CHEMISTRY GLOSSARY

Daniell cell    →   Daniellov članak

In 1836 the British chemist John Frederic Daniell (1790-1845) proposed an improved electric cell that supplied an even current during continuous operation. Daniell cell consisted of a glass jar containing copper and zinc electrodes, each immersed in their respective acidic sulphate solutions. The two solutions were separated by a porous clay cylinder separator. It was a galvanic cell in which the spontaneous electrodissolution of zinc and electroplating of copper provided the electrical current.

dialysis    →   dijaliza

Dialysis is a method by which large molecules (such as starch or protein) and small molecules (such as glucose or amino acids) may be separated in a solution by selective diffusion through a semipermeable membrane. Through this kind of membrane dissolved particles pass and colloid dimension particles fall behind. For example, if a mixed solution of starch and glucose is placed in a closed container made of a semipermeable substance (such as cellophane), which is then immersed in a beaker of water, the smaller glucose molecules will pass trough the membrane into the water, while the starch molecules remain behind.

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.

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

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.

neutral substance    →   neutralna tvar

Neutral substance is a substance that shows no acid or base properties, has an equal number of hydrogen and hydroxyl ions and does not change the colour of litmus-paper.

neutralisation    →   neutralizacija

Neutralisation is the process in which an acid reacts with a base to form a salt and 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.