Page 40 - Essentials of physical chemistry

2 Essentials of Physical Chemistry

FIGURE 1.1 Sir Robert Boyle (1627–1691). An English natural philosopher and chemist, known mainly
for careful experiments on gases. He is recognized as the first modern chemist. He was the 14th child of Richard
of Cork, an Englishman, but Robert was sent to live with an Irish family at an early age and learned the
separate Irish language as a child. He later owned land in Ireland but came to Oxford University for his
research. He was a founding member of the Royal Society of London. (Courtesy of the Chemical Heritage
Foundation. The Shannon Portrait of the Hon. Robert Boyle, F. R. S. oil on canvas by Johann Kerseboom,
British, 1689. Photograph by Will Brown.)

equations. In 1660, Sir Robert Boyle studied gases in Ireland. Much of early scientific research was
conducted only by wealthy individuals who had time and funds to conduct special experiments,
many of which seem simple today but were of considerable innovation at the time. Consider that
today we can routinely purchase glass tubing of uniform bore from several chemical supply
companies. However, Boyle’s experiments depended on using the height difference of mercury in
a glass ‘‘J-tube’’ so that the volume ‘‘V ’’ he wanted to study was related to the height of the mercury
column ‘‘h’’ by the equation

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V ¼ pr h

Here we see in Figure 1.2 that by measuring ‘‘h’’ and assuming a uniform radius for the tubing, the
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data can be related to the volume ‘‘V ’’ of the trapped gas sample. Note that the essential r part of
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the formula would provide cm while ‘‘h’’ provides ‘‘cm’’ to produce a volume unit of cm . Thus,
measuring the one-dimensional height difference can give a volume only if there is a proportionality
with a uniform bore of the tubing whether the units are in cubic inches or cubic centimeters. In
Figure 1.2, the same ruler can be used to measure the volume of the trapped gas (assuming a flat top
of the gas chamber) and the difference in height of the mercury levels. While this is only a schematic
diagram of Boyle’s apparatus, we have captured the key features.
To properly appreciate this seemingly simple device can you imagine how you would melt sand
to make glass and draw it into a piece of tubing with uniform bore? By adding more and more liquid
mercury to the open end of the right side of the J-tube, the air trapped on the closed left side of the
apparatus was gradually compressed. Because of the uniform bore of the tubing, the volume V of
the air in the closed end can be calculated by the height of the air bubble space while the difference
in the height of the mercury in the left and right sides gives the pressure P applied to the gas
expressed in inches of mercury.