Chemistry

Hafnium (Hf): properties, occurrence, history and uses Hafnium Hafnium (Hf, atomic number 72) is a silver-gray transition metal related to zirconium, used in nuclear control rods, high-temperature alloys and modern microelectronics.

Half-life (radioactive decay) Half-life (element) Half-life is the time required for half of a given quantity of unstable atoms to decay. It describes exponential decay, applies to radioisotopes, drugs, and processes with constant…

Halide (chemistry) Halide Halide compounds contain a halogen atom bonded to another element. They range from ionic salts like sodium chloride to organic halides, hydrogen halides and light-sensitive silver…

Hallucinogens: types, effects, history, uses and risks Hallucinogen Overview of hallucinogens: categories (psychedelics, dissociatives, deliriants), typical effects and mechanisms, cultural history, medical and research uses, risks and emergency re…

Haloalkane (alkyl halide) Haloalkane Haloalkanes are organic compounds in which one or more hydrogen atoms of an alkane are replaced by halogen atoms; they are important in synthesis, industry, and environmental chemi…

Halogens (Group 17 elements) Halogen Halogens are the reactive Group 17 elements—fluorine, chlorine, bromine, iodine, astatine and tennessine—known for forming salts and halide compounds with many uses in industry, me…

Hard water: causes, characteristics, effects and treatment Hard water Hard water contains high levels of dissolved minerals—mainly calcium and magnesium. This article explains its chemistry, types, impacts on homes and industry, health considerations…

Hashish: resin from the cannabis plant — composition, history, and uses Hashish Hashish is a concentrated cannabis resin used historically and today for recreational and medicinal purposes; its composition, production methods, cultural history, and legal statu…

Hassium: a synthetic, highly radioactive transactinide element (Hs, Z=108) Hassium Hassium (Hs, atomic number 108) is a synthetic, highly radioactive transactinide element produced in particle accelerators and studied only in tiny amounts for research.

Heat of Combustion (Calorific Value): Definition, Measurement, and Uses Heat of combustion The heat released when a substance burns completely. Defined as a calorific or energy value, measured by calorimetry or calculated from enthalpies, and used to compare fuels and ev…

Heavy metals: definitions, properties, uses and environmental impact Heavy metals Heavy metals are dense metallic elements or their compounds. This article explains definitions, physical and chemical properties, common examples, uses, environmental and health co…

Heavy water (deuterium oxide) Heavy water Heavy water (D2O) is water in which hydrogen atoms are replaced by deuterium. It has distinct physical and nuclear properties and is used as a neutron moderator, research tracer, a…

Helium — properties, occurrence, uses, and history Helium Helium is the light, inert noble gas with symbol He (atomic number 2). This article describes its properties, isotopes, origins, common uses, safety issues and notable low-temperat…

Heptachlor: persistent organochlorine insecticide and environmental pollutant Heptachlor Heptachlor is a highly persistent organochlorine insecticide formerly used against termites, ants and mosquitoes. It bioaccumulates, is toxic to liver and nervous system, and is li…

Heptane (C7H16): properties, isomers, uses, and safety Heptane Heptane is a colorless hydrocarbon (C7H16) used as a solvent and fuel component. This article summarizes its structure, isomers, physical properties, applications, and handling con…

Hexachlorobenzene (HCB) Hexachlorobenzene Hexachlorobenzene, an organochlorine compound once used as a fungicide, is persistent in the environment, bioaccumulative, toxic to humans and wildlife, and is now widely banned un…

Hexadecene (cetene): structure, isomers, properties and uses Hexadecene Hexadecene (C16H32) is a 16‑carbon alkene family including linear alpha‑olefins and internal isomers. This article covers structure, isomerism, production, applications and safety…

Hexafluorosilicic acid (H2SiF6) Hexafluorosilicic acid A corrosive inorganic acid used industrially to produce fluorosilicates and in water fluoridation; formed during hydrofluoric acid production and notable for its reactivity and saf…

Hexane Hexane, C6H14, is a six‑carbon alkane used as an industrial solvent and fuel component. It exists as several isomers, with n‑hexane being the straight‑chain form and a known neurot…

Holmium (chemical element) Holmium Holmium (Ho, atomic number 67) is a lanthanide rare-earth metal. It is silvery, malleable, usually trivalent, and used in medical lasers, optical calibration glasses, and specializ…

Homogeneity: Meaning, Examples, and Importance Across Sciences Homogeneity Homogeneity denotes uniformity or similarity throughout a substance, space, or dataset. This article explains its meanings, examples in chemistry and mathematics, assessment method…

Homologous series (chemistry) Homologous series A homologous series is a family of compounds sharing a general formula and repeating unit; members differ by a constant increment and show gradual, predictable changes in physical…

Household ammonia (dilute ammonium hydroxide) Household ammonia A dilute aqueous ammonia solution commonly used for household cleaning and degreasing. Weakly alkaline, typically 5–10% NH3; effective on glass and grease but corrosive to some mat…

Hückel's rule: aromaticity in cyclic conjugated systems Hückel's rule A guideline from molecular orbital theory that predicts when planar, cyclic, conjugated molecules exhibit aromatic stability based on the 4n+2 π electron count.

Hydration (chemical hydrates) Hydration Hydration in chemistry is the incorporation of water molecules into a compound’s structure, producing hydrates with physical and chemical properties often distinct from the anhydro…

Hydrazine (N2H4): properties, production, uses, and safety Hydrazine Hydrazine is a reactive, nitrogen-rich chemical (N2H4) used in synthesis, propulsion, and water treatment; it is highly toxic and handled under strict controls.

