Rotting is fundamentally a chemical change, a natural process where complex organic matter breaks down into simpler substances. This decomposition is driven by microorganisms, which fundamentally alter the molecular structure of the original material. The process transforms materials like food scraps or fallen leaves, recycling the components back into the environment in a new chemical form.
Defining Chemical Versus Physical Change
A chemical change is defined by the formation of one or more new substances with different chemical properties than the starting materials. This transformation involves the breaking of old chemical bonds and the creation of new ones, and the process is difficult or impossible to reverse. Burning wood, for example, converts cellulose into ash, carbon dioxide, and water vapor, permanently changing its composition.
In contrast, a physical change alters the form, state, or appearance of a substance without changing its chemical identity. The composition of the material remains the same, and the change is often reversible. When an ice cube melts into liquid water, its physical state changes, but the molecules are still H2O.
The Mechanisms of Decomposition
Rotting, or decomposition, is a complex series of chemical reactions driven by decomposers like bacteria and fungi. These organisms secrete extracellular enzymes that break down large, complex organic molecules, such as proteins, carbohydrates, and lipids. This enzymatic action is a form of chemical digestion that occurs outside the microbial cell.
Hydrolytic enzymes like cellulases break down complex carbohydrates such as cellulose into simpler sugars like glucose. Similarly, proteases cleave proteins into amino acids. This process, called catabolism, involves breaking the chemical bonds of the organic matter and altering its molecular structure. The result is the transformation into simpler organic and inorganic compounds, such as carbon dioxide and water.
Observable Evidence of Chemical Change
The chemical changes occurring during rotting produce several observable indicators that confirm a chemical reaction has taken place.
A significant sign is the production of foul odors, caused by the release of new volatile compounds. For example, protein breakdown produces amines like putrescine and cadaverine, which are responsible for the characteristic stench of decay.
Another clear piece of evidence is the formation of gases, which can manifest as bubbles or swelling in sealed environments. In anaerobic conditions, bacteria produce methane and hydrogen sulfide, the latter contributing a distinct rotten-egg smell.
Furthermore, the dramatic discoloration, such as the browning of a rotting apple, is a result of enzymatic oxidation reactions that produce new pigments. Decomposition is often an exothermic process, meaning it releases heat as the chemical bonds are broken and reformed.