Thermodynamics and Chemistry

CHAPTER 2 SYSTEMS AND THEIR PROPERTIES

2.3 SOMEBASICPROPERTIES ANDTHEIRMEASUREMENT 38

Table 2.2 Representative measurement methods

Physical

Method

Typical Approximate

quantity value uncertainty

Mass analytical balance 100 g 0:1mg

microbalance 20 mg 0:1 ùg

Volume pipet, Class A 10 mL 0:02mL

volumetric flask, Class A 1 L 0:3mL

Density pycnometer, 25-mL capacity 1 g mL^12 mg mL^1

magnetic float densimeter 1 g mL^1 0:1mg mL^1

vibrating-tube densimeter 1 g mL^1 0:01mg mL^1

Pressure mercury manometer or barometer 760 Torr 0:001Torr

diaphragm gauge 100 Torr 1 Torr

Temperature constant-volume gas thermometer 10 K 0:001K

mercury-in-glass thermometer 300 K 0:01K

platinum resistance thermometer 300 K 0:0001K

monochromatic optical pyrometer 1300 K 0:03K

2.3.3 Density

Density, an intensive property, is defined as the ratio of the two extensive properties mass
and volume:

def
D

m

V

(2.3.3)

The molar volumeVmof a homogeneous pure substance is inversely proportional to its
density. From Eqs.2.1.2,2.3.1, and2.3.3, we obtain the relation

VmD

M



(2.3.4)

Various methods are available for determining the density of a phase, many of them
based on the measurement of the mass of a fixed volume or on a buoyancy technique.
Three examples are shown in Fig.2.4on the next page. Similar apparatus may be used for
gases. The density of a solid may be determined from the volume of a nonreacting liquid
(e.g., mercury) displaced by a known mass of the solid, or from the loss of weight due to
buoyancy when the solid is suspended by a thread in a liquid of known density.

2.3.4 Pressure

Pressure is a force per unit area. Specifically, it is the normal component of stress exerted
by an isotropic fluid on a surface element.^4 The surface can be an interface surface between
the fluid and another phase, or an imaginary dividing plane within the fluid.
Pressure is usually a positive quantity. Because cohesive forces exist in a liquid, it may
be possible to place the liquid under tension and create anegativepressure. For instance,
the pressure is negative at the top of a column of liquid mercury suspended below the closed

(^4) A liquid crystal and a polar liquid in a electric field are examples of fluids that are not isotropic—that is, that
have different macroscopic properties in different directions.