CHEMISTRY GLOSSARY

dielectric constant    →   dielektrična konstanta

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 utilises 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:

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.

dioxin    →   dioksin

Dioxin is a general term that describes a group of hundreds of chemicals that are highly persistent in the environment. The most toxic compound is 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD. The toxicity of other dioxins and chemicals like PCBs that act like dioxin are measured in relation to TCDD. Dioxin is formed as an unintentional by-product of many industrial processes involving chlorine such as waste incineration, chemical and pesticide manufacturing and pulp and paper bleaching. Dioxin was the primary toxic component of Agent Orange, found at Love Canal in Niagara Falls, NY and was the basis for evacuations at Times Beach, MO and Seveso, Italy.

Dioxin is formed by burning chlorine-based chemical compounds with hydrocarbons. The major source of dioxin in the environment comes from waste-burning incinerators of various sorts and also from backyard burn-barrels. Dioxin pollution is also affiliated with paper mills which use chlorine bleaching in their process, with the production of Polyvinyl Chloride (PVC) plastics, and with the production of certain chlorinated chemicals (like many pesticides).

dipole    →   dipol

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 molecule    →   dipolna molekula

Dipole molecules are created when mutual electronic pair at covalent bond is asymmetrical. If different atoms are bonded by a covalent bond, which can have different electron affinity, then the the atom with greater electron affinity will attract the electron pairs more strongly. In this way an asymmetrical distribution of negative charge appears in a molecule, so one part of the molecule becomes relatively negatively (the one closer to the electron pair) and the other becomes relatively positively charged.

dissociation    →   disocijacija

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 a solution.

An example of dissociation is the reversible reaction of hydrogen iodide at high temperatures

The term dissociation is also applied to ionisation reactions of acids and bases in water. For example

which is often regarded as a straightforward dissociation into ions

natural resins    →   prirodne smole

Natural resins are organic compounds which are secreted by some plants and insects. They are used as adhesives, and are very often unsoluble in water.

neptunium    →   neptunij

Neptunium was discovered by Edwin M. McMillan and P. H. Abelson (USA) in 1940. Named after the planet Neptune. It is rare, silvery radioactive metal. Resists alkalis; reacts with oxygen and acids. Attacked by steam. Radiotoxic. Neptunium was produced by bombarding uranium with slow neutrons.

neutral oxide    →   neutralni oksid

Neutral oxides are those oxides which show neither basic nor acidic properties when they react with water. Examples include carbon monoxide (CO) and nitrous oxide (N₂O) which are only slightly soluble in water, and nitric oxide (NO) which is appreciably soluble in cold water.

dissociation constant    →   konstanta disocijacije

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

which is often regarded as a straightforward dissociation into ions

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

is also a dissociation; with the base dissociation constant or basicity constant, given by

K_a (K_b) is a measure of the strength of the acid (base).

dysprosium    →   disprozij

Dysprosium was discovered by Paul Emile Lecoq de Boisbaudran (France) in 1886. The origin of the name comes from the Greek word dysprositos meaning hard to obtain. It is soft, lustrous, silvery metal. Reacts with oxygen. Reacts rapidly with water; dissolves in acids. Metal ignites and burns readily. Reductant. Dysprosium usually found with erbium, holmium and other rare earths in some minerals such as monazite sand. Dysprosium uses are limited to the experimental and esoteric. Some isotopes of dysprosium are effective absorbers of thermal neutrons and are being considered for use in the control rods in nuclear reactors.