DRYNESS, CHEMICAL. The majority of chemical ac tions take place in the presence of water or some other solvent which plays an important part in the reaction ; in fact, water ap pears to be a substance whose presence in minute amounts is es sential for the occurrence of most chemical reactions. So far back as 1794, Mrs. Fulhame, in "An Essay on Combustion with a View to a New Art of Dyeing and Painting," recognized that presence of water, either as liquid or vapour, was necessary for the reduction of salts of gold and silver by hydrogen. In 1812 Cluzel discovered that the reaction between sulphuretted hydro gen and sulphur dioxide would not take place unless liquid water was present.
In 1869 Wanklyn brought together pure chlorine gas and me tallic sodium without any appreciable interaction taking place even when the sodium was molten, but he did not realize that the impurity which he had removed was water.
Some of the earliest experiments on the effect of moisture on gaseous combustion were carried out in 188o by H. B. Dixon, who discovered that a mixture of carbon monoxide and oxygen which had been most carefully dried would not explode when an electric spark was passed through the mixture, but on addition of a drop of water a violent explosion took place on again passing the spark. In 1888 H. B. Baker showed that water exerts some influence on the combustion of charcoal, since the carefully dried substance could be heated to redness in dry oxygen without appearance of a flame and only underwent partial oxidation to carbon monoxide and dioxide. He also showed that the elements boron, sulphur and phosphorus do not burn in dried oxygen, the latter not even becom ing luminous. He further showed (1902) that a well-dried mixture of hydrogen and chlorine does not explode on exposure to sunlight, as does an undried one; similarly, the gases ammonia and hydro , gen chloride, if perfectly dry, do not combine when mixed at the ordinary temperature, whilst pure dry electrolytic gas may be heated to redness or exposed to ultra-violet light without any measurable amount of water being formed. The introduction of a mere trace of moisture, e.g., by allowing a small bubble of ordinary moist air to enter the apparatus, causes the normal ac tion to take place. In 1887 E. W. Morley showed that the mere passing of a gas slowly through a long tube of such a vigorous dehydrating agent as phosphoric oxide leaves in it only 3 milli grams of water vapour per million litres, and yet this is sufficient to allow chemical action to proceed.
Not only is the rate of a chemical reaction materially retarded by removal of water from the system, but in many cases the phys ical properties of substances are modified by intensive drying. Carefully purified liquids of such widely different constitution as nitrogen trioxide, nitrogen tetroxide, bromine, mercury, hexane, benzene, carbon bisulphide, carbon tetrachloride, ether and various aliphatic alcohols were submitted to the influence of the drying agent, phosphorus pentoxide, for a period of several years. They were then examined and their boiling points showed a very con siderable rise. In the case of mercury the rise was 62°, i.e., the boiling point was 42o--425° C. instead of 358° C.; for benzene the rise was 38° (from 8o° to 118° C.). The melting point of bromine dried for 1 o years, was found to be — 4.5 ° C., while that of bromine dried for only a few days was — 7.3 ° C. ; that of sul phur was 117.5 ° C., showing a rise of 5.5 °. The melting point of pure, dried sulphur trioxide was found to be 61 ° C., compared with 5o° C. for the undried sample, and the dried sulphur trioxide changed on melting into the (3-modification which had a melt ing point of 15.5° C. compared with the normal 14° C. The vapour densities of dried ether and methyl alcohol were 81.7 and 45 re spectively, i.e., more than double and treble the normal values; this is in agreement with the change of surface tension for these and other intensively dried liquids.
These results point to a change, generally an increase in the complexity of the molecule, which is due to the removal of the last traces of water from the substance. In the same way that particles of water vapour collect together and condense to form droplets which in their turn coalesce to give large drops, so the single molecules of a substance may coalesce into aggregates of molecules. This aggregation of single molecules into a more com plex molecule is termed association (q.v.) and the breaking up of complex molecules into more simple ones is dissociation. It is obvious, then, that the effect of intensively drying a substance is to increase its molecular association to such an extent as to cause a modification of its physical properties. Since in every instance drying has produced increased association, it is evident that asso ciation is less affected by the process of desiccation than is dis sociation.
Practically all gases, liquids, and solids are soluble in water, some being very soluble, whereas others dissolve to such a small extent that only the most searching examination will give any indication of their presence in water. Conversely, water is soluble to a similar extent in most substances. This fact, and also the property of water of condensing in a thin film on liquid and solid surfaces—as observed in the "sweating" of walls in wet weather— in such a way that complete removal of the liquid film of water from surfaces is very difficult, gives some idea of the difficulty of removing the last, generally invisible, traces of water from sub stances. A clean, dry, polished glass carries on its surface an in visible film of water.
This property of retaining liquids or gases in an invisible form on the surface of solids is known as occlusion, and it is this power, which all substances possess, of occluding water to a greater or less extent, which renders so difficult the exact determination of the part played by water, both liquid and vapour, in chemical actions.
It has been shown above that a carefully dried mixture of cer tain gases will not react under ordinary conditions, whereas intro duction of a trace of water causes instantaneous reaction. That the presence of liquid water is essential to the reaction between sulphuretted hydrogen and sulphur dioxide is demonstrated by an experiment described by H. B. Baker. The gases in ques tion were dried by calcium chloride before mixing. This leaves about 4 milligrams of water vapour per litre of gas. The gases were then mixed and a small, open tube introduced con taining about 2 milligrams of dried radium bromide. There was no apparent change after six hours; i.e., no sulphur had been de posited on the walls of the jar, and it seemed as if no reaction had taken place. On opening the jar, however, there was an inrush of air and on heating the radium tube a large quantity of water was driven off and there was a considerable sublimate of sulphur. The whole of the gaseous contents of the jar had condensed in the small tube of radium bromide, and this was due to the fact that the ions from the radium salt cause condensation of the water vapour (left after desiccation with calcium chloride), to drops of liquid water in which the chemical action takes place.
This property possessed by water of helping a chemical action without itself being changed in the process is also characteristic of other substances, and the phenomenon is known as catalysis (q.v.).
Desiccation, or the removal of water from a substance, may be performed either by mechanical devices such as centrifuging, freezing or evaporation, or by chemical means. The choice of agent in this case is determined by the degree of dryness required and the nature of the substance to be dried. The most generally useful substances are anhydrous calcium chloride, concentrated sulphuric acid and phosphorus pentoxide, the last being the most powerful desiccator known. By its action over varying periods of time the degree of intensive dehydration described above has been attained.