Thermodynamics and Chemistry

CHAPTER 2 SYSTEMS AND THEIR PROPERTIES

2.3 SOMEBASICPROPERTIES ANDTHEIRMEASUREMENT 43

H 2 O(g)

H 2 O(l)

H 2 O(s)

H 2 O(l)

thermometer bulb

Figure 2.5 Cross-section of a water triple-point cell. The cell has cylindrical sym-

metry. Pure water of the same isotopic composition as H 2 O in ocean water is distilled

into the cell. The air is pumped out and the cell is sealed. A freezing mixture is placed

in the inner well to cause a thin layer of ice to form next to the inner wall. The freezing

mixture is removed, and some of the ice is allowed to melt to a film of very pure water

between the ice and inner wall. The thermometer bulb is placed in the inner well as

shown, together with ice water (not shown) for good thermal contact.

h

gas

Hg

Figure 2.6 Simple version of a constant-volume gas thermometer.

different systems: a reference system of known temperatureT 1 (such as one of the systems
listed in Table2.3), and the system whose temperatureT 2 is to be measured. The pressures
p 1 andp 2 are measured at these temperatures. In the two equilibrations the amount of gas
is the same and the gas volume is the same except for a small change due to effects ofT
andpon the bulb dimensions.
If the gas exactly obeyed the ideal gas equation in both measurements, we would have
nRDp 1 V 1 =T 1 Dp 2 V 2 =T 2 orT 2 DT 1 .p 2 V 2 =p 1 V 1 /. Since, however, the gas approaches