Experiment 7.

1
Experiment 7. Thermal Expansivity by
Dilatometry and Pycnometry
The dilatometer and pycnometer used in this
experiment are illustrated here. The
dila-tometer is in the form of a coiled tube, to
facilitate rapid thermal equilibration, and in-
cludes a capillary extension having a ruled scale
for precise measurement of small volume changes.
The pycnometer has been modified from the usual
Weld design it, too, has a capillary extension.
This makes its volume variable, but in a
calculable way, since the capillary extension has
a precisely known diameter. The extension
permits one to achieve equilibrium from either
below or above (in T), whereas the usual design
requires equilibration from below. Both devices
have approximate volume 25 mL.
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The pycnometry measurements are carried out over
the range 10-40C using a thermostatted bath to
achieve the desired temperatures for the sample.
At the beginning and again at the end of the
period, the volume of the pycnometer is
determined using water as a known. Each
measure-ment for the unknown then involves (1)
equilibrating the sample at a known T (measured
with a thermom-eter) and recording the height h
on the capillary and (2) weighing the
pyc-nometer with sample on the analytical
balance. The two measurements give V and m for
the sample, yielding a precise determination of
the density r.
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3
The dilatometry measurements employ a large bell
jar type beaker full of water as thermal bath.
The bell jar is mounted on a stir plate that must
be used only to operate the stir bar, NEVER to
heat. Heating is accomplished with an immersion
heater connected to a Variac. (This way heating
starts and stops quickly when the power is turned
on and off.) The temperature is monitored with a
thermometer graduated in units 0.1C. These
components are all shown in more detail in a
picture on the following page. The bath water is
cooled initially by adding ice. Then
measurements are done working up in temperature
by heating with the immersion heater.
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4
The dilatometer and thermometer should be
mounted such that they are both at least an inch
or so from the wall of the bell jar, so that
their temperatures will be close to that of the
bath. (Since the jar is not insulated, the
temperature of the glass will be intermediate
between those of the air and of the water at the
center of the bath.) The immersion heater should
be similarly mounted, so that it delivers its
heat to the water and not to the glass wall. The
movies on the following page show what happens
when the heater is turned on for about a minute
at 80 power (120V), for a starting T of 16C.
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The initial slow rise in both T and h is due to
the slow warming of the bath in air. Note that
after the heater is turned off, there is a lag
time of 2 minutes before T and h return to a
slow rate of rise comparable to that at the
start. You must wait until then to record your
values of T and h.