Physical world and mathematics

Hydra–Centaurus Supercluster

The Hydra–Centaurus Supercluster, or Hydra–Centaurus concentration, is a nearby complex of galaxy clusters in the southern sky that supplies much of the mass of the Great Attractor.

Hydrate

A hydrate, also called a crystallohydrate, is a compound whose crystal structure includes water molecules in a fixed ratio, such as copper(II) sulfate pentahydrate, CuSO₄·5H₂O.

Hydration force

The hydration force is a short-range repulsive force between hydrophilic surfaces in water, arising from the energy needed to remove bound water as the surfaces approach.

Hydration reaction

A hydration reaction is a chemical reaction in which water, or its H and OH elements, is added to a molecule, most often converting an alkene into an alcohol.

Hydraulic conductivity

Hydraulic conductivity, also called hydraulic permeability, measures how easily water moves through a porous or fractured material, and it has the units of a velocity.

Hydraulic control

Hydraulic control is the condition in stratified or open-channel flow where long waves cannot travel upstream, typically at a sill crest, governing exchange through straits and fjords.

Hydraulic diameter

The hydraulic diameter is a characteristic length used in fluid mechanics for noncircular ducts, defined as four times the flow cross-sectional area divided by the wetted perimeter.

Hydraulic geometry

Hydraulic geometry is an empirical method in fluvial geomorphology expressing a river channel's width, depth, and velocity as power functions of discharge, introduced by Leopold and Maddock in 1953.

Hydraulic head

Hydraulic head, also called piezometric head, is a measure of the total energy that moves ground water through an aquifer, expressed as a height above a datum.

Hydraulic jump

A hydraulic jump is the sudden transition of open channel flow from fast, shallow supercritical to slow, deep subcritical flow, dissipating energy in rivers, canals, and kitchen sinks.

Hydraulic press

A hydraulic press, also known as a Bramah press, is a machine that uses confined liquid under pressure to compress or shape a workpiece via Pascal's principle.

Hydraulic tomography

Hydraulic tomography is an aquifer characterization method that maps spatially varying hydraulic conductivity and storage by inverting head responses from sequential pumping or injection tests.

Hydrazide

Hydrazide, also called acylhydrazine, is a compound derived from an oxoacid in which acidic hydroxy groups are replaced by hydrazine-derived groups; isoniazid is a well-known example.

The hydrazine oxidation reaction is the electrochemical oxidation of hydrazine at an electrode, used in direct hydrazine fuel cells and as a low-voltage substitute for oxygen evolution in hydrogen production.

Hydrazoic acid

Hydrazoic acid, also called hydrogen azide or hydronitric acid, is the colorless, volatile, toxic and explosive liquid nitrogen hydride (HN₃) used to make heavy metal azides for shell detonators.

Hydride

Hydride is the anion of hydrogen, H−, and by extension the broad class of compounds hydrogen forms with other elements, grouped as ionic, covalent, and metallic hydrides.

Hydride generation

Hydride generation is a sample-introduction technique in atomic spectrometry that converts elements like arsenic, selenium, and tin in acidified solution to volatile hydrides for detection, first applied analytically in 1969.

Hydride superconductivity

Hydride superconductivity is superconductivity in hydrogen-rich compounds compressed to megabar pressures. It holds the highest known transition temperatures, with lanthanum decahydide superconducting at about 250 K near 170 GPa.

Hydroacylation

Hydroacylation is an organic reaction in which the formyl C–H bond of an aldehyde adds across an alkene or alkyne to give a ketone, typically catalyzed by transition-metal complexes such as rhodium.

Hydroalkylation

Hydroalkylation is a catalytic reaction in which hydrogen and an alkyl group add across an alkene's double bond, converting it directly into a substituted alkane.

Hydroamination

Hydroamination is the addition of an amine N–H bond across a carbon–carbon double or triple bond, forming a new C–N bond in one step with 100% theoretical atom economy.

Hydroaminoalkylation

Hydroaminoalkylation (HAA) is a catalytic reaction that inserts an inexpensive alkene directly into the C–H bond beside an amine's nitrogen, an atom-economic route to substituted amines found in pharmaceuticals.

Hydroarylation

Hydroarylation is the addition of an aryl group and hydrogen across a carbon-carbon double or triple bond, an atom-economical alternative to Friedel-Crafts alkylation that preferentially gives linear, anti-Markovnikov products.

Hydrobiologia

Hydrobiologia is a peer-reviewed scientific journal of aquatic sciences published by Springer since 1948, covering the biology of freshwater and marine environments from molecular to ecosystem level.

Hydroboration

Hydroboration is the addition of a boron–hydrogen bond, typically from borane (BH₃), across a carbon–carbon double or triple bond, discovered by H.C. Brown in 1959 and used to convert alkenes into alcohols with anti-Markovnikov regiochemistry.

Hydroboration–oxidation reaction

The hydroboration–oxidation reaction is a two-step organic reaction that converts alkenes into alcohols by anti-Markovnikov, syn addition of water, discovered by H.C. Brown in 1959.

Hydrocarbon

A hydrocarbon is an organic compound consisting of only carbon and hydrogen. Found in petroleum, coal, and natural gas, hydrocarbons serve mainly as fuels and as feedstocks for plastics.

Hydrochloric acid

Hydrochloric acid, also known as muriatic acid, is a strong corrosive acid that dissolves metals, pickles steel, and forms the gastric acid in the human stomach.

Hydrocyanation

Hydrocyanation is a catalytic reaction that adds hydrogen cyanide across carbon–carbon double or triple bonds to form organonitriles, used industrially to make adiponitrile, the precursor to nylon-66.