AcidEdit

An acid is a substance that interacts with water and other substances to increase the concentration of hydronium ions or, more broadly, a substance capable of donating a proton or accepting an electron pair in a chemical reaction. The way chemists describe acids has evolved over time, moving from the early Arrhenius notion—acids yield hydrogen ions in water—to broader theories such as the Brønsted-Lowry concept, which defines acids as proton donors, and the Lewis view, which emphasizes electron-pair acceptance. In aqueous solutions, the pH scale provides a practical gauge of acidity: lower pH means higher acidity, and a wide variety of substances—from simple inorganic minerals to complex organic molecules—participate in acid-base chemistry. See pH for a fuller treatment of the measurement and interpretation of acidity.

Acidic substances occur naturally and play essential roles in biology, geology, and everyday life, while many acids are manufactured for commerce. Common examples include hydrochloric acid hydrochloric acid in the stomach, as well as organic acids such as acetic acid in vinegar and citric acid in citrus fruits. These substances differ in strength, stability, and reactivity, but all share the property of engaging in reactions that release protons or accept electron pairs. In the laboratory and in industry, acids are used for etching, cleaning, digestion of samples, refinement of metals, and as catalysts in chemical synthesis. For readers seeking specific chemical species, see entries such as sulfuric acid, nitric acid, and phosphoric acid.

From a policy and practical standpoint, acids touch many domains: health and safety in laboratories and workplaces, environmental stewardship, and the economics of manufacturing and energy use. A pragmatic approach to problems involving acids emphasizes clear evidence, cost-effectiveness, and innovations that improve safety and efficiency. See environmental regulation for the broader policy context in which acid-related issues arise, industrial policy for the economic framework that shapes investment in chemical processing, and workplace safety for standards governing handling and exposure.

Definitions and concepts

Arrhenius definition: An acid is a substance that increases the concentration of hydronium ions in water. This widely taught model helps explain many everyday reactions but is most accurate for aqueous systems. See Arrhenius acid.
Brønsted-Lowry definition: An acid donates a proton to another substance, which broadens the class of acids beyond aqueous solutions. See Bronsted-Lowry acid.
Lewis definition: An acid accepts an electron pair, which allows chemists to discuss reactions that do not involve protons at all. See Lewis acid.
pH and acidity: The measure of acidity in a solution, reflecting the balance of hydronium ions, is central to chemistry, biology, medicine, and environmental science. See pH.

Different acids vary in strength (how completely they donate protons in a given solvent) and in the number of protons they can donate (monoprotic versus polyprotic). They also differ in their organic or inorganic nature, which influences volatility, corrosivity, and reactivity in synthetic pathways. For examples of specific acids and their properties, consult entries such as hydrochloric acid (a strong mineral acid), sulfuric acid (a strongly dehydrating, highly exothermic acid used in many industries), and acetic acid (a weak organic acid common in food chemistry).

Types of acids

Mineral acids: Inorganic acids derived from minerals. Prominent examples include hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. These acids are central to many industrial processes, including metal processing, chemical synthesis, and metallurgy.
Organic acids: Carbon-containing acids often found in biological systems and in food and flavor chemistry. Notable examples are acetic acid, citric acid, and oxalic acid.
Aqueous versus non-aqueous behavior: In non-water solvents or in gas phase, acids may behave differently or reveal different aspects of acidity, leading to the broader concept of Lewis acidity in catalysis and materials science.
Monoprotic and polyprotic acids: Some acids donate a single proton per molecule, while others can donate two or more; polyprotic acids can participate in multiple successive acid-base steps.

See also entries for the individual acids and their roles in synthesis, materials science, biology, and environmental processes, such as sulfuric acid, nitric acid, phosphoric acid, acetic acid, and citric acid.

Properties, reactions, and measurement

Acid-base reactions: Acids react with bases in neutralization reactions to form salts and water, a foundational concept in chemistry. See acid-base reaction.
Safety and handling: Acids can be corrosive and reactive; proper storage, ventilation, and protective equipment are essential in laboratories and industry. See chemical safety and hazardous material handling guidelines.
Indicators and titration: Indicators help determine the extent of neutralization, which is a common analytical technique in chemistry labs and industrial quality control.
Biological relevance: In living organisms, acids participate in digestion, metabolism, and signaling, and enzymes can be sensitive to pH. See gastric acid and enzyme biology for context.

Uses and applications

Industrial chemistry: Acids are used in leaching, electroplating, catalyst preparation, and polymerization processes. For example, lead-acid battery electrolytes rely on sulfuric acid, and phosphoric acid is central to fertilizer production.
Food and beverage: Food-grade acids such as acetic acid and citric acid contribute to flavor, texture, and preservation.
Medicine and science: Acids are employed in pharmaceutical manufacturing, analytical chemistry, and laboratory research. See entries for specific acids and their applications.
Environmental concerns and mitigation: The release of acidic compounds into air and water can lead to ecological effects, including soil and water acidification. This has driven regulatory programs and technological solutions, such as emission control devices and scrubbers on industrial stacks. See acid rain and environmental regulation.

Environmental aspects and regulation

Acid rain and atmospheric emissions: Emissions of sulfur dioxide and nitrogen oxides contribute to acidification of soils and water bodies in downwind regions. The topic has been central to environmental policy debates and regulatory action. See acid rain and sulfur dioxide and nitrogen oxides.
Policy response: Policy approaches range from emission reductions and performance standards to market-based mechanisms like cap-and-trade. Proponents argue that well-designed programs yield environmental benefits at reasonable costs and encourage innovation, while critics caution about compliance costs and the distribution of burden. See cap-and-trade and Clean Air Act.
Debates and perspectives: The discussion around how best to balance environmental protection with industrial competitiveness is ongoing. A pragmatic approach emphasizes measurable outcomes, flexible compliance paths, and technological progress—factors that can reduce the long-run costs of cleaner production while safeguarding jobs and energy security.

Safety, handling, and hazards

Work practice and PPE: Proper handling procedures, ventilation, and personal protective equipment are essential when dealing with acids in the lab or in industry.
Storage and transport: Acids require appropriate containers, separation from incompatible materials, and appropriate labeling to reduce the risk of spills and reactions.
Health and environmental risks: Acute exposure can cause burns and respiratory irritation; long-term exposure and improper disposal can lead to environmental harm. See occupational safety and environmental impact for broader context.