« PreviousContinue »
oxygen combine with the nitrogen of the air to produce nitric acid, and this dissolves in the water. If this hypothesis is correct, removal of nitrogen from contact with the decomposing water should be attended with cessation of the production of * nitric acid; re-introduction of nitrogen should be accompanied by re appearance of nitric acid.
Davy placed a gold vessel containing pure water on a plate of glass, and covered it with a strong glass jar connected with an air-pump; he exhausted the air from the jar, admitted hydrogen, again exhausted, and again filled the jar with hydrogen; he continued this treatment until he could feel sure that the whole of the air had been withdrawn from the jar. He then filled the jar with hydrogen, and passed the electric current; not a trace of acid was produced; hydrogen and oxygen, and these gases only, appeared at the electrodes. He admitted air into the jar; the acid began to form at the positive electrode.
But he had already proved that the production of acid was not connected with the presence of substance in the water, nor with the nature of the vessels containing the water, nor with the material of the electrodes; hence the production of acid always accompanied the presence of nitrogen. The latter was the cause of the former. “It seems evident then,” says Davy, "that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen.”
This remarkable research is a type of all scientific inquiry. 94 Facts were noticed and verified, conclusions were drawn and tested by experiments; hypotheses were framed on the basis of the experimentally determined facts, and were used to explain these facts by suggesting fresh lines of inquiry. The result which Davy obtained was not a barren fact; it at once prompted him to further discoveries. The electric current had slowly decomposed the glass vessels; probably it would also decompose other substances more or less resembling glass in composition. Water was electrolysed in cups of gypsum ; lime appeared at one electrode and sulphuric acid at the other. Other substances were employed; he generally obtained an alkaline body at the negative, and an acid at the positive, electrode. This led Davy to regard many compounds as built up of two parts, one positively, the other negatively, electrified.
This conception prompted him to make more experiments; these furnished him with new hypotheses; these in turn led
to further inquiry; and this reaction of experiment on theory
and theory on experiment proceeded until he had framed a general conception of the composition of salts, and of the relations between salts, acids, and alkalis, which had a most important influence on the development of chemistry. The facts noticed during the electrolysis of water also led Davy to investigate the action of the current on various substances which were included in the class of elements; many of these he succeeded in decomposing; he obtained new elements, and to a large extent changed the whole course of the chemical study of matter.
95 The synthesis of water by passing hydrogen over hot copper oxide has been already mentioned. The synthesis was carried out in the most accurate manner by Dumas. Fig. 14 represents the apparatus employed. A is a flask in which hydrogen is produced by the interaction of zinc and dilute sulphuric acid (the cylinder to the left of A contains mercury; it, and the tube dipping into it, serve as a means for allowing the hydrogen to pass away without taking the apparatus to pieces): the seven large U tubes contain materials by passing through which the hydrogen is purified and dried: the small U tube B contains phosphorus pentoxide, a substance which greedily absorbs moisture; this tube is weighed before and after each experiment, should it increase in weight after an experiment, the results of that experiment are rejected,
as the increase in the weight of B tells Fig. 14.
that the hydrogen was not perfectly dry when it passed into C: C is a bulb of hard glass con
taining a weighed quantity of perfectly pure and dry copper oxide; the neck of this bulb is drawn out and passes into the next bulb D: D is a dry bulb of glass destined to contain the water produced in the reaction; it is weighed before and after each experiment: the U tubes E contain materials to absorb any traces of water which may not be retained in D: the small tube F also contains drying materials; it is weighed before and after each experiment; should it shew an increase in weight after an experiment is finished the results of that experiment are rejected, because a doubt arises as to whether the whole of the water produced has been retained in the apparatus and weighed.
Let the weight of the copper oxide before an experiment = x; the weight of the copper remaining in C after the experiment
=y; the weight of water produced (that is the increase in weight of D and E) = =z: then x - y gives the weight of oxygen which has combined with hydrogen to produce the weight z of water; let this weight of oxygen =a: then 2- a gives the weight of hydrogen which has combined with a of oxygen to produce z of water.
Dumas' result was that 1 part by weight of hydrogen combines with 7.9804 parts by weight of oxygen, to produce 8.9804 parts by weight of water.
The volumetric synthesis of water has already been briefly 96 described : when this synthesis is conducted with all precautions the result is that one volume of hydrogen combines with half a volume of oxygen to produce water; as careful determinations have shewn that oxygen is 15.96 times heavier than hydrogen, the result of the volumetric synthesis, altogether confirms that of the gravimetric synthesis, of water.
