Химические тексты для перевода (тренировочные)

PHOSPHORUS

We have selected as the element to be studied here phosphorus. It is located below nitrogen in group V of the Periodic Table. These two elements resemble each other in many respects, but there are some properties in which they differ radically. The most striking difference between these two elements is that nitrogen is quite inactive under ordinary conditions, while phosphorus reacts readily both with metals and non-metals. They also differ in that nitrogen is a gas at ordinary temperature, whereas phosphorus is a solid.

Ordinarily obtained phosphorus is a soft solid, white when first prepared but slowly turning yellow. It is ordinary stick phosphorus which is called white phosphorus. It is to be distinguished from the other allotropic forms of the element. When heated much above its boiling point, the molecules begin to dissociate according to the reaction P₄ - 2P₂, one per cent of the molecules being dissociated at 800°C and more than 50 per cent at 1200°C. Phosphorus is almost insoluble in water. Phosphorus dissolves in many solvents, the best solvent being carbon disulfide. One part of carbon disulfide will dissolve nine parts of phosphorus.

It is to be noted that phosphorus exists in several allotropic forms and only two of them are of great interest. One form is white phosphorus. The other form is red phosphorus.

It should be remembered that the most striking property of white phosphorus is its activity with oxygen. When exposed to air at room temperature the oxidation of phosphorus begins slowly but with the rise of temperature spontaneous combustion may result. As it is easily inflammable phosphorus must be kept under water. Care should be taken not to handle phosphorus with bare hands as heat of the body is sufficient to cause ignition. White phosphorus is known to be very' poisonous. If one remains for a long time in the atmosphere containing phosphorus vapour chronic poisoning may result.

Red phosphorus is a more stable form and its reactions are much less violent. It does not ignite in the air until heated to 240°C.

SILICON

We are to study the element silicon which is located in column IV В of the Periodic Table following carbon. Its chemistry like that of carbon is considered to be complex.

Silicon does not occur free in nature. Compounds of silicon are very abundant making up about 87 per cent of the matter in the outer layer of the solid earth. The element ranks next after oxygen in abundance. Silicon plays an important role in the inorganic world. Its importance in the inorganic world results from the fact that its molecules exist in chains and more complex structures, in which the silicon atoms are characterized as being connected by oxygen atoms. It should be noted that carbon is of great importance in organic chemistry. Its importance is due to the fact that it possesses the ability to form carbon-carbon bonds which permit complex molecules with various properties to exist.

The compounds of silicon and carbon to be of importance are not similar. These differences are due to the fact that the silicon atom has a much larger radius than the carbon atom. Thus the attraction of the nucleus for electrons is found to be less in the silicon atom than in the carbon atom.

Although the compounds of silicon had been used for many centuries, it was only at the beginning of the 19th century that the element itself was prepared. Now many methods of preparing silicon are known. One of them is to heat silicon dioxide with magnesium. One way of preparing silicon industrially is by reducing the dioxide (SiO2) with carbon in an electric furnace. The product containing up to 98 per cent of silicon is obtained by this reaction.

Silicon resembles carbon in having crystalline as well as amorphous form. The latter is a brown powder consisting of very small crystals. Crystalline silicon has a structure resembling that of diamond. They are alike in being very hard, crystalline silicon being hard enough to scratch glass. Crystalline silicon is less active in chemical reactions than its amorphous form. Besides being used in the steel industry free silicon has few uses but its compounds have a wide application.

RUBIDIUM AND CAESIUM

These two elements belong to the elements of Group I A of the Periodic Table. The elements of this group which includes lithium, sodium, potassium, rubidium and caesium are called the alkali metals. From the chemical point of view the alkali metals are the most active metals known. They are too active to be found free in nature.

Rubidium and caesium are known to be the fourth and the fifth members of the alkali metal group. Both of them were discovered in 1860 and 1861 respectively. Rubidium and caesium got their names owing to their colour in their spectra. Rubidium was named so because of its having the prominent red lines in its spectrum. Caesium was called so because of its having blue lines in its spectrum. So, it is the colour of their lines in spectrum which gives an explanation of their names.

Both elements are alike in being soft, silvery white in colour. They have low density and low melting points. In physical properties and in their chemical behaviour rubidium and caesium resemble other alkali metals. They are monovalent in their compounds, the latter being very stable to oxidation and reduction.

Rubidium is to be ranked as the sixteenth most abundant element in the Earth's cruet but it is not found in any mineral as a principal constituent. It often occurs widely dispersed in potassium 'minerals in very low concentrations. This fact accounts for the scarcity of production and application of this rather abundant element.

Caesium is to be ranked as the fortieth most abundant element in the Earth's crust. It is found in some minerals.

In their chemical reactions rubidium and caesium are very similar to potassium. Both metals prove to react vigorously with air and water.

We know rubidium and caesium to form alloys with the alkaline earth metals, mercury, antimony, bismuth and gold. When alloyed with the last three metals both rubidium and caesium display the property of releasing electrons under the influence of light. This effect can be employed in photoelectric tubes.

SELEHIUM AMD TELLURIUM

A discussion of sulphur would be incomplete without at least a brief consideration of these two closely related elements selenium and tellurium.

Selenium is widely distributed over the Earth's surface. Tellurium is approximately one half as abundant as selenium. Usually these two elements are classed among the rare elements. But from the point of view of industry they are readily available since they occur in most of the sulphide ores of copper, silver, gold and nickel. In the refining of these ores it is essential that selenium should be removed. Because of their being abundant these elements could be produced in moderately large quantities if there were sufficient commercial demand.

Both these elements resemble sulphur in having a number of allotropic forms, those of selenium having been studied extensively. Selenium may be metallic and amorphous, both forms being soluble in carbon disulphide. Metallic selenium is steel grey in colour. It is this form of the element which has unusual properties of electric conductivity. Amorphous selenium is a supercooled liquid of great viscosity. Tellurium is both amorphous and crystalline. Crystalline tellurium is brittle and is easily ground to powder.

It should be noted that the metallic properties of the elements of the Sulphur Family increase with atomic weight. Tellurium is known to be the most metallike of the four elements.

Both selenium and tellurium are alike in being monoatomic at 2000°C. In their chemical behaviour these two elements resemble sulphur in uniting with many metals forming selenides and tellurides. They burn in air forming dioxides and after further oxidation they give trioxides.

Selenium and tellurium have a great variety of industrial uses at present.