P> Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically . According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials . This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828 . Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, who, among many discoveries, established the mechanisms of substitution reactions . </P> <P> Chemical equations are used to graphically illustrate chemical reactions . They consist of chemical or structural formulas of the reactants on the left and those of the products on the right . They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds . A double arrow (⇌) pointing in opposite directions is used for equilibrium reactions . Equations should be balanced according to the stoichiometry, the number of atoms of each species should be the same on both sides of the equation . This is achieved by scaling the number of involved molecules (A, B, C (\ displaystyle (\ ce (A, B, C))) and D (\ displaystyle (\ ce (D))) in a schematic example below) by the appropriate integers a, b, c and d . </P> <Dl> <Dd> a A + b B ⟶ c C + d D (\ displaystyle (\ ce (((\ mathit (a)) A) + (\ mathit (b)) B -> ((\ mathit (c)) C) + (\ mathit (d)) D))) </Dd> </Dl>

What names are given to the materials at the end of a chemical reaction