Experiment 7.
Thermal Expansivity by Dilatometry and Pycnometry
The dilatometer and pycnometer used in
this experiment are illustrated here. The dilatometer is in the form of a coiled tube, to
facilitate rapid thermal equilibration, and includes 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.
2
The pycnometry measurements
are carried out over the range 10-40°C
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 measurement for the unknown then involves
(1) equilibrating the sample at a
known T (measured with a thermometer) and recording the height h on the
capillary; and (2) weighing the pycnometer with sample on the analytical
balance. The two measurements give
V and m for the sample, yielding a
precise determination of the density r.
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 0.1°C.
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.
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 ~16°C.
5
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
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.