The ability of an acid to dissolve or destroy materials is known as corrosivity, rooted in its chemical composition and reactivity. Corrosion describes the gradual degradation of a material, often a metal, by chemical or electrochemical reaction. Acids possess a unique molecular structure that makes them aggressive toward a wide variety of substances, including inorganic materials like metals and and organic matter like living tissue.
The Chemical Definition of Acidity
A substance is chemically defined as an acid based on its capacity to release hydrogen ions, symbolized as H+, when dissolved in water. These highly reactive, positively charged particles are essentially protons that drive the corrosive reactions. The concentration of these hydrogen ions determines the acid’s strength, which is measured on the logarithmic pH scale.
The pH scale ranges from 0 to 14; any value below 7 is acidic, and a lower number indicates a greater concentration of H+ ions. Because the scale is logarithmic, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4. Strong acids, such as hydrochloric or sulfuric acid, release nearly all their hydrogen ions into a solution, making them highly reactive and more corrosive than weaker acids.
How Acids Corrode Metals
Acids corrode metals through a specific chemical interaction known as an oxidation-reduction or redox reaction. Metals are composed of atoms that hold their electrons loosely, making them susceptible to attack by the hydrogen ions in the acid. The H+ ions act as electron acceptors, seeking to gain an electron to become a stable hydrogen atom.
When an acid contacts a metal surface, the H+ ions strip electrons from the metal atoms. This process, called oxidation, converts the solid, neutral metal atoms into positively charged metal ions that dissolve into the solution. Simultaneously, the H+ ions that accepted the electrons are reduced, often combining to form molecules of hydrogen gas, which can be seen bubbling away. This results in the dissolution of the metal, forming a metal salt and degrading the material.
This reaction continues as long as the acid is in contact with the metal and H+ ions are available to accept electrons. The rate of destruction depends on the particular metal’s tendency to lose electrons and the concentration of the acid. Metals like iron and zinc react vigorously with common strong acids, whereas noble metals like gold and platinum resist this corrosive attack due to their stable electron structures.
How Acids Damage Organic Tissue
The mechanisms by which acids damage organic tissue are chemically distinct from their reaction with metals, primarily involving the disruption of complex biological molecules. Living tissue is composed largely of proteins, fats, and carbohydrates held together by specific chemical bonds. Acidic hydrogen ions catalyze a destructive process called hydrolysis.
During hydrolysis, the H+ ions accelerate the breaking of molecular bonds in biological polymers, such as the amide bonds in proteins and the ester bonds in fats. The acid uses water molecules present in the tissue to split these larger molecules into smaller, dysfunctional components. This process essentially dissolves the structural integrity of the cells and tissues, leading to a chemical burn.
Concentrated strong acids, such as sulfuric acid, inflict damage through a process of dehydration. These acids have a high affinity for water and chemically extract water molecules directly from the organic tissue. Removing water from cellular structures results in a severe thermal burn because the dehydration reaction is highly energetic, rapidly charring the tissue and causing deep cellular damage.