In this lesson, we will learn how to test
for the presence of hydrogen, oxygen, carbon
dioxide, ammonia, and chlorine.
We test for gases in the laboratory because
it is nearly impossible to determine the identity
of a gas just by its appearance.
As an example, hydrogen, oxygen, and carbon
dioxide are all colourless and odourless.
How would we be able to determine which is
which?
To test for hydrogen, place a lit splint at
the mouth of the reaction vessel.
You should hear a very distinctive �squeaky
pop�, which confirms its presence.
This is due to the combustion reaction of
hydrogen in the presence of oxygen, creating
water as the only product.
To test for oxygen, place a glowing splint
at the mouth of the reaction vessel.
Keep in mind that the splint should be �glowing�,
not lit.
A glowing splint relights in the presence
of oxygen.
Why does hydrogen burn with a �squeaky�
pop?
Why does oxygen relight the glowing splint?
Hint: recall some properties of hydrogen and
oxygen.
Please pause the lesson to think about this
and resume once you are done.
Hydrogen is highly flammable and the pop sound
that you hear is actually a mini-explosion.
The glowing splint relights in the presence
of oxygen as there is a higher concentration
of oxygen in the reaction vessel than compared
with air, which is only 21% oxygen.
Carbon dioxide will extinguish a lit splint,
but the same occurs in the presence of ammonia.
A more accurate test is to bubble carbon dioxide
though limewater, which is an aqueous solution
of calcium hydroxide, also known as slaked
lime.
Limewater turns milky in the presence of carbon
dioxide due to the formation of calcium carbonate.
Ammonia extinguishes a lit split, and turns
damp red litmus paper blue.
In the presence of concentrated hydrochloric
acid, a white smoke will form.
This is ammonium chloride, and confirms the
presence of ammonia.
Chlorine turns damp blue litmus paper red
and eventually bleaches it to white.
In conclusion, hydrogen burns with a �squeaky�
pop, oxygen relights a glowing splint, and
carbon dioxide turns limewater milky.
Ammonia turns damp red litmus paper blue and
forms a white smoke of ammonium chloride in
the presence of concentrated hydrochloric
acid.
Chlorine turns damp blue litmus paper red
and continues to bleach it white.
