the temperature falls,42 (3) that many of the properties of solutions of hydrochloric acid vary distinctly according as they contain more or less than 25 p.c. of hydrochloric acid (for instance, antimonious sulphide gives hydrogen sulphide with a stronger acid, but is not acted on by a weaker solution, also a stronger solution fumes in the air, &c.), and (4) that the composition HCl,6H2O corresponds with 25·26 p.c. HCl—judging from all these data, and also from the loss of tension which occurs in the combination of hydrochloric acid with water, it may be said that they 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.