Glossary of chemical terms

calomel electrode
	Calomel electrode (calomel half cell) is a type of half cell in which the electrode is mercury coated with calomel (Hg2Cl2) and the electrolyte is a solution of potassium chloride and saturated calomel. In the calomel half cell the overall reaction is Hg2Cl2(s) + 2e- 2Hg(l) + 2Cl- The standard electrode potential is   Electrode potential SHE, V T, °C 0.1 M KCl 3.5 M KCl sat. KCl 15 0.3362 0.254 0.2511 20 0.3359 0.252 0.2479 25 0.3356 0.250 0.2444 30 0.3351 0.248 0.2411 35 0.3344 0.246 0.2376
candela
	Candela (cd) is the SI base unit of luminous intensity. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.
carboxylic acids
	Carboxylic acids are organic compounds characterized by the presence of one or more RC(=O)OH groups (the carboxyl group). In systematic chemical nomenclature carboxylic acids names end in the suffix -oic (e.g. ethanoic acids, CH3COOH). The carbon of the terminal group being counted as part of the chain. They are generally weak acids. Carboxylic acids include the large and important class of fatty acids and may be either saturated or unsaturated. There are also some natural aromatic carboxylic acids (benzoic, salicylic).
Carnot cycle
	Carnot cycle is the most efficient cycle of operations for a reversible heat engine. Published in 1824 by N. L. S. Carnot (1796-1832), it consists of four operations on the working substance in the engine: a) Isothermal expansion at thermodynamic temperature T1 with heat q1 taken in. b) Adiabatic expansion with a fall of temperature to T2. c) Isothermal compression at temperature T2 with heat q2 given out. d) Adiabatic compression at temperature back to T1. According to the Carnot principle, the efficiency of any reversible heat engine depends only on the temperature range through which it works, rather than the properties of the working substances.
catalyst
	Catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Catalyst that have the same phase as the reactants are homogenous catalysts (e.g. enzymes in biochemical reactions). Those that have a different phase are heterogeneous catalyst (e.g. metals or oxides used in gas reactions). The catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy is lower. In thus increases the rate at which the reaction comes to equilibrium, although it does not alter the position of the equilibrium.
cathode
	Cathode is a negative electrode of an electrolytic cell; to which positively charged ions (cations) migrate when a current is passed as in electroplating baths. In a primary or secondary cell (battery or accumulator) the cathode is the electrode that spontaneously becomes negative during discharge, and form which therefore electrons emerge. In vacuum electronic devices, electrons are emitted by the cathode and flow to the anode.
cation
	Cation is a positively charged atomic or molecular particle.
cation exchange
	A cationic resin has positive ions built into its structure and therefore exchanges negative ions. In cation exchange, the side groups are ionized acidic groups, such as (-SO3H, -COOH, -OH) to which cations H+ are attached. The exchange reaction is one in which different cations in the solution displace the H+ from the solid.
ceramics
	Ceramics are an inorganic material of very high melting point. Ceramics are metal silicates, oxides, nitrides, etc.
cgs system of units
	Cgs system of units is a system of units based upon the centimetre, gram, and second. The International System (SI) has supplanted the cgs system.
chalcogens
	Chalcogens are the Group 16 elements: oxygen (O), sulphur (S), selenium Se), tellurium (Te), and polonium (Po). Compounds of these elements are called chalcogenides.
Charles' law
	The volume of a fixed mass of gas at constant pressure expand by constant fraction of its volume at 0 °C for each Celsius degree or kelvin its temperature is raised. For any ideal gas fraction is approximately 1/273. This can expressed by the equation were V0 is the volume at 0°C and V is its volume at t°C. This is equivalent to the statement that the volume of a fixed mass of gas at constant pressure is proportional to its thermodynamic temperature V = kT This law also know as Gay-Lussac's law. An equation similar to that given above applies to pressures for ideal gases:
chelate
	Chelate is a compound characterized by the presence of bonds from two or more bonding sites within the same ligand to a central metal atom.
chemical equation
	Chemical equation is a way of denoting a chemical reaction using the symbol for the participating particles (atoms, molecules, ions, etc.); for example, aA + bB cC + dD The single arrow is used for an irreversible reaction; double arrows are used for reversible reactions. When reactions involve different phases it is usual to put the phase in brackets after the symbol. s = solid l = liquid g = gas aq = aqueous The numbers a, b, c, and d, showing the relative numbers of molecules reacting, are called the stoichiometric coefficients. The convention is that stoichiometric coefficients positive for reactants and negative for products. If the sum of the coefficients is zero the equation is balanced.
chemical potential
	For a mixture of substances, the chemical potential of constituent B (μB) is defined as the partial derivative of the Gibbs energy G with respect to the amount (number of moles) of B, with temperature, pressure, and amounts of all other constituents held constant. Also called partial molar Gibbs energy. Components are in equilibrium if their chemical potentials are equal.
chiral molecule
	Chiral molecule is a molecule which cannot be superimposed on its mirror image. A common example is an organic molecule containing a carbon atom to which four different atoms or groups are attached. Such molecules exhibit optical activity, i.e., they rotate the plane of a polarized light beam.
chromatography
	Chromatography is a method for separation of the components of a sample in which the components are distributed between two phases, one of which is stationary while the other moves. In gas chromatography, the gas moves over a liquid or solid stationary phase. In liquid chromatography, the liquid mixture moves through another liquid, a solid, or a gel. The mechanism of separation of components may be adsorption, differential solubility, ion-exchange, permeation, or other mechanisms.
Clapeyron equation
	Clapeyron equation is a relation between pressure and temperature of two phases of a pure substance that are in equilibrium, where ΔtrsS is the difference in entropy between the phases and ΔtrsV the corresponding difference in volume.
