-------------+-----------------+--------------+-------------------- | | | Temperature. | Pressure in cm. | Temperature. | Pressure in cm. | mercury. | | mercury. -------------+-----------------+--------------+-------------------- | | | -10° | 0.213 | 120° | 148.4 0° | 0.458[29] | 130° | 201.9 +20° | 1.752 | 150° | 356.8 40° | 5.516 | 200° | 1162.5 60° | 14.932 | 250° | 2973.4 80° | 35.54 | 270° | 4110.1 100° | 76.00 | 364.3° | 14790.4 (194.6 atm.) | | (critical | (critical pressure). | | temperature) | -------------+-----------------+--------------+-------------------- The pressure is, of course, independent of the relative or absolute volumes of the liquid and vapour; on increasing the volume at constant temperature, a certain amount of the liquid will pass into vapour, and the pressure will regain its former value. If, however, the pressure be permanently maintained at a value different from that corresponding to the temperature employed, then either all the liquid will pass into vapour, or all the vapour will pass into liquid, and we shall have either vapour alone or liquid alone. Upper Limit of Vaporization Curve.--On continuing to add heat to water contained in a closed vessel, the pressure of the vapour will gradually increase. Since with increase of pressure the density of the vapour must increase, and since with rise of temperature the density of the liquid must decrease, a point will be reached at which the density of liquid and vapour become identical; the system ceases to be heterogeneous, and passes into one homogeneous phase. The temperature at which this occurs is called the _critical temperature_. To this temperature there will, of course, correspond a certain definite pressure, called the _critical pressure_. The curve representing the {24} equilibrium between liquid and vapour must, therefore, end abruptly at the critical point. At temperatures above this point no pressure, however great, can cause the formation of the liquid phase; at temperatures above the critical point the vapour becomes a gas. In the case of water, the critical temperature is 364.3°, and the critical pressure 194.6 atm.; at the point representing these conditions the vapour-pressure curve of water must cease.