5-14 SHENSTONE AND CUNDALL INFLUENCE OF TEMPERATURE XL.-The InJlueme ?f Temperatwe on the Composition and Solubility of Hydrated Calcium Szclrphate and of Calcium Hydroxide. By W. A. SHENSTONE and J. TUDOR CIJNDALL. I. SUCH facts as the extraordinary soliibility of silver nitrate at temperatiires above loo",* and the evidence of a distinct relation between the solubility of salts a t high temperatures and their melting points,* together with the circumstance that high temperature, though favourable to dissociation diminishes the solubility of only a few substances are apparently inconsistent' with the idea that in all cases the soliibility of salts in water is due to the formation of definite hydrates. Yet on the other hand even before the recent publications of Mendel6eff and others there was considerable reason for supposing that in many cases the solubility of anhydrous salts in water is f Tilden and Shenstone Phil.Trans. 1884 Part I ON HYDRATED CALCIUM SULPHATE ETC. 545 accompanied by and influenced by the formation of such hydrates.* And it has been supposed that the influence of high temperature in reducing the solubility of several salts is due to dissociation of soluble hydrated compounds the products of the dissociation in these cases being less soluble than the compounds from which they have been formed. This view receives some support from the fact that several of the salts in question for instance sodium sulphate and copper sulphate form well-known hydrates which are readily d ecom-posed on heating even in the presence of a large excess of water.? On the other hand however in the case of some othem such as sulphate of calcium the only known hydrates have been supposed to be comparatively stable even in dry air.As a satisfactory theory of solut.ion must be wide enough t o include an explanation of the peculiar behaviour of these salts we have thought it worth while to spend some time on the experiments described in this paper and we think that our results will be found to be a useful contribution to this side of the " solution question," although in the case of calcium hydroxide t,hey are at present partly of a negative character. 11. The In8uence of Temperature on Hydrated 0alciu.m 8ulphate.-The statements on this subject in works on chemistry vary consider-ably but all agree so far that they lead to the conclusion that a temperature above 100" is necessary to stmt the dehydration of the salt and that a considerably higher temperature is required for its complete desiccation.For example Hannay ( J . Clzem. Xoc. 1887 ii, 381) states that in dry air dehydration of calcium sulphate does not commence till a temperature of 118" is reached but that subsequently it proceeds at as low a temperature as 100" (Bar. = 760 mm.) until 15 per cent. of water has been expelled. Our results do not agree very well with any of these statements ; for in consequence of conducting our experiments with the special view of detecting small changes (viz. by employing it set of weights whose exact relations were known and by resorting to the method of vibrations in weighing) we observed at an early stage a peculiarity in the behaviour of the substance that has previously been over-looked.Experiment I.-A known mass of pure hydrated calcium sulphate of our own preparation was placed in a flask A and after the removal of hygroscopic moisture by means of a current of dry air was heated in a slow current of air which had been dried by means of phosphoruf; f It is not unlikely that in some cases for instance in that of calcium sulphate, -f We find this to be true in t'he case of CuS04,5H20 at temperatures above solubility depends entirely on the existence of one or more hydrated cornpounda. 