Analytical Methods - Gas Tests (SQA National 5 Chemistry): Revision Note
Exam code: X813 75
Gas tests
In many chemical reactions, a gas is produced
The three most common gases to test for are:
Oxygen
Hydrogen
Carbon dioxide
Test for oxygen (O2)
Oxygen gas is produced during the decomposition of hydrogen peroxide:
hydrogen peroxide → water + oxygen
To test for oxygen:
Place a glowing splint is placed inside the test tube of gas
If the gas is oxygen, the glowing splint will relight
Test for hydrogen (H2)
Hydrogen gas is often produced when an acid reacts with a reactive metal
For example:
magnesium + hydrochloric acid → magnesium chloride + hydrogen
To test for hydrogen:
A lit (burning) splint is held at the mouth of the test tube containing the gas
If the gas is hydrogen, it will burn with a "squeaky pop" sound
Examiner Tips and Tricks
Examiners often report that students confuse the tests for hydrogen and oxygen. Make sure you learn the difference:
Oxygen:
You are testing if the gas will support combustion
A glowing splint relights
Hydrogen:
You are testing if the gas itself will burn
A lit splint causes a "squeaky pop"
Test for carbon dioxide (