CHEMISTRY GLOSSARY

acid    →   kiselina

Acid is a type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction for an acid HA is commonly written:

In fact, the hydrogen ion (the proton) is solvated, and the complete reaction is:

This definition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and produce colour changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water, (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. acetic acid and hydrogen sulphide). The strength of an acid depends on the extent to which it dissociates, and is measured by its dissociation constant.

In the Lowry-Brønsted theory of acids and bases (1923), the definition was extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). An important feature of the Lowry-Brønsted concept is that when an acid gives up a proton, a conjugate base is formed that is capable of accepting a proton.

Similarly, every base produces its conjugate acid as a result of accepting a proton.

For example, acetate ion is the conjugate base of acetic acid, and ammonium ion is the conjugate acid of ammonia.

As the acid of a conjugate acid/base pair becomes weaker, its conjugate base becomes stronger and vice versa.

A further extension of the idea of acids and bases was made in the Lewis theory. In this, a G. N. Lewis acid is a compound or atom that can accept a pair of electrons and a Lewis base is one that can donate an electron pair. This definition encompasses "traditional" acid-base reactions, but it also includes reactions that do not involve ions, e.g.

in which NH₃ is the base (donor) and BCl₃ the acid (acceptor).

antioxidant    →   antioksidans

Antioxidants are compounds that slow down oxidation processes that degrade foods, fuels, rubber, plastic, and other materials. Antioxidants like butylated hydroxyanisole (BHA), are added to food to prevent fats from becoming rancid and to minimize decomposition of vitamins and essential fatty acids; they work by scavenging destructive free radicals from the food.

aqueous solution    →   vodena otopina

Aqueous solutions are those solutions where water is the solvent. An aqueous solution found in an equation describing a chemical reaction is denoted by the state symbol, (aq).

biochemistry    →   biokemija

Biochemistry is the study of the chemistry of living organisms, especially the structure and function of their chemical components (principally proteins, carbohydrates, lipids and nucleic acids).

bioelement    →   bioelement

Bioelement is any chemical element that is found in the molecules and compounds that make up living organism.

Bronsted acid    →   Bronstedova kiselina

Brøsted acid is a material that gives up hydrogen ions in a chemical reaction.

adenosine triphosphate    →   adenozin trifosfat

Adenosine triphosphate (ATP) is nucleotide that is of fundamental importance as a carrier of chemical energy in all living organisms. It consists of adenin linked to D-ribose).

allotrope    →   alotrop

Allotropes are the elements which exist in two or more different forms in the same physical state. Allotropes generally differ in physical properties and may also differ in chemical activity.

Diamond, graphite and fullerenes are three allotropes of the element carbon. Graphite is a soft, black, slippery substance; by contrast, diamond is one of the hardest substances known. The different properties of the allotropes arise from their chemical structures. Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally bonded carbon atoms. Graphite crystallizes in the hexagonal system. In the fullerenes, the carbon atoms taking the form of a hollow sphere, ellipsoid, or tube.

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.

The term allotropes may also be used to refer to the molecular forms of an element. Ozone is a chemically active triatomic allotrope of the element oxygen.

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.

Bronsted base    →   Bronstedova baza

Brønsted base is a material that accepts hydrogen ions in a chemical reaction.