colloid
	Colloids are systems in which there are two or more phases, with one (the dispersed phase) distributed in the other (the continuous phase). Moreover, at least one of the phases has small dimensions, in the range between 1 nanometer and 1 micrometer (10-9 - 10-6 m). Dimension, rather than the nature of the material, is characteristic. In this size range, the surface area of the particle is large with respect to its volume so that unusual phenomena occur, e.g., the particles do not settle out of the suspension by gravity and are small enough to pass through filter membranes. Macromolecules (proteins and other high polymers) are at the lower limit of this range; the upper limit is usually taken to be the point at which the particles can be resolved in an optical microscope. Colloidal particles may be gaseous, liquid, or solid, and occur in various types of suspensions: Sols - dispersions of small solid particles in a liquid. Emulsions - colloidal systems in which the dispersed and continuous phases are both liquids. Gels - colloids in which both dispersed and continuous phases have a three-dimensional network throughout the material. Aerosols - colloidal dispersions of liquid or solid particles in a gas. Foams - dispersions of gases in liquids or solids.
colloid mills
	Colloid mills are machines used to grind aggregates into very fine particles or to apply very high shearing within a fluid to produce colloid suspensions or emulsions in which the particle sizes are less than 1 micrometer. One type of colloid mill is called a disc mill, in which a mixture of a solid and liquid (or two liquid) is passed between two discs a small distance apart, which rotate very rapidly relative to each other. Applications of colloid mills occur in food processing, in paint manufacture, and in the pharmaceutical industry.
concentration
	Concentration is the amount of given substance in a stated unit of a mixture, solution, or ore. The concentration of an atom, ion, or molecule in a solution may be symbolized by the use of square brackets, as [Ca2+].
copolymer
	Copolymers are also known as heteropolymers. They made from two (or more) different monomers, which usually undergo a condensation reaction with the elimination of a simple molecule, such as ammonia or water. A typical example is the condensation of 1,6-diaminohexane (hexamethylenediamine) with hexanedioic acid (adipic acid) to form nylon 6,6. The properties of a polymeric plastic can most easily be modified if it is a copolymer of two or more different monomers, e.g. acrylonitrile-butadiene-styrene copolymer (ABS). Varying the proportions of the component monomers can preselect its properties.
cosmic rays
	Cosmic rays are high energy (1015 - 1017 eV) nuclear particles, electrons, and photons, originating mostly outside the solar system, which continually bombard the Earth's atmosphere.
coulomb
	Coulomb (C) is the SI unit of electric charge. It is equal to the charge transferred by a current of one ampere in one second (C = A s). The unit is named after French physicist C. A. Coulomb (1736-1806).
Coulomb's law
	Coulomb's law is the statement that the force F between two electrical charges q1 and q2 separated by a distance r is where ε0 is the permittivity of a vacuum, equal to ε0 = 8.8542×10-12 F/m.
covalent bond
	Covalent bond is a chemical bond between two atoms whose stability results from the sharing of two electrons, one from each atom.
critical mass
	Critical mass is the minimum mass of a fissionable material (235U or 239Pu) that will initiate an uncontrolled chain reaction as in an atomic bomb. The critical mass of pure 239Pu is about 4.5 kg, and of 235U about 15 kg.
critical point
	In general, critical point is the point on the phase diagram of a two-phase system at which the two coexisting phases have identical properties and therefore represent a single phase. At the liquid-gas critical point of a pure substance, the distinction between liquid and gas vanishes, and the vapour pressure curve ends. The coordinates of this point are called the critical temperature and critical pressure. Above the critical temperature, it is not possible to liquefy the substance.
critical pressure
	Critical pressure is the pressure of a fluid in its critical point; i.e. when it is at its critical temperature and critical volume.
critical temperature
	Critical temperature is the temperature of the liquid-vapour critical point, that is, the temperature above which a gas cannot be liquefied by an increase of pressure.
critical volume
	Critical volume is the volume of a fixed mass of a fluid at critical temperature and pressure.
cross-linking
	Cross-linking is attachment of two chains of polymer molecules by bridges, composed of either an element, a group, or a compound, that join certain carbon atoms of the chains by primary chemical bonds, as indicated in the schematic diagram Cross-linking occurs in nature in substances made up of polypeptide chains that are joined by the disulfide bonds of the cysteine residue, as in keratins or insulin. Cross-linking can be effected artificially, either adding a chemical substance (cross-linking agent), or by subjecting the polymer to high-energy radiation. Examples are: vulcanisation of rubber with sulphur, cross-linking of polystyrene with divinylbenzene, or cross-linking of polyethylene by means of high-energy radiation. Cross-linking has the effect of changing a plastic from thermoplastic to thermosetting. Thus, it also increases strength, heat and electrical resistance, and especially resistance to solvents and other chemicals.
crust
	Crust is the outer layer of the solid earth, above the Mohorovicic discontinuity. Its thickness averages about 35 km on the continents and about 7 km below the ocean floor.
cryoscopic constant
	Cryoscopic constant (Ef) is the constant that expresses the amount by which the freezing point Tf of a solvent is lowered by a non-dissociating solute, through the relation ΔTf = Ef m where m is the molality of the solute.
Curie temperature
	For a ferromagnetic material, Curie temperature or Curie point (TC) is the critical temperature above which the material becomes paramagnetic. For iron the Curie point is 760 °C and for nickel 356 °C. It is named after French physicist Pierre Curie (1859-1906).
cycloalkanes
	Cycloalkanes are cyclic saturated hydrocarbons containing a ring of carbon atoms joined by single bonds. They have the general formula CnH2n, for example cyclohexane, C6H12. In general, they behave like the alkanes but are rather less reactive.

 

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