Physical Properties of Carbon Dioxide

Pure carbon dioxide is a colourless and odourless gas with a sharp taste. It is about one and a half times as heavy as air, and on this account accumulates in valleys, wells, and other low-lying spots..Thus the Poison Valley of Java is so named because carbon dioxide, issuing from fissures in the earth, remains in the valley, and may suffocate an unwary traveller; and the Grotto of Dogs, near Naples, similarly contains air richly laden with carbon dioxide to the depth of two or three feet, so that dogs or other small animals entering the Grotto are compelled to breathe the gas, whilst a man, walking with his head above the gas, is safe. The heaviness of carbon dioxide may be illustrated by pouring, ladling, or siphoning the gas from one vessel to another, its presence being shown by the extinction of a taper. The density of the gas at 0° C. and 760 mm. (air = 1) is 1.52909 (Lord Rayleigh) or 1.52894 (Leduc), whilst the value calculated from the molecular weight is 1.5201. One litre of carbon dioxide at 0° C. and 760 mm. Weighs 1.9678 grams.

The coefficient of thermal expansion of the gas at constant pressure has been determined by Chappuis to be as follows:

Pressure	0-20° C.	0-40° C.	0-100° C.
At 518 mm	0.0037128	0.0037100	0.0037073
At 998 mm	0.0037602	0.0037536	0.0037410
At 1377 mm	0.0037972	0.0037906	0.0037703

As will be shown below, carbon dioxide is an easily condensable gas; consequently when it is compressed it departs considerably from Boyle's law. The compressibility at various temperatures has been measured by Regnault, Roth, and Amagat. The results of Amagat are given in the following table, which shows the values of pv when pv at 0° C. and 1 atm. = 1.

Atm.	0°	10°	20°	30°	40°	60°	80°	100°	137°	198°	258°
50	0.105	0.114	0.680	0.775	0.850	0.964	1.096	1.206	1.380	-	-
100	0.202	0.213	0.229	0.255	0.309	0.661	0.873	1.030	1.259	1.582	1.847
150	0.295	0.309	0.326	0.346	0.377	0.485	0.681	0.878	1.159	1.530	1.818
200	0.385	0.401	0.419	0.440	0.468	0.543	0.660	0.815	1.096	1.496	1.804
300	0.559	0.578	0.599	0.623	0.649	0.710	0.790	0.890	1.108	1.493	1.820
400	0.728	0.748	0.771	0.795	0.823	0.884	0.956	1.039	1.218	1.563	1.883
500	0.891	0.913	0.938	0.963	0.990	1.054	1.124	1.201	1.362	1.678	-
700	1.206	1.232	1.259	1.289	1.319	1.383	1.454	1.529	1.676	1.956	-
1000	1.656	1.685	1.716	1.748	1.780	1.848	1.921	1.999	-	-	-

It will be seen from these figures that values of pv at a given temperature fall to a minimum as the pressure increases and then rise again; or the compressibility reaches a maximum at an intermediate pressure and then diminishes again. The following table shows the pressures at which pv reaches a minimum value at different temperatures.

Temperature ° C.	Pressure Atm.	Temperature ° C.	Pressure Atm.
0	35	70	162
10	45	80	179
20	57	90	196
30	76	100	211
40	101	137	247
50	124	198	255
60	143	258	218

Isothermals of carbon dioxide, illustrating the critical phenomena.

The above phenomena are closely connected with those of the critical state investigated by Andrews, who showed that above 31.1° C., which is called the critical temperature, no sensible liquefaction of carbon dioxide takes place, whatever the pressure; whilst below that temperature any isothermal, i.e. a curve showing the connection between volume and pressure at a particular temperature, consists of three parts representing: (i) compression of gas, (ii) liquefaction, (iii) compression of liquid. This is illustrated in Fig.

An examination of Amagat's results in the light of these considerations is instructive. The compressibility of carbon dioxide at 0° C. from 50 to 1000 atmospheres relates entirely to the liquid, and the same applies to the compressibility at 10° C. Carbon dioxide at 20° C. becomes liquid at about 57 atm.; consequently the first value for pv in the third column of the table relates to gaseous carbon dioxide, and the remaining values to the liquid; and the same is true for the values of pv at 30° C., which lies just below the critical temperature. All the other values for pv in the table relate only to gaseous carbon dioxide.

