Glossary - Hh

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Half life	The half life of a reaction is the time required for the amount of reactant to drop to one half its initial value.
Halide	halide ion. A compound or ion containing fluorine, chlorine, bromine, iodine, or astatine.
Halogen	An element of group VIIA (a. k. a. Group 18). The name means "salt former"; halogens react with metals to form binary ionic compounds. Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) are known at this time.
Hartree	(E_h) The atomic unit of energy, equal to 4.359 743 81 × 10^-18 J ± 0.000 000 34 × 10^-18 J [1998 CODATA values].
Heat capacity	The heat required to raise the temperature of an object by 1°C is called the heat capacity of the object. Heat capacity is an extensive property with units of J K^-1.
Heat	Heat is a transfer of energy that are placed into contact. Heat is a process, not a property of a material occurs when objects with different temperatures.
Heavy water	(D₂O) Water that contains ²H, rather than ¹H. Heavy water is about 11% denser than ordinary water.
Helium	He. Element 2, atomic weight 4.0026. A colourless, odourless, inert gas, first discovered in the emission spectrum of the sun in 1868.
Helmholtz free energy	(A) Arbeitfunktion. A thermodynamic property that can be used to predict whether a process will occur spontaneously at constant volume and temperature. Helmholtz free energy A is defined as A = U - TS where U, T and S are the internal energy, temperature, and entropy. Changes in A correspond to changes in free energy for processes occuring at constant temperature and volume. The sign of Delta A is negative for spontaneous processes and zero for processes at equilibrium.
Henry's Law	Henry's law constant. Henry's law predicts that the solubility (C) of a gas or volatile substance in a liquid is proportional to the partial pressure (P) of the substance over the liquid: P = k C where k is called the Henry's law constant and is characteristic of the solvent and the solute.
Hertz	(Hz, s^-1) frequency. The SI unit of frequency equal to one cycle of the wave per second (s^-1).
Hess's law	law of constant heat summation; Hess's law of heat summation. The heat released or absorbed by a process is the same no matter how many steps the process takes. For example, given a reaction AB, Hess's law says that H for the reaction is the same whether the reaction is written as ACB or as AB. This is the same as writing that H(AB) = H(AC) + H(CB).
Heterocyclic	heterocycle; heterocyclic ring. An organic group or molecule containing rings with at least one noncarbon atom on the ring.
Heterogenous mixture	heterogenous. A common misspelling of heterogeneous.
Heterogeneous mixture	A sample of matter consisting of more than one pure substance and more than one phase. Blood, protoplasm, milk, chocolate, smoke, and chicken soup are examples of heterogeneous mixtures.
High performance liquid chromatography	HPLC. An efficient form of column chromatography that pumps a liquid solution of the sample at very high pressure through a column packed with a stationary phase made of very tiny particles. The high pressure pumps required make HPLC an expensive technique.
High spin complex	A metal-ligand complex with the same number of unpaired electrons as the uncomplexed metal ion. When a weak ligand complexes the metal ion, the crystal field splitting is small and the electrons can still occupy all of the d orbitals without pairing.
Homogeneous	Having uniform properties or composition.
Homogenous	A common misspelling of homogeneous.
Homogeneous mixture	Solution A sample of matter consisting of more than one pure substance with properties that do not vary within the sample.
Homolog	homologue; homologous; homologous series A compound belonging to a series of compounds that differ by a repeating group. For example, propanol (CH₃CH₂CH₂OH), n-butanol (CH₃CH₂CH₂CH₂OH), and n-pentanol (CH₃CH₂CH₂CH₂CH₂OH) are homologs; they belong to a homologous series CH₃(CH₂)_nOH.
Hormone	A molecule produced by endocrine glands that controls specific biological processes like growth and metabolism.
Humectant	A substance that absorbs or retains moisture, added to a product to keep it from drying out.
Hund's rule	rule of maximum multiplicity. A rule of thumb stating that subshells fill so that the number of unpaired spins is maximized, or "spread them out and line them up."
