The object
of this experiment is to measure the percentage concentration of the
oxygen in air. Basically a wet piece of steel wool is stuck into
a 100 mL graduated cylinder. The cylinder is turned upside down
and the open end is immersed in a cup of water. The air in the
graduated cylinder is sealed off from the rest of the atmosphere.
The oxygen reacts with the steel wool to form rust and is removed from
the air sample (it turns from a gas and becomes part of the rust, a
solid).
If you simply try to immerse the open end of the cylinder in a cup
of water you would find that the water doesn't enter the
cylinder. Air pressure keeps the water out. You want the
water to enter partway into the cylinder so that the water level can be
read on the cylinder scale.
Note that it isn't that the cylinder is full of air
that
keeps the
water out (as shown above at left), there's actually a lot of empty
space in the cylinder. Rather it is the fact that the air
molecules are moving around inside the cylinder at 100s of miles per
hour and they strike the water molecules with enough force that the
water can't move into the cylinder.
The solution to this problem is to insert a small piece of
flexible tubing into the cylinder as shown above. If you lower
the cylinder into the water while keeping the two ends of the tubing
out of the water, water will enter the cylinder. When the water
level can be read on the scale (ideally between the 90 and 100 ml
marks), the tubing is removed. This seals off the air sample and
the experiment is underway.
You can carefully rest the cylinder against bottom and side of the
cup. Be sure to tell any friends or roommates to leave your
experiment materials alone.
Periodically lift the cylinder just enough to be able to read the
water level. Don't lift the open end of the cylinder out of the
water as this would break the seal and you would need to restart the
experiment (extra pieces of steel wool will be available in class
should this happen). Also make a note of the time.
After some time you will notice that the water level doesn't
change between readings. All of the oxygen in the sample has been
removed and the experiment is over. The figure below shows you
one way of removing the steel wool (which should then be
discarded). Return the materials to class and pick up the
supplementary information handout.
Straighten the paper clip supplied with the experiment
and
then bend
about 2/3 rds of it around the end of a pencil to form a
corkscrew. Attach the corkscrew to the end of the pencil and then
insert it into the cylinder. With a list twisting the corkscrew will
snag the steel wool and you will be able to pull it out of the cylinder
and dispose of it.
