Glossary of chemical terms

half-life
	For a given reaction the half life t1/2 of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) value. In nuclear chemistry, (radioactive) half life is defined, for a simple radioactive decay process, as the time required for the activity to decrease to half its value by that process. N = N0/2
halocarbon
	Halocarbon is a compound containing no elements other than carbon, one or more halogens, and sometimes hydrogen. The simplest are compounds such as tetrachloromethane (CCl4), tetrabromomethane (CBr4), etc. The lower members of the various homologous series are used as refrigerants, propellant gases, fireextinguishing agents, and blowing agents for urethane foams. When polymerized, they yield plastics characterized by extreme chemical resistance, high electrical resistivity, and good heat resistance.
halogens
	The halogens are the elements fluorine (F) chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are non-metals, and make up part of the 17 group in the periodic table. Compounds of these elements are called halogenides or halides. The halogens all have a strong unpleasant odor and will burn flesh. They do not dissolve well in water. The five elements are strongly electronegative. They are oxidizing agents, with fluorine being the strongest and astatine being the weakest. They react with most metals and many non-metals. Halogens form molecules which consists of atoms covalently bonded. With increasing atomic weight there is a gradation in physical properties. For example: Fluorine is a pale green gas of low density. Chlorine is a greenish-yellow gas 1.892 times as dense as fluorine. Bromine is a deep reddish-brown liquid which is three times as dense as water. Iodine is a grayish-black crystalline solid with a metallic appearance. And astatine is a solid with properties which indicate that it is somewhat metallic in character.
hardness
	Hardness is the resistance of a material to deformation of an indenter of specific size and shape under known load. This definition applies to all types of hardness scales except Mohs scale, which is a based on the concept of scratch hardness and is used chiefly for minerals. The most generally used hardness scales are Brinell (for cast iron), Rockwell (for sheet metal and heat-treated steel), Knoop (for metals).
heat
	Heat is a form of energy associated with and proportional to molecular motion. It can be transferred from one body to another by radiation, conduction, or convection. Heat of atomization is the energy required to dissociate one mole of a given substance into atoms. Heat of combustion is the heat evolved when a definite quantity of a substance is completely oxidized (burned). Heat of crystallization is the heat evolved or absorbed when one mole of given substance crystallizes from a saturated solution of the same substance. Heat of formation is the heat evolved or absorbed when one mole of a compound is formed in their standard state from its constituent elements. Heat of fusion is the heat required to convert a substance from the solid to the liquid state with no temperature change (also called latent heat of fusion or melting). Heat of hydration is the heat evolved or absorbed when a hydrate of a compound is formed. Heat of reaction is the heat evolved or absorbed as a result of the complete chemical reaction of molar amounts of the reactants. Heat of sublimation is the energy required to convert one mole of a substance from the solid to the gas state (sublimation) without the appearance of the liquid state. Heat of vaporization is the heat required to convert a substance from the liquid to the gaseous state with no temperature change (also called latent heat of vaporization).
heat capacity
	Heat capacity is defined in general as dQ/dT, where dQ is the amount of heat that must be added to a system to increase its temperature by a small amount dT. The heat capacity at constant pressure is Cp = (∂H/∂T)p; that at constant volume is CV = (∂E/∂T)V, where H is enthalpy, E is internal energy, p is pressure, V is volume, and T is temperature. An upper case C normally indicates the molar heat capacity, while a lower case c is used for the specific (per unit mass) heat capacity.
hectare
	Hectare (ha) is a unit of area equal to 104 m2. The unit is still used in agriculture.
Heisenberg uncertainty principle
	Heisenberg uncertainty principle is the principle that it is not possible to know with unlimited accuracy both the position and momentum of a particle. German physicist Werner Heisenberg (1901-1976) discovers this principle in 1927.
Helmholz free energy
	Helmholz free energy (A) is a thermodynamic function defined by A = U - TS, where U is the internal energy, S the entropy, and T the thermodynamic temperature. For a reversible isothermal process ΔA represents the useful work available.
henry
	Henry (H) is the SI derived unit of inductance equal to the inductance of a closed circuit in which an e.m.f. of one volt is produced when the electric current in the circuit varies uniformly at the rate of one ampere per second (H = V∙s/A). The unit is named after the US physicst Joseph Henry (1797-1878).
Henry's law
	Henry's law is discovered in 1801 by the British chemist William Henry (1775-1836). At a constant temperature the mass of gas dissolved in a liquid at equilibrium is proportional to the partial pressure of the gas. It applies only to gases that not react with the solvent. xi = Kx∙pi where pi is the partial pressure of component i above the solution, xi is its mole fraction in the solution, and Kx is the Henry's law constant (a characteristic of the given gas and solvent, as well as the temperature).
hertz
	Hertz (Hz) is the SI derived unit, with a special name, for frequency. The hertz is the frequency of a periodic phenomenon of which the period is one second (Hz = 1 s-1). It is named after the German scientist Heinrich Hertza (1857 - 1894).
heterocyclic compounds
	Heterocyclic compounds are cyclic compounds having as ring members atoms of at least two different elements, e.g., quinoline, 1,2-thiazole.
Heusler alloys
	Heusler alloys are alloys of manganese, copper, aluminum, nickel, and sometimes other metals which find important uses as permanent magnets.
holography
	Holography is a technique for creating a three-dimensional image of an object by recording the interference pattern between a light beam diffracted from the object and a reference beam. The image can be reconstructed from this pattern by a suitable optical system.
Hooke's law
	When a load is applied to any elastic body is deformed or strained, then the resulting stress (the tendency of the body to resume its normal condition) is proportional to the strain. Stress is measured in units of force per unit area, strain is the extent of the deformation.
horse power
	Horse power is obsolete non-SI of power introduced by James Watt in 1782 to allow to describe the power of steam machinery. It was equal to the work effort of a horse needed to raise vertically 528 cubic feet of water to one metre in one minute (HP = 735.498750 W).
HPLC
	HPLC is abbreviation for high-performance liquid chromatography. HPLC is a variation of liquid chromatography that utilizes high-pressure pumps to increase the efficiency of the separation.
hydration
	Hydration is addition of water or the elements of water (i.e. H and OH) to a molecular entity. The term is also used in a more restricted sense for the process: A+B- A+(aq) + B-(aq)
hydrocarbon
	Hydrocarbon is a compound containing only carbon and hydrogen.
hydrolysis
	Hydrolysis is a chemical reaction in which water reacts with another substance to form two or more new substances. This involves ionization of the water molecule as well as splitting of the compound hydrolyzed, e.g. CH3COOC2H5 + H2O CH3COOH + C2H5OH Examples are conversion of starch to glucose by water in presence of suitable catalysts and reaction of the ions of a dissolved salt to form various products, such as acids, complex ions, etc.
hydrophilic
	Hydrophilic is having a strong tendency to bind or absorb water, which results in swelling and formation of reversible gels. This property is characteristic of carbohydrate.
hydrophobic
	Hydrophobic is antagonistic to water, incapable of dissolving in vater. This property is characteristic of oill, fats, waxes, and many resins.
hydrophobic interaction
	Hydrophobic interaction is the tendency of hydrocarbons (or of lipophilic hydrocarbon-like groups in solutes) to form intermolecular aggregates in an aqueous medium, and analogous intramolecular interactions. The name arises from the attribution of the phenomenon to the apparent repulsion between water and hydrocarbons. Use of the misleading alternative term hydrophobic bond is discouraged.

 

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

R

S

T

U

V

X

Z