Hybridization	hybrid orbitals; hybridized orbitals. The combination of a set of atomic orbitals to produce a new set of "hybrid" orbitals. Hybridized orbitals are theoretical constructions that make molecular structures easier to explain. For example, combining the valence s and p orbitals of carbon produces a set of four "sp³" hybrid orbitals that can be used to explain the tetrahedral bonding in CH₄.
Hydrate	A hydrate is an addition compound that contains water in weak chemical combination with another compound. For example, crystals of CuSO₄·5 H₂O (copper sulfate pentahydrate) are made of regularly repeating units, each containing 5 molecules of water weakly bound to a copper(II) ion and a sulfate ion.
Hydration	Combination with water.
Hydrazine	(NH₂NH₂) A colourless, fuming, corrosive liquid that is a powerful reducing agent. NH₂NH₂ is used in jet and rocket fuels, and as an intermediate in the manufacture of agricultural, textile, photographic, and industrial chemicals.
Hydride ion	(H^-) hydride. A -1 ion formed from hydrogen. Hydride ions and hydride ionic compounds react instantly and sometimes violently with water.
Hydrocarbon	Hydrocarbons are organic compounds that contain only hydrogen and carbon. The simplest hydrocarbons are the alkanes.
Hydrogen	(H) H; protium. Element 1, atomic weight 1.00797. The most abundant element in the universe. Hydrogen occurs as H₂ at ambient temperature and pressure, a colourless, odourless, and extremely flammable gas. Discovered in 1766 by Cavendish.
Hydrogen bond	An especially strong dipole-dipole force between molecules X-H...Y, where X and Y are small electronegative atoms (usually F, N, or O) and ... denotes the hydrogen bond. Hydrogen bonds are responsible for the unique properties of water and they loosely pin biological polymers like proteins and DNA into their characteristic shapes.
Hydrolysis	A catch-all term for any reaction in which the water molecule is split.
Hydrometer	An instrument for measuring the specific gravity of liquids. A hydrometer is a weight with a vertical scale attached. When placed into a liquid, the hydrometer bobs upright, and sinks to a certain level. The specific gravity or solution composition can be read from the liquid level on the vertical scale. Hydrometers are often calibrated in degrees Baumé.
Hydronium ion	(H₃O⁺) hydronium. The H3O+ ion, formed by capture of a hydrogen ion by a water molecule. A strong covalent bond is formed between the hydrogen ion and water oxygen; all hydrogen ions in aqueous solution are bound inside hydronium ions.
Hydrophilic	hydrophilicity; hydrophilic group. A polar molecule or group that can form strong hydrogen bonds with water.
Hydrophobic	hydrophobicity; hydrophobic group. A nonpolar molecule or group that has little affinity for water. Hydrophobic groups on molecules in solution tend to turn in on themselves or clump together with other hydrophobic groups because they are unable to disrupt the network of strong hydrogen bonds in the water around them.
Hydroxyl	1. An -OH group within a molecule. 2. A free radical formed by abstraction of a hydrogen atom from water.
Hydroxide	(OH^-) hydroxide ion. 1. The OH- ion. 2. Compounds containing the OH- ion. See also: hydroxide compounds.
Hygroscopic	Able to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air.
Hygroscopically	By absorbing moisture from air.
Hygroscopicity	The ability of a substance to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air.
Hypergolic mixture	hypergolic fuel; hypergolic. An oxidizing agent and a fuel that react or ignite instantly and spontaneously on contact. Methylhydrazine and dinitrogen tetroxide is a hypergolic mixture used as a propellant in the space shuttle's orbital manoeuvring engines.
Hypertonic	Describes a solution which has higher osmotic pressure than some other solution (usually, higher osmotic pressure than cell or body fluids). Freshwater fish die if placed in seawater because the seawater is hypertonic, and causes water to leave the cells in fish's body.
Hypotonic	Describes a solution which has lower osmotic pressure than some other solution (usually, lower osmotic pressure than cell or body fluids). Washing your contact lenses with distilled water rather than saline is painful because distilled water is hypotonic; it causes water to move into cells, and they swell and burst.
Hypothesis	A hypothesis is a conjecture designed to guide experimentation. Hypotheses are extremely useful in problem solving, and are essential in developing new theories.

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