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 10-40°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 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 .

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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.

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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.

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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.