When two volumes of hydrogen are caused to combine with one volume of oxygen in a vessel the temperature of which is above the boiling point of water, that is when the conditions are arranged so that the water produced is maintained in the state of gas, the result is that two volumes of hydrogen combine with one volume of oxygen to produce two volumes of water-gas.
Water then is a compound, not a mixture, since it has been shewn to be of constant composition, and to conform to the laws of chemical combination.
The results of experiments on the composition of water 97 are summed
in the formula H,O, and in the equation H+0=H.O.
Assuming that the combining weight of oxygen is 16 (in round numbers), and that the symbol o represents 16 parts by weight of oxygen, then this formula, and this reaction, tell us, that the masses of hydrogen and oxygen which combine to produce water are in the ratio 1:8 (in round numbers); that the reacting weight of water is 18, that this reacting weight is composed of two combining weights of hydrogen and one combining weight of oxygen, and that this quantity of water can be decomposed and either one or two combining weights of hydrogen removed, but that if oxygen is removed the whole of the oxygen must be removed; and that two volumes of water-gas are formed by the union of, or can be resolved into two volumes of hydrogen-gas and one volume of oxygen-gas (comp. pars. 86 and 87).
We may here inquire, what would be the composition of other compounds of hydrogen and oxygen if such compounds existed ? The law of multiple proportions tells us that the composition of such compounds would be expressed by the general formula H,Oy, where H represents a combining weight of hydrogen, O represents a combining weight of oxygen, and and
y are whole numbers. Two compounds of hydrogen and oxygen are known; one is water, H,O; the other is hydrogen peroxide H,O,.
But neither the formula H,O, nor the equation H, +0=H,O, says anything as to the conditions under which water is produced by the union of hydrogen and oxygen; they do not tell, or even suggest, the properties of water; nor do they indicate the physical changes which accompany the chemical change from a mixture of hydrogen and oxygen to the compound water.
We have already learned something of the conditions under which the reactions expressed by the equations
(1) H, +0= H,0; (2) H,0 = H, +0 proceed; we know a few of the properties of water; and we are not wholly ignorant of the fact that the production of water is an event which has a physical as well as a chemical aspect. But we ought now to look a little more closely at some of these points. Conditions under which the equations (1) H, +0=H,0 and
(2) HO = H, + O are realised. (1) When two volumes of hydrogen are mixed with one
volume of oxygen, and an electric spark is passed through the mixture, or a lighted taper is applied.
(2) When an electric current is passed through 18 parts by weight (say 18 grams) of water until the whole of the water has disappeared. Or the water is raised to a very high temperature (say 1500°—2000") under conditions such that the oxygen and hydrogen are removed from contact with the yet undecomposed water as quickly as they are produced. Or the water in the form of steam is passed over hot finely divided iron, or hot magnesium (or certain other metals); the hydrogen is collected; the oxygen combines with the iron or magnesium to form an oxide of either metal; by decomposing this oxide by suitable means the oxygen may be obtained. Physical changes which accompany the chemical change from the
mixture H, +0 to the compound H,O. (1) A large quantity of heat is produced. When 2 grams of hydrogen combine with 16 grams of oxygen to form 18 grams of water, the heat produced is sufficient to raise the temperature of (in round numbers) 68,000 grams of water from 0° to 1° C.; in other words 68,000 gram-units of heat are produced.
This statement tells that the chemical change H, +0= H,O, i.e. the change of certain masses of two definite kinds of matter into a mass of another kind of matter equal to the sum of the masses of the two kinds of matter, is accompanied by the degradation of a large quantity of energy (s. Chap. xiv.). The system H,0 possesses much less energy, it can do much less work, than the system H, +0; the difference between the energies of the two systems is approximately represented by 68,000 gram-units of heat.
(2) A contraction of volume occurs. The mixture of 2 grams of hydrogen with 16 grams of oxygen occupies about 44,000 c.c. at 0° and 760 mm.; the 18 grams of water produced occupy about 18 c.c.
If the temperature is kept a little above 100°, the 18 grams of water-gas produced occupy about 30,000 c.c.
(3) The mixture of hydrogen and oxygen is gaseous ; the water formed is a liquid below 100° at 760 mm.
(4) The change is accompanied by the production of a flash of light.
These are some of the more important conditions under which the changes represented by the chemical equations we