Making and Testing Gases
Carbon dioxide gas
Making carbon dioxide gas
To make carbon dioxide, you need two ingredients - a
carbonate of some sort (a carbonate has carbon dioxide
trapped in it as part of its structure) like marble chips
(calcium carbonate), and an acid to react with the
carbonate, which lets the carbon dioxide out. We can also
use kitchen chemicals – vinegar (acid) and baking soda
(sodium bicarbonate). Carbon dioxide is heavier than air so
it isn't too hard to trap some carbon dioxide in a test tube.
Chemicals used to make
carbon dioxide gas
Testing for carbon dioxide gas
The easiest way to test carbon dioxide gas is to mix it with some
limewater. Limewater is usually clear. When carbon dioxide is added,
the limewater goes a milky white colour for a short while (if you keep
adding carbon dioxide, the limewater goes back to colourless again so
watch out for that). This test is also known as the 'limewater test'.
Hydrogen gas
Making hydrogen gas
You just need a small piece of metal (tin, zinc or magnesium work
really well) and some acid, such as hydrochloric acid. Put them
carefully together in a test tube and you'll see the liquid fizzing as the
hydrogen gas gets released -the metal disappears as it gets used.
The colour of limewater
for a positive result
Testing for hydrogen gas
Hydrogen gas is explosive so if we bring the gas near a flame, it
makes a squeaky pop. This test is also known as the 'pop test'.
Why is hydrogen gas useful? It burns easily and so makes a good
fuel - liquid hydrogen can mix with liquid oxygen to make rocket fuel.
In fact, hydrogen is the fuels that stars use to produce their heat and
light, although they don't burn it, rather they get much more energy
out of it by making a nuclear reaction called fusion.
Magnesium metal and
hydrochloric acid
Oxygen gas
Making oxygen gas
The easiest way to make oxygen gas in a lab is
to use two ingredients - hydrogen peroxide (a
colourless liquid that looks like water but is much
more reactive), and manganese dioxide. (Note
that the blackish brown powder called
manganese dioxide (MnO2) that is very good at
leaving a messy stain on your equipment!)
The easiest way to capture some oxygen gas is
Chemicals used to make
to use a technique known as "the downward
oxygen gas
displacement of water". The downward
displacement of water means that if your test
tube is upside down and full of water (in a big container of water too) then as the oxygen
floats up into the test tube from the end of your delivery tube, it displaces the water
downwards by pushing it down and out of the way, until all the water is outside in the big
container of water. We also used this technique for the other gases.
Testing for oxygen gas
The easiest way to test oxygen uses the fact that flames burn really well in pure oxygen,
better than they do in air. We light a small, thin piece of wood called a splint, and then we
blow it out so that it's just glowing orange a little, but not actually got any flames left (like an
incense stick). Then we put the glowing end of the splint into our gas sample. If the gas is
pure oxygen, then the oxygen feeds the fire and a flame starts up again or relights. If the
gas sample isn't oxygen, or if your sample has too much air in it, then the glowing orange end
does nothing or goes out completely. This test is also known as the 'glowing splint test'.
Testing for oxygen gas
 Read Year 9 text book, pages 44 to 53, for more information
Uses of Gases
Use textbook to find at least 2 uses
Oxygen
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Carbon dioxide
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Hydrogen
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Summary of Gases
Name of Gas
How we made it
Test for gas &
expected result
Two important ways
the gas can be
used.