Physical Properties of Acetylene

Pure acetylene is a colourless gas with an ethereal smell. It is liquefied more easily than ethylene or methane, and was first obtained in the liquid state by Cailletet in 1877. The vapour-pressures of solid and liquid acetylene at various temperatures have been determined by Ansdell, Villard, Kuenen, and Mcintosh. Some of the values determined by Villard, with which those of Kuenen and Mcintosh are in close agreement, are:

Temp. ° C.	Vapour Pressure.	Temp. ° C.	Vapour Pressure.
-90	0.69 atm (solid)	-40	7.7 atm
-85	1.00 atm (solid)	0	26.05 atm
-81	1.25 atm (M.P.)	11.5	34.8 atm
-70	2.22 atm (liquid)	20.2	42.8 atm

It is noteworthy that at the melting-point of solid acetylene ( -81°) the vapour-pressure is equal to 1.25 atmosphere, and that at -85° solid acetylene vapourises at atmospheric pressure. That is to say, solid acetylene, like solid arsenic, for instance, sublimes when heated under atmospheric pressure, and can only be melted under greater pressure. When liquid acetylene evaporates in the air it solidifies to a snowlike mass.

According to Ladenburg and Krugel the sublimation temperature of acetylene is -82.4° C., and according to Mcintosh -83.6° C. at 760 mm. pressure. This latter observer found -81.5° C. at 895 mm. to be the triple point at which solid, liquid, and gaseous acetylene can co-exist in equilibrium.

The critical data according to Villard (1), Kuenen (2), and Mcintosh (3) are:

	(1)	(2)	(3)
Critical pressure in atm	61.1	61.0	61.6
Critical temperature ° C	35.0	35.25	36.5
Critical volume	-	-	83 c.c.

According to Mathias the critical temperature is 37.05° C. and the critical density 0.2346, but Cardoso and Baume find critical temperature 35.5° C. and critical pressure 61.5 atm.

The constants for van der Waals' equation are: a = 0.0880; b = 0.00230.

The density of acetylene under normal conditions is 0.92 (air = 1), the theoretical value being 0.8988; the ratio of the specific heats; Cp/Cv=1.26; the molecular heat of vaporisation for liquid acetylene at -81.2° C. = 5080 calories; for solid acetylene at -83.6° C. = 5490 calories.

The density of liquid acetylene at -80° C. is 0.613, and of the solid at the same temperature 0.72. Thus the molecular volume of liquid acetylene is 42, and this value supports the acetylidene formula C:CH₂ of Nef.

On the other hand, Mathews has shown physical reasons for rejecting the acetylidene formula in favour of the more usual and symmetric formula CH≡CH.

The heat of combustion of acetylene, according to Thomsen, is 310,050 calories; and this value subtracted from the sum of the heats of formation of water and carbon dioxide (262,280) gives - 47,770 calories, as the heat of formation of acetylene.

Berthelot and Matignon obtained the values 315,700 and - 58,000 calories and Mixter 313,800 and -51,400 calories, respectively.

Acetylene is more soluble in water and other solvents than ethylene, and much more soluble than methane. One volume of water dissolves about 1.1 vol. of acetylene at atmospheric temperature; and the following coefficients are given by Winkler:

0° C	1.73
10° C	1.31
20° C	1.03
30° C	0.84

At low temperature acetylene forms with water the crystalline hydrate C₂H₂.6H₂O, whose dissociation pressure is as follows:

Temperature, ° C	0	7	15
Dissociation pressure	5.75 atm	12 atm	33 atm

Acetylene is much more soluble in organic solvents such as methyl and ethyl alcohol, ethyl acetate and acetone, than it is in water. Alcohol dissolves about six times its volume of acetylene, and acetone twenty-five times its volume at 15° C. and atmospheric pressure, and 300 times its volume under a pressure of 12 atmospheres. These facts are made use of for the storage of acetylene as an illuminant. At -80° C. acetone dissolves 2000-2500 its volume of acetylene, and thereby expands to four or five times its original volume.

A solution of acetylene in benzene shows a normal molecular weight cryoscopically. Acetylene forms crystalline compounds with alcohol, ether, and acetone. Palladium-black and colloidal palladium adsorb acetylene.