Text 22 A Factors Affecting the Boiling Point - раздел Образование, Part One THE PREDICATE The Temperature At Which A Liquid Boils, Is Dependent Not Only Upon The Natur...
The temperature at which a liquid boils, is dependent not only upon the nature of the liquid itself, but also upon the pressure prevailing at the time the boiling point is determined. An increase in the atmospheric pressure causcs a rise in the boiling point of a liquid; a dccrcasc results in a lowering of the boiling point.
Each individual substancc capable of existing in the liquid state has its own definite boiling point. Consequently, this property is used for identifying or characterizing a substancc in the same way that density and other physical properties arc employed.
Inasmuch as the boiling point of a substancc is affcctcd by changes in pressure, it has been found ncccssary to set up a standard pressure at which all boiling points can be compared. A pressure of 760 mm Hg has been selcctcd as the standard. Thus, if the boiling point of water is given as 100° C, the measurement is understood to have been made at a pressure
of 760 mm Hg. Nevertheless, it is quite true to say that the boiling temperature of water is 70°C at a pressure of 233.8 mm Hg; it is also correct to say that carbon tetrachloride boils at a temperature of 70°C, but in this ease the pressure is 620 mm Hg. Boiling temperature should not therefore be used for distinguishing one substancc from another unless the values used have been determined at the same pressure. Unless otherwise stated[8], a boiling point value is understood to be the temperature at which a substancc boils under a pressure of 760 mm Hg.
One atmosphere (standard atmospheric pressure) is equal to 101.325 kNnr2. It is not an approved SI (International System) unit of pressure, but it is especially important in chcmistry bccausc many properties of substanccs have been measured and recorded at 1-atm pressure. An approved SI unit of pressure is the pascal (1 Pa = 1 H/m2).
Another unit of pressure that has been much used in the past is the torr, which is the height in millimetres of a column of mercury that balances the pressure. The symbol mmHg is sometimes used for torr. The name of the unit is taken from the name of the Italian physicist Evangclista Torricelli (1609-1647), known for having invented the mercury barometer. One atmosphere is equal to 760 torr.
