In this lesson, we will learn how to collect
gases based on their density. Have you ever wondered why some balloons float
and some don’t? This is due to the density of the gas inside
the balloon. Balloons that float contain helium, which
is less dense than air. Balloons that we blow up ourselves contain
mostly carbon dioxide, which is denser than air.
Therefore, these balloons fall to the ground. From the introduction, we already know that
a gas less dense than air rises. These gases are collected using a method called
upward delivery. The collection vessel is inverted so that
the gas is able to displace the air inside it as it rises upwards.
Air is pushed downwards, so this method is also called downward displacement.
Hydrogen gas and ammonia are collected using this method.
Recall the example of the balloon falling to the ground in the introduction.
Gases denser than air are collected by downward delivery.
The collection vessel is placed upright so the denser gas collected is able to displace
the air inside it as it sinks. Air is pushed upwards, so this method is also
called upward displacement Carbon dioxide and chlorine gas are collected
using this method. If we are unsure of the density of a gas,
we can collect it over water. Since all gases are less dense than water,
displacement of water is possible. The gas produced is bubbled through water
and collected in a gas jar. Note that this method does not work for gases
that are soluble in water, such as ammonia. What do you think is the best method to collect
hydrogen chloride? Let’s pause the lesson momentarily to think
about this. Hydrogen chloride dissolves in water to produce
hydrochloric acid, so collecting over water is not possible.
Since it is denser than air, it can be collected by downward delivery.
This brings us to the next topic and addresses a very important question.
What if you are unsure of both the density and solubility of the gas produced?
A method to collect a gas irrespective of these properties is by using a gas syringe.
It is a closed container so the density of the gas does not need to be taken into consideration.
A gas syringe is also used when the volume of a gas produced needs to be measured. In summary, gases less dense than air are
collected by upward delivery. Downward delivery is used to collect gases
denser than air. Gases can also be collected over water, but
this method only works for gases that are insoluble in water.
A gas syringe can be used to collect all gases and is useful for measuring the volume of