Thermodynamics and Chemistry

CHAPTER 2 SYSTEMS AND THEIR PROPERTIES

2.3 SOMEBASICPROPERTIES ANDTHEIRMEASUREMENT 39

(a)

C

D

B

S

(b) (c)

Figure 2.4 Three methods for measuring liquid density by comparison with samples

of known density. The liquid is indicated by gray shading.

(a) Glass pycnometer vessel with capillary stopper. The filled pycnometer is brought

to the desired temperature in a thermostat bath, dried, and weighed.

(b) Magnetic float densimeter.a BuoyB, containing a magnet, is pulled down and

kept in position with solenoidSby means of position detectorDand servo control

systemC. The solenoid current required depends on the liquid density.

(c) Vibrating-tube densimeter. The ends of a liquid-filled metalU-tube are clamped to

a stationary block. An oscillating magnetic field at the tip of the tube is used to make it

vibrate in the direction perpendicular to the page. The measured resonance frequency

is a function of the mass of the liquid in the tube.

aRef. [ 69 ].

end of a capillary tube that has no vapor bubble. Negative pressure in a liquid is an unstable
condition that can result in spontaneous vaporization.
The SI unit of pressure is thepascal. Its symbol is Pa. One pascal is a force of one
newton per square meter (Table1.2).
Chemists are accustomed to using the non-SI units of millimeters of mercury, torr, and
atmosphere. One millimeter of mercury (symbol mmHg) is the pressure exerted by a col-
umn exactly 1 mm high of a fluid of density equal to exactly13:5951g cm^3 (the density
of mercury at 0 C) in a place where the acceleration of free fall has its standard valuegn
(see AppendixB). One atmosphere is defined as exactly1:01325 105 Pa (Table1.3). The
torr is defined by letting one atmosphere equal exactly 760 Torr. One atmosphere is approx-
imately 760 mmHg. In other words, the millimeter of mercury and the torr are practically
identical; they differ from one another by less than 2  10 ^7 Torr.
Another non-SI pressure unit is thebar, equal to exactly 105 Pa. A pressure of one
bar is approximately one percent smaller than one atmosphere. This book often refers to a
standard pressure,p. In the past, the value ofpwas usually taken to be 1 atm, but since
1982 the IUPAC has recommended the valuepD 1 bar.
A variety of manometers and other devices is available to measure the pressure of a
fluid, each type useful in a particular pressure range. Some devices measure the pressure of
the fluid directly. Others measure the differential pressure between the fluid and the atmo-
sphere; the fluid pressure is obtained by combining this measurement with the atmospheric
pressure measured with a barometer.
Within asolid, we cannot define pressure simply as a force per unit area. Macroscopic