The apparatus shown in the diagram
is used to measure atmospheric pressure. Which of the following occupies
region A of the test tube? (a) Air, (b) mercury vapor, (c)
glass, (d) vacuum.
The apparatus in the diagram is
from the Torricelli experiment, what we would call today a barometer. It has a dish on the bottom that is
filled with mercury that is open to the atmospheric pressure and a test tube in the
middle that is also filled with mercury. When the test tube is first placed
into the dish, it is filled with mercury all the way to the top. The mercury then flows out of the
test tube and into the dish below, leaving the space that we see in our diagram
between the top of the mercury and the test tube. Because the test tube was full
before and now it’s not, that means that there is a vacuum left in this space,
leading us to answer choice (d). Vacuum is what occupies region A of
the test tube.