Lab #10
GAS LAWS
In 1660, Robert Boyle, an English philosopher, performed an experiment in which he
showed that the volume of a trapped amount of air varied inversely with the pressure applied to
it. However, the relationship between the volume of a trapped amount of air and its temperature
was not noted at the same time since the accurate thermometers required to make this
observation were not constructed until approximately 1750. It was some time after 1780 that the
French scientist Jacques Charles made the observation that the volume and temperature of a gas
are directly proportional; or “at a constant pressure, the volume of a fixed mass of gas is directly
proportional to its temperature expressed in K.” In today’s lab, you will investigate both of these
Gas Laws.
For the Charles’ Law Experiment, you will be given a piece of capillary tubing permanently
sealed at one end and containing a moveable plug of mercury at the other end. This plug traps a
fixed volume of air and moves up and down as the air expands or contracts with a change in
temperature. A plot of the volume of the gas versus the various temperatures, when properly
extrapolated, will yield the temperature at which the air should have zero volume.
For the Boyle’s Law Experiment, you will be using a specially designed apparatus, that will
allow you to quickly and easily measure volume and gas pressure. The apparatus will be
connected to the faucet aspirator, thus lowering the pressure of a trapped air sample. The
aspirator will then be turned off and the pressure increased by small increments, and the new
volume of the trapped air sample will be successively measured.
PROCEDURE:
Part A: Charles’ Law:
Obtain a capillary tube, thermometer and a ruler from your instructor. The capillary will
have a small plug of mercury trapped in the upper end. Be sure to never turn the capillary
upside-down since there is a possibility that the mercury may fall out. Mercury is poisonous and
may be absorbed through the skin. In case an accident should happen, contact your lab instructor
at once.
The capillary and the thermometer should be attached to the metric ruler by means of a
rubber band or twist tie. Before making any measurements, and several times during the course
of the experiment, check to see that the bottom inside of the capillary is exactly on the “zero
line” of the ruler. The same capillary must be used for the entire experiment in order to get
consistent results.
Make a mixture of ice and water in a 600 mL beaker. Place the capillary and ruler into the
water bath so that the entire capillary up to the mercury plug is immersed. Be careful not to get
water into the top of the capillary. Wait several minutes for the tube to reach the temperature of
the bath. Record the height of the mercury column to the nearest 0.1 mm and the temperature of
the bath to the nearest 0.1°C. Repeat this procedure over the temperature range 0°C to 100°C
using water baths at about 10°C intervals. Start with the highest temperature reading then add
cold tap water or ice to lower the temperature. After room temperature has been reached, ice can
be added to get a lower reading. Record the temperature of the water and the height of the
column of air for each determination.
