Physical world and mathematics

Hydrographic office

A hydrographic office is a national agency responsible for surveying a country's waters and producing the nautical charts, tide tables, and Notices to Mariners that ships need to navigate safely.

Hydrographic survey is the measurement of water depth and seafloor features to produce bathymetric models and nautical charts, using single-beam and multibeam echo sounders, airborne LiDAR, and satellite-derived bathymetry.

Hydrography

Hydrography is the applied science of measuring and describing the physical features of oceans, seas, coastal waters, lakes, and rivers, chiefly to chart hazards and support safe navigation.

Hydrohalogenation

Hydrohalogenation is the addition of a hydrogen halide (HCl, HBr, or HI) across the π bond of an alkene or alkyne, giving alkyl or vinyl halides with Markovnikov regiochemistry.

Oct 21, 2026

Hydrologic balance

Hydrologic balance (water balance): accounting where storage change equals precipitation minus evapotranspiration and runoff; basis of catchment hydrology, the Budyko framework, and water balance models.

Hydrological model

A hydrological model, also called a hydrological transport model, is a mathematical representation of water movement in a catchment, supporting flood forecasting and water resource management.

Hydrology

Hydrology, or hydrologic science, is the science of Earth's water, studying its occurrence, movement, and properties through the water cycle, from precipitation and runoff to evaporation.

Hydrolysis

Hydrolysis is a chemical reaction in which water breaks a bond in a molecule, covering salt pH changes, ester saponification, polymer degradation, and ATP-driven cell reactions.

Hydrometer

A hydrometer, also spelled areometre, is an instrument that measures a liquid's density or specific gravity by floating in it, used in brewing, battery testing, and petroleum measurement.

Hydronium

Hydronium, also known as the hydroxonium ion, is the cation H3O+ formed when a proton bonds to a water molecule; it is the form all hydrogen ions take in water.

Hydroperoxy

Hydroperoxy refers to the univalent –OOH group, formed by replacing one hydrogen atom of hydrogen peroxide; its ROOH compounds are hydroperoxides, and the neutral radical HOO• is the hydroperoxyl radical, HO2.

Hydroperoxyl

Hydroperoxyl, also called the perhydroxyl radical and written HO2, is the protonated form of the superoxide anion and a key radical in atmospheric oxidation and combustion chemistry.

Hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) is a liquid chromatography method, named by Andrew Alpert in 1990, that retains polar compounds too hydrophilic for reversed-phase LC, coupling well to mass spectrometry.

Hydrophilic-lipophilic balance

The hydrophilic-lipophilic balance (HLB) is a numerical scale from 1 to 20, introduced by William C. Griffin in 1949, showing how strongly a surfactant attracts water relative to oil.

Hydrophobe

A hydrophobe is a nonpolar molecule or surface, such as a hydrocarbon, that lacks favorable interactions with water; hydrophobicity is measured by contact angle or Log Kow.

Hydrophobic effect

The hydrophobic effect, or hydrophobic interactions, is the tendency of oil and water to segregate, arising from water's behavior around solutes that cannot hydrogen-bond with it.

Hydrophobic interaction chromatography

Hydrophobic interaction chromatography (HIC) is a liquid chromatography method that separates proteins by surface hydrophobicity using high-salt buffers, keeping them native and active, and is widely used for antibody aggregate removal and ADC analysis.

Hydrophytic vegetation

Hydrophytic vegetation, also called hydrophytes, is the plant life of areas where saturated soils control which species grow, and one of three wetland delineation criteria.

Hydroquinone

Hydroquinone, also known as benzene-1,4-diol, is a white crystalline aromatic compound used as a photographic developer and skin-lightening agent that inhibits tyrosinase to treat melasma.

HydroSHEDS

HydroSHEDS is a free suite of global hydrographic datasets, including catchment boundaries, river networks, and lakes, derived from satellite elevation data and initiated by World Wildlife Fund US in 2006.

Hydrosilane reagents in organic synthesis

Hydrosilane reagents in organic synthesis are organosilicon compounds with Si–H bonds, including triethylsilane, PMHS, and TMDS, that reduce alkenes, carbonyls, and other groups under mild, catalytic conditions.

Hydrosilylation

Hydrosilylation is the addition of a silicon–hydrogen bond across an alkene or alkyne to form an organosilane, first reported in 1947 and now central to the multibillion-dollar silicones industry.

Hydrosphere

The hydrosphere is all of Earth's water in liquid and solid form at and beneath the surface, including oceans, rivers, lakes, groundwater, glaciers, and ice sheets.

Hydrostatic equilibrium

Hydrostatic equilibrium, also called hydrostatic balance, is the state of a fluid at rest where internal pressure balances gravity. It applies to oceans, atmospheres, and stars.

Oct 22, 2026

Hydrostatic mass bias

Hydrostatic mass bias is the systematic underestimate of galaxy-cluster masses inferred from X-ray observations assuming hydrostatic equilibrium, an assumption violated by non-thermal pressure from turbulent gas motions.

Hydrostatic pressure

Hydrostatic pressure is the pressure in a fluid at rest, produced by the weight of fluid above, increasing linearly with depth according to p = p₀ + ρgh.

Hydrostatic test

A hydrostatic test, also called hydrotesting, is a pressure test in which a vessel, pipe, or component is filled with water and pressurized above its operating pressure to check for leaks.

Hydrostatics

Hydrostatics is the branch of fluid mechanics that studies fluids at rest, explaining how pressure increases with depth and giving rise to Pascal's principle and Archimedes' principle of buoyancy.