100". VOL. LIlI. 2 546 SHENSTONE AND CUNDALL INFLUENCE OF TEMPERATURE Total loss per cent. by artificial CaS04,2H20.Temperature. pentoxide until its weight became constant a t 70° loo" and 150". A similar experiment was made on a portion of powdered selenite. Total loss per cent by selenite. I n these experiments the admission of moisture to the contentls of A during cooling and weighing was prevented by means of well-fitting stoppers B C and the t w o tubes were securely plugged with glass-wool to prevent the mechanical conveyance of particles of salt from A by the current of air. We assured ourselves of the efficiency of these precautions by repeated experiments. As hot water attacks glass vessels in the prolonged exposure necessary in these experiments to an extent sufficient t o interfere with the accuracy of the work and as paraffin is inconvenient to work with A was plunged into a copper-bath filled with shot the copper-bath being itself placed in a vessel containing water or paraffin.The temperature of A was determined by means of a thermometer whose bulb was immersed in the shot and in contact with A. TABLE I.-Showing the Results of Experiment I. I I ___- /---I- --70°C. . 100" c . 150" C . Low red heat . 20 *67 20.89 21 *C'r .-19'99 20.45 20 '67 ON HYDRATED CALCIUM SULPHATE ETC. 547 The formula CaSO4,2H2O requires 20.9 per cent. of water. It has frequently been observed by others that the total loss of water in drying this salt slightly exceeds that required by theory. The temperature 70" was selected as the starting point in this experiment because whilst that temperature is not too far below loo", the solubility of calcium sulphate at 70" is distinctly lower than at 35" which is probably the temperature of maximum solubility.Experiment 11.-This experiment was made at do" which is only slightly above the temperature of maximum solubility of calcium sulphate. The results obtained are given in Table 11. The same method as that described above was employed. TABLE 11. Water lost by 2.0460 gram of calcium sulphate at 40". Time. I G hours . . . . 30 . . . . . . . . 48 . . . . . . . . 72 . . . . . . . . 96 . . . . . . . . 120 . . . . . . . . 144 . . . . . . . . I . 0 *0008 gram 0-0068 ,, 0*0309 ,, 0-0376 ,, 0'0492 ,, 0.0600 ,, 0.0202 ), The loss in this case at the end of 144 hours amounted to about 3 per cent.but the change was not yet complete. Experiment III.-The above experiments seem to establish that, contrary to previous statements hydrated calcium sulphate parts with the greater part of its water of crystallisation a t 70" in dry air and that even at as low a temperature as 40" it is not absolutely permanent. Whilst we were making them we were led to think that the results of a careful examination of the rate of loss in equal periods of time would be interesting. Another experiment was therefore made at 70", in which powdered selenite was heated in A during periods of three hours in a current of dry air which was passed through the apparatus at an average rate of 111.6 C.C. per minute. The results of this experi-ment which aro given in Table 111 and curve No.I (p. 550) show that not only is the rate at which this salt dissociates a slow one but that i t is much slower in its earlier stages than afterwards.* We think that these two facts and especially the latter help to account for the divergent statements that have been made on this subject. f At higher temperatures the three-hour intervals between the weighings are too long to permit of the detection of this peculiarity in the behaviour of caloiuw sulphate. 2 P 548 SHENSTONE AND CUNDALL INFLTJENCE OF TEMPERATURE Owing to the fact that most salts part with their water of crystallisa-tion as rapidly during the earlier stages as afterwards (or eveE more rapidly) chemists frequently neglect slight changes in experi-ments of this kind. Thus in his careful experiments Hannay heated calcium sulphate for periods of 20 minutes only to the following temperatures 100°7 103"' 105" 110" 115"' and finding no perceptible loss till 118" was reached concluded that lower temperatures than this were without effect on it.TABLE I11 (Curve No. 1). Time. 3 hours 6 9 ) . . . . . . . . . 12 ) 15 18 21 24 27 30 33 36 Loss of water in dry air at 70". 0 -94 per cent. 2-86 ), 4.87 )) 7.25 ), 9-73 ), 12-87 :? 14 94 )) 18-20 ,, 19.14 ), 19.87 ,, 20'14 )) 16 -7.2 ,, Experiment IV.-This experiment shows even more markedly the slowness with which calcium sulphate responds to the influence of heat. Table IV gives the results obtained by heating a specimen of powdered selenite a t 100' in a water-oven during periods of three houm each.Only a very SIOW current of undried air was allowed to pass through the oven during the experiment. TABLE IV (Curve Xo. 2). Loss of water after 3 hours' heating was 0.04 per cent. ?? ? 7 6 77 7 0.075 7 7 7 7 7 9 9 ?7 7 7 0.85 9 9 7 7 9' 12 31 > ? 4.06 7 7 7 ) 9 7 15 7 7 7 7 8-30 7 7 ? ? '? 18 7 ) 7 7 11.25 9 , 7 7 7 7 21 7 ) > ? 13.86 7 7 7 7 7 7 24 7 7 , 14.69 ,, 7 3 7 7 29 9 7 ) 14-73 ,, 1 7 7 33 7 1 7 7 14.93 ,) 9 7 ,) 39 7 7 7 9 14-93 9 ON HYDRATED CALCIUN SULPHATE ETC. 549 Ezperirnent V.-The divergent results t o which we have alluded are, however partly to be attributed t o the circumstance that different specimens of calcium sulphate (CaSO4,2H2O) do not behave in exactly the same manner when submitted to identical treatment.Table V gives the results of exposing three different specimens of hydrated calcium sulphate to a temperature of 100" simultaneously under absolutely identical conditions. In respect to the total loss in undried air it will be observed that these results agree with those of other observers as well as could be expected under somewhat variable conditions. Loss of Waterper cent. at 100". Time. 2 hours 4 6 8 ) Artificia.1 CaSO4,2H2O. I -4.3 15 '0 15 *8 15.8 Selenite. Selenite, T. 1.3 7 - 8 11 -5 13.8 1 '0 5 '8 10'0 12-8 Xummaq.-The results we have obtained show that both in dry and moist air hydrated calcium sulphate is less stable than has been supposed. Its peculiar behaviour under the influence of heat,* suggests the idea that in the case of this substance we have to deal with molecular aggregates of great stability and that it is only after the gradual breaking up of these that dissociation can take place.It will not perhaps be unreasonable in future to place calcium sulphate with those salts whose diminished solubility in hot water may possibly be due to dissociation of definite hydrates. It is true that calcium sulphate is thrown down from a hot solution at a much more rapid rate than that at which dehydration occurs even in perfectly dry air at similar temperatures. But if there be any truth in the above suggested explanation of the slow rate at which the solid salt dissociates this circumstance is of less importance since it is not unlikely that in the liquid state there is a comparatively simple state of affairs in regard t o molecular aggregation and moreover the liquid condition is more favourable to the rapid progress of change than the solid state.It will be noticed that the curves fail to afford any support to the idea that other hydrates of calcium sulphate exist besides that to which we give the formula CaSO,,B&O. * Zinc sulphate behaves in a somewhat similar manner. See Hannay loc. cit 550 SKINNER AKD RUHEMANN THE ACTION OF IV.-In$uence of Temperature o n the Composition and Solubility of Culciunz Hydroxide. Our experiments with this substance are much fewer than in the Case of calcium sulphate. A specimen of pure calcium hydroxide, dried a t 16" in air free from carbon dioxide lost only 0.3 per cent.of moistmure after prolonged heating to temperatures ranging from 70" to 150" in a current of air thoroughly dried by means of phosphorus pentoxide. This led us to doubt the accuracy of the statements that have been made as to its solubility in hot water especially as all experiments on the subject, so far as we are aware have been made in Fessels of glass which might possibly be attacked by the hydroxide as it is at high temperatures by calcium sulphate (Tilden and Shenstone Zoc. cit.). An experiment was therefore made to test this point. This experiment was made with the platinum tube employed for similar work by one of us in conjunction with W. A. Tilden in 1885 (Proc. Roy. Xoc. vol. 38 p. 331). The utmost care was taken and a carefully prepared specimen of the hydroxide was employed. We found that a t 15U0 3081 parts of solution contained only one part of calcium hydroxide. A t 19" one part of hydroxide was found to be present in 640 parts of solution. No donbt therefore remains on this point; calcium hydroxide is very decidedly less soluble in hot water than in cold but we have a t present no information as to the cause of its diminished solubility