Hydrazoic acid (HN3): properties, preparation, uses and safety Hydrazoic acid Hydrazoic acid (HN3), also called hydrogen azide, is a volatile, highly toxic acid that yields the azide ion (N3−). It appears in laboratory synthesis but poses severe explosive an…

Hydrazone: structure, synthesis, reactions and applications Hydrazone Hydrazones are organic compounds with the general formula R1R2C=NNH2, formed from hydrazines and carbonyls. This article covers structure, preparation, key reactions (Wolff–Kishner…

Hydride (compounds of hydrogen with other elements) Hydride Hydride: compounds in which hydrogen bonds to other elements. Discusses major types (ionic, covalent, interstitial), properties, formation, applications, and safety considerations.

Hydrobromic acid (HBr): properties, preparation, and uses Hydrobromic acid Hydrobromic acid (HBr) is a strong, corrosive hydrogen halide in aqueous solution. This article summarizes its properties, common methods of preparation, principal uses, safety con…

Hydrocarbon: Chemistry, Types, Sources and Uses Hydrocarbon Hydrocarbons are organic compounds of hydrogen and carbon that form fuels and petrochemical feedstocks. This article summarizes their types, bonding, sources, industrial processing…

Hydrochloric acid (HCl) — properties, production, uses, and safety Hydrochloric acid Hydrochloric acid is an aqueous solution of hydrogen chloride used widely in industry and biology. This article explains its composition, production, common uses, biological role a…

Hydrofluoric acid (HF) Hydrofluoric acid Hydrofluoric acid is an aqueous solution of hydrogen fluoride. It is chemically HF, used in industry for etching, fluorochemical manufacture, and petroleum processing, and is uniqu…

Hydrofluorocarbons (HFCs): chemistry, uses, and environmental effects Hydrofluorocarbon An overview of hydrofluorocarbons (HFCs): chemical nature, common applications as refrigerants and propellants, atmospheric behavior, climate impacts, and regulatory responses.

Hydrogen: properties, isotopes, history, and common uses Hydrogen Hydrogen (H) is the lightest and most abundant chemical element; essential in stars, water, fuels, and modern industry. This article summarizes its properties, isotopes, history, a…

Hydrogen atom: structure, isotopes, and scientific importance Hydrogen atom An overview of the hydrogen atom — the simplest atom — covering its structure, isotopes, quantum description, spectral properties and roles in chemistry, astrophysics and precision…

Hydrogen bond: definition, mechanism, examples, and significance Hydrogen bond A hydrogen bond is a directional, noncovalent attraction in which a hydrogen atom bound to an electronegative atom interacts with another electronegative atom; crucial in water, bi…

Hydrogen bromide (HBr): properties, production, and uses Hydrogen bromide Hydrogen bromide (HBr) is a colorless, pungent hydrogen halide gas that forms hydrobromic acid in water. This article covers its properties, preparation methods, applications and s…

Hydrogen chloride (HCl) Hydrogen chloride Hydrogen chloride is a colorless, pungent gas that dissolves in water to form hydrochloric acid; it has important industrial uses and a long history as 'spirit of salt' or muriatic…

Hydrogen cyanide (prussic acid) Hydrogen cyanide Hydrogen cyanide (HCN), also called prussic acid, is a colorless, highly toxic, volatile chemical used as a precursor in industry and occurring naturally in some plants. It inhibit…

Hydrogen deuteride (HD): properties, formation, and significance Hydrogen deuteride Hydrogen deuteride (HD) is a heteronuclear diatomic molecule made of protium (1H) and deuterium (2H). It exhibits unique spectral and chemical behavior and is important in spectros…

Hydrogen fluoride (HF): properties, uses, and safety Hydrogen fluoride Hydrogen fluoride (HF) is a colorless, corrosive chemical used to make hydrofluoric acid and fluorine, and in etching and industrial catalysis. It is highly toxic and requires stri…

Hydrogen halide Hydrogen halides are simple diatomic compounds of hydrogen with a halogen; in water they form hydrohalic acids with strong acidity (except HF). They have distinct physical, chemica…

Hydrogen iodide (HI) Hydrogen iodide Hydrogen iodide (HI) is a colorless gas that forms hydroiodic acid in water. It is a strong reducing agent and a precursor to iodide salts, used in synthesis and industry; it must…

Hydrogen ion (hydron): forms, properties, and significance Hydrogen ion Hydrogen ion (hydron) is the general name for all charged species of hydrogen and its isotopes. Covers positive (H+, deuteron) and negative (hydride) forms, common environments, no…

Hydrogen peroxide (H2O2): properties, uses, and safety Hydrogen peroxide A concise overview of hydrogen peroxide, covering its chemistry, common uses, production, and safety precautions for household and industrial concentrations.

Hydrogen selenide (H2Se) Hydrogen selenide Hydrogen selenide (H2Se) is a colorless, highly toxic gas and the hydrogen analog of hydrogen sulfide; it forms hydroselenic acid in water and is used in synthesis and semiconducto…

Hydrogen sulfate (bisulfate): the HSO4− ion — properties and uses Hydrogen sulfate Hydrogen sulfate (bisulfate, HSO4−) is the singly deprotonated form of sulfuric acid. Overview of its structure, acid–base behavior, common salts, practical uses and how it differs…