CHEMISTRY GLOSSARY

allotropy    →   alotropija

Allotropy (Gr. allos, other, and tropos, manner) is the phenomenon of an element existing in two or more physical forms in the same physical state. The difference between the forms involves either crystaline structure (white, red and black phosphorus), the number of atoms in the molecule of a gas (diatomic oxygen and triatomic ozone), or the molecular structure of a liquid (liquid helium an helium II).

In some cases, the allotropes are stable over a temperature range, with a definite transition point at which one changes into the other. For instance, tin has two allotropes: white (metallic) tin stable above 13.2 °C and grey (nonmetallic) tin stable below 13.2 °C. This form allotropy is called enantiotropy. Form of allotropy, in which there is no transition temperature at which the two are in equilibrium, is called monotropy.

Allotropy does not apply to the substance existing in different physical states as, for example, when ice melts and changes from solid ice to liquid water.

Allotropy is generally restricted to describing polymorphic behaviour in elements, while polymorphism may refer to any material having multiple crystal structures.

azeotrope    →   azeotropi

Azeotrope is a mixture of two liquids that boils at constant composition, i.e. the composition of the vapour is the same as that of the liquid. Azeotropes occur because of deviations in Raoult’s law leading to a maximum or minimum in the boiling point - composition diagram. The composition of an azeotrope depends on the pressure.

Arrhenius equation    →   Arheniusova jednadžba

In 1889, Svante Arrhenius explained the variation of rate constants with temperature for several elementary reactions using the relationship

where the rate constant k is the total frequency of collisions between reaction molecules A times the fraction of collisions exp(-E_a/RT) that have an energy that exceeds a threshold activation energy E_a at a temperature of T (in kelvin). R is the universal gas constant.

balance    →   vaga

Balance is an instrument to measure the mass (or weight) of a body. Balance beam type scales are the oldest type and measure weight using a fulcrum or pivot and a lever with the unknown weight placed on one end of the lever, and a counterweight applied to the other end. When the lever is balanced, the unknown weight and the counterweight are equal. The equal-arm balance consists of two identical pans hung from either end of a centrally suspended beam. The unequal-arm balance is made with one arm of the balance much longer than the other.

More modern substitution balances use the substitution principle. In this calibrated weights are removed from the single lever arm to bring the single pan suspended from it into equilibrium with a fixed counter weight. The substitution balance is more accurate than the two-pan device and enables weighing to be carried out more rapidly.

Electromagnetic force restoration balances also use a lever system but a magnetic field is used to generate the force on the opposite end of the lever and balance out the unknown mass. The current used to drive the magnetic coil is proportional to the mass of the object placed on the platform.

barytes    →   barit

Barytes is an orthorhombic mineral, form barium sulphate, BaSO₄, the chief of barium. It is usually white but my also be yellow, grey or brown. Large deposits occur in Andalusia, Spain and in the USA.

bcc    →   bcc

bcc is abbreviation for body-centered cubic lattice.

Boltzmann equation    →   Boltzmannova jednadžba

Boltzmann equation is a statistical definition of entropy, given by

where S and k are the entropy and Boltzmann’s constant, respectively, and W is the probability of finding the system in a particular state.

calcination    →   kalciniranje

Calcination is a process of driving out crystal water from crystal salts by heating.

coherence    →   koherentnost

If two overlapping light waves are to interfere detectably, the phase difference between them must remain constant with time, i.e. the waves must be coherent.

covalentity    →   kovalentnost

Covalentity is a maximum number of covalent bond that one atom can make, it is equal to the number of hydrogen atoms that are combined with other atoms. It is usually constant.