form a definite hydrate of the composition HCl,6H2O. Besides this hydrate there exists also a crystallo-hydrate, HCl,2H2O,43 which is formed by the absorption of hydrochloric acid by a saturated solution at a temperature of -23°. It crystallises and melts at -18°.44
The mean specific gravities at 15°, taking water at its maximum density (4°) as 10,000, for solutions containing p per cent. of hydrogen chloride are—
| p | S | p | S |
|---|---|---|---|
| 5 | 10,242 | 25 | 11,266 |
| 10 | 10,490 | 30 | 11,522 |
| 15 | 10,744 | 35 | 11,773 |
| 20 | 11,001 | 40 | 11,997 |
The formula S = 9,991·6 + 49·43p + 0·0571p2, up to p = 25·26, which answers to the hydrate HCl,6H2O mentioned above, gives the specific gravity. Above this percentage S = 9,785·1 + 65·10p - 0·240p2. The rise of specific gravity with an increase of percentage (or the differential ds/dp) reaches a maximum at about 25 p.c.45 The intermediate solution, HCl,6H2O, is further distinguished by the fact that the variation of the specific gravity with the variation of temperature is a constant quantity, so that the specific gravity of this solution is equal to 11,352·7(1 - 0·000447t), where 0·000447 is the coefficient of expansion of the solution.46 In the case of more dilute solutions, as with water, the specific gravity per 1° (or the differential
ds / dt )
rises with a rise of temperature.47
| p = | 0 | 5 | 10 | 15 | 20 |
|---|---|---|---|---|---|
| S 0 – S 15 = | 7·2 | 23 | 38 | 52 | 64 |
| S 15 – S 130 = | 34·1 | 42 | 50 | 59 | 67 |
Whilst for solutions which contain a greater proportion of hydrogen chloride than HCl,6H2O, these coefficients decrease with a rise of temperature; for instance, for 30 p.c. of hydrogen chloride S0 - S15 = 88 and S15 - S30 = 87 (according to Marignac's data). In the case of HCl,6H2O these differences are constant, and equal 76.