Isothermals of carbon dioxide, showing the derivation of this gas from Boyle's law (Amagat). (If pv = constant, the isothermal is a horizontal straight line.)

These relationships appear clearly from the diagram in which isothermals are drawn, with pressures in atmospheres as abscissae and the values of pv as ordinates (Fig.). If carbon dioxide obeyed Boyle's law these isothermals would be horizontal straight lines. Thus the increasing departure of this substance from the condition of an ideal gas with lowering of temperature is illustrated, as well as the continuity of the gaseous and liquid states, which is especially shownby the parallel lines to the right of the diagram, the last four of which pertain to liquid and the rest to gas. The vertical portions of the curve below 30° C. represent the progress of liquefaction, and so far correspond to the horizontal portions of the curves in the previous figure.

Carbon dioxide is a colourless gas which shows no visible absorption spectrum; two absorption bands occur, however, in the infra-red part of the spectrum, the maximum intensities of which correspond to the wave lengths 2.6 and 4.36μ. The atmosphere shows these lines, and they are shown also by a flame in which carbon dioxide is being produced. Since they represent the absorption of radiant energy, they are the visible expression of the power possessed by carbon dioxide of hindering terrestrial radiation, which, according to Arrhenius, has had a great influence on climate.

Water at atmospheric temperature dissolves about its own volume of carbon dioxide. The solubility coefficient c was determined by Bunsen at different temperatures with the following results:

Temperature ° C.	0°	5°	10°	15°	20°
c	1.7967	1.4497	1.1847	1.0020	0.9014

the general equation being:

c = 1.7967 – 0.07761t + 0.0016424t².

The following values have been obtained more recently by Bohr and Bock (c = vol. per vol. ; g = gram per 100 grams.):

Temp. ° C.	c	g	Temp. ° C.	c	g
0	1.713	0.3347	18	0.928	0.1789
1	1.646	0.3214	19	0.902	0.1736
2	1.584	0.3091	20	0.878	0.1689
3	1.527	0.2979	21	0.854	0.1641
4	1.473	0.2872	22	0.829	0.1591
5	1.424	0.2774	23	0.804	0.1541
6	1.377	0.2681	24	0.781	0.1494
7	1.331	0.2590	25	0.759	0.1459
8	1.282	0.2494	26	0.738	0.1407
9	1.237	0.2404	27	0.718	0.1367
10	1.194	0.2319	28	0.699	0.1328
11	1.154	0.2240	29	0.682	0.1293
12	1.117	0.2166	30	0.665	0.1259
13	1.083	0.2099	35	0.592	0.1106
14	1.050	0.2033	40	0.530	0.0974
15	1.019	0.1971	45	0.479	0.0862
16	0.985	0.1904	50	0.436	0.0762
17	0.956	0.1845	60	0.359	0.0577

At low pressures the solubility of carbon dioxide in water accords with Henry's law; but at high pressures solubility does not keep pace with pressure, so that the ratio S/P diminishes instead of remaining constant. The following results were obtained by Wroblewski:

P. (atm).	S at 0° C.	S/P
1	1.797	1.797
5	8.65	1.730
10	16.03	1.603
15	21.95	1.463
20	26.65	1.332
25	30.55	1.222
30	33.74	1.124

Sander finds, however, that Henry's law is the more nearly followed the higher the temperature, and that at 100° C. the solubility of carbon dioxide in most solvents is proportional to the pressure.

When carbon dioxide has been dissolved in water under pressure - as, for instance, in the manufacture of mineral waters - the excess does not immediately leave the solution upon removal of the excess of pressure. The state of supersaturation is, however, disturbed by particles of dust or a rough surface.

A crystalline hydrate, CO₂.8H₂O, is formed from water at about 0° C. and carbon dioxide at 25 atm., which is permanent until the pressure is reduced to 12.7 atm. The hydrate may, however, be CO₂.6H₂O.

The solubility of carbon dioxide in alcohol is more than twice its solubility in water; it is given by the expression:

c = 4.3294 – 0.09426t + 0.0012354t².

This gas also dissolves in many other organic solvents, in all of which it is distinctly more soluble than in water. The depression of the freezing-point of benzene and acetic acid by dissolved carbon dioxide shows that this substance possesses the normal molecular weight in these solutions. The solubility of carbon dioxide in colloidal liquids and fine suspensions has been investigated by Findlay, Creighton, Shen, and Williams, and the influence of non-electrolytes on the solubility of this gas in water by Usher.