Glossary of chemical terms

oersted
	Oersted (Oe) is a non-SI unit of magnetic field (H), equal to 79.57747157 A/m. The unit is named after the H.C. Oersted (1777-1851).
ohm
	Ohm (W) is the SI derived unit of electric resistance. The ohm is the electric resistance between two points of a conductor when a constant difference of potential of one volt, applied between these two points, produces in this conductor a current of one ampere, this conductor not being the source of electromotive force (W = V/A). The unit is named after the German physicist Georg Simon Ohm (1789-1854).
olefins
	Olefins are acyclic and cyclic hydrocarbons having one or more carbon-carbon double bonds, apart from the formal ones in aromatic compounds. The class olefins subsumes alkenes and cycloalkenes and the corresponding polyenes.
oligomer
	Oligomer is a substance consisting of molecules of intermediate relative molecular mass (molecular weight), the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. In contrast to a polymer, the properties of an oligomer can vary significantly with the removal of one or a few of its units.
Onsager relations
	Onsager relations are an important set of equations in the thermodynamics of irreversible processes. They express the symmetry between the transport coefficients describing reciprocal processes in systems with a linear dependence of flux (Ji) on driving forces (Xj). In Onsager's theory the coupling coefficients are equal, Lij = Lji. This are known as reciprocal relations. The theory was developed by the Norwegian chemist Lars Onsager (1903-1976) in 1931.
orbital
	Orbital is the area in space about an atom or molecule in which the probability of finding an electron is greatest. The possible atomic orbitals correspond to subshells of the atom. Thus there is one s-orbital for each shell (orbital quantum number l = 0). There are three p-orbitals (corresponding to the three values of l) and five d-orbitals. The shapes of orbitals depend on the value of l.
osmosis
	Osmosis is the flow of a solvent in a system in which two solutions of different concentration are separated by a semipermeable membrane which cannot pass solute molecules. The solvent will flow from the side of lower concentration to that of higher concentration, thus tending to equalize the concentrations. The pressure that must be applied to the more concentrated side to stop the flow is called the osmotic pressure.
osmotic pressure
	Osmotic pressure (Π) is the excess pressure necessary to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent. In an ideal dilute solution Π = cB RT where cB is the amount-of-substance concentration of the solute, R is the molar gas constant, and T the temperature.
Ostwald's dilution law
	Ostwald's dilution law is a relation for the concentration dependence of the molar conductivity Λ of an electrolyte solution, viz. where c is the solute concentration, Kc is the equilibrium constant for dissociation of the solute, and L0 is the conductivity at cΛ = 0. The law was first put forward by the German chemist Wilhelm Ostwald (1853-1932).
overpotential
	Overpotential (η) is a potential that must be applied in an electrolytic cell in addition to the theoretical potential required to liberate a given substance at an electrode. The value depends on the electrode material and on the current density.
oxidation
	The term oxidation originally meant a reaction in which oxygen combines chemically with another substance. More generally, oxidation is a part of a chemical reaction in which a reactant loses electrons (increase oxidation number). Simultaneous reduction of a different reactant must occur (redox reaction).
oximes
	Oximes are organic compounds of structure R2C=NOH derived from condensation of aldehydes or ketones with hydroxylamine. Oximes from aldehydes may be called aldoximes; those from ketones may be called ketoximes.
oxo compounds
	Oxo compounds are organic compounds that contain the karbonyl group, C=O. The term thus embraces aldehydes, carboxylic acids, ketones, amides, and esters.
ozone
	Ozone is an alotropic form of oxygen. It is unstable blue gas with pungent odor. It is a powerful oxidizing agent. The gas is made by passing oxygen through a silent electric discharge. 3O2(g) 2O3(g)

 

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