Glossary of chemical terms

degree of polymerisation
	Degree of polymerisation is the number of monomeric units in a macromolecule or an oligomer molecule.
density
	In the most common usage, density (ρ) is mass density or mass per unit volume. ρ = m / V More generally, the amount of some quantity (mass, charge, energy, etc.) divided by a length, area, or volume. Relative density is the ratio of the density of a substance to the density of some reference substance. For liquids or solids, it is the ratio of the density (usually at 20 °C) to the density of water at 4 °C. This quantity was formerly called specific gravity.
diamagnetism
	In diamagnetism the magnetization is in the opposite direction to that of applied field, i.e. susceptibility is negative. It results from changes induced in the orbits of electrons in the atoms of a substance by the applied field, the direction of the change opposing the applied flux.
diazo compounds
	Diazo compounds are compounds having the divalent diazo group, =N+=N-, attached to a carbon atom. The term includes azo compounds, diazonium compounds, and also such compounds as diazomethane, CH2=N2.
dielectric constant
	Dielectric constant or permittivity (ε) is an index of the ability of a substance to attenuate the transmission of an electrostatic force from one charged body to another. The lower the value, the greater the attenuation. The standard measurement apparatus utilizes a vacuum whose dielectric constant is 1. In reference to this, various materials interposed between the charged terminal have the following value at 20 °C: air 1.00058 glass 3 benzene 2.3 acetic acid 6.2 ammonia 15.5 ethanol 25 glycerol 56 water 81 The exceptionally high value for water accounts for its unique behaviour as a solvent and in electrolytic solutions. Dielectric constant values decrease as the temperature rises.
dienes
	Dienes are unsaturated organic compounds that contain two fixed double bonds between carbon atoms. Dienes in which the two double-bond units are linked by one single bond are termed conjugated.
diffusion
	Diffusion is the spontaneous mixing of one substance with another when in contact or separated by a permeable membrane. Diffusion is a result of the random motions of their component atoms, molecules, ions, or other particles. Diffusion occurs most readily in gases, less so in liquids, and least in solids. The rate of diffusion is proportional to the concentration of the substance and increases with temperature. The theoretical principles are stated in Fick's laws.
dipole
	Dipole is a pair of separated opposite electric charges. Electric dipole is an assemblage of atoms or subatomic particles having equal electric charges of opposite sign separated by a finite distance. In the case of HCl, the electrons are attracted towards the more electronegative chlorine atom.
dipole moment
	Electric dipole moment (μ) is the product of the positive charge and the distance between the charges. Dipole moments are often stated in debyes; The SI unit is the coulomb metre. In a diatomic molecule, such as HCl, the dipole moment is a measure of the polar nature of the bond; i.e. the extent to which the average electron charges is displaced towards one atom (in the case of HCl, the electrons are attracted towards the more electronegative chlorine atom). In a polyatomic molecule, the dipole moment is the vector sum of the dipole moments of the individual bonds. In a symmetrical molecule, such as tetrafluoromethane (CF4) there is no overall dipole moment, although the individual C-F bonds are polar.
disaccharides
	Disaccharides are compounds in which two monosaccharides are joined by a glycosidic bond. For example, sucrose comprises one glucose molecule and one fructose molecule bonded together.
dissociation
	Dissociation is the process by which a chemical combination breaks up into simpler constituents as a result of either added energy (dissociated by heat), or the effect of a solvent on a dissolved polar compound (electrolytic dissociation). It may occur in the gaseous, solid, or liquid state, or in solution. An example of dissociation is the reversible reaction of hydrogen iodide at high temperatures 2HI(g) H2(g) + I2(g) The term dissociation is also applied to ionisation reactions of acids and bases in water. For example HCN + H2O H2O+ + CN- which is often regarded as a straightforward dissociation into ions HCN H+ + CN-
dissociation constant
	Dissociation constant is a constant whose numerical value depends on the equilibrium between the undissociated and dissociated forms of a molecule. A higher value indicates greater dissociation. The term dissociation is also applied to ionisation reactions of acids and bases in water. For example HCN + H2O H2O+ + CN- which is often regarded as a straightforward dissociation into ions HCN H+ + CN- The equilibrium constant of such a dissociation is called the acid dissociation constant or acidity constant, given by The concentration of water [H2O] can be taken as constant. Similarly, for a base, the equilibrium NH3 NH4+ + OH- is also a dissociation; with the base dissociation constant or basicity constant, given by Ka (Kb) is a measure of the strength of the acid (base).
distillation
	Distillation is the process of boiling a liquid and condensing and collecting the vapour. The liquid collected is the distillate. The usual purpose of distillation is purification or separation of the components of a mixture. This is possible because the composition of the vapour is usually different from that of liquid mixture from which it is obtained. Gasoline, kerosene, fuel oil, and lubricating oil are produced from petroleum by distillation.

 

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