Video Transcript
Fill in the blank. During the preparation of nitrogen
gas from atmospheric air in a laboratory, nitrogen is collected over blank to keep
the nitrogen dry.
In this question, we want to know
what substance nitrogen is collected over in order to keep it dry in the isolation
process. In the process of isolating
nitrogen gas from atmospheric air, we begin with water and progressively move
along. Atmospheric air consists primarily
of nitrogen, about 78 percent, then about 21 percent oxygen, about 0.04 percent
carbon dioxide, less than one percent mixture of other gases, such as argon, and
some water vapor.
The content of water vapor differs
according to the humidity of the location, but on average it is about one
percent. The atmospheric air is bubbled
through water, then through a solution of caustic soda, also known as sodium
hydroxide. In this second container, a
reaction occurs between the sodium hydroxide and carbon dioxide from the air. This reaction essentially removes
carbon dioxide from the air mixture.
The remaining gas mixture is then
passed through concentrated sulfuric acid in a third container, which dehydrates the
air mixture and removes any water vapor. What remains of the air mixture
then passes into a horizontal open-ended glass tube where the gases pass over a
small pile of strongly heated copper turnings. Oxygen is removed from the gas
mixture as it reacts with the heated copper turnings. The remaining gas, which is almost
pure nitrogen, is collected over mercury in order to keep it dry within the gas
cylinder.
Mercury is suitable for this
purpose as water vapor from the air is not soluble in the mercury. Mercury essentially forms a
physical barrier to water vapor in the air, preventing it from entering the
collected, dried, nitrogen gas.
Finally, during the preparation of
nitrogen gas from atmospheric air in a laboratory, nitrogen is collected over
mercury to keep the nitrogen dry. So, the correct answer is
mercury.
