A fundamental type of chemical change. In simple decomposition, one substance breaks down into two simpler substances, e.g., water yields hydrogen and oxygen. In double decomposition, two compounds break down and recombine to form two different compounds, e.g., 2HCl + CaCO3 → CaCl2 + H2CO3. In some cases, heat is absorbed, and in others it is evolved.Decomposition may occur as a result of (1) reaction at room temperature (NaOH + HCl → NaCl + H2O), (2) heating in air (C + H2O → CO + H2), (3) electrolysis (inorganic compounds), (NaClaq + e → Na+ + Cl−), (4) bacterial or enzymic action (fermentation, C6H12O6 → 2C2H5OH + 2CO2), (5) radiation (photodecomposition), as in the breakdown of chlorofluorocarbons in the upper atmosphere and of biodegradable polymers exposed to sunlight (6) heating in absence of air (thermal decomposition), in which carbonaceous raw materials such as coal and natural gas are converted into carbon and volatile organic compounds without undergoing combustion (coal → coke, coal, tar and coal gas). The term thermal decomposition is virtually synonymous with pyrolysis and destructive distillation.See Degradation; Pyrolysis; Destructive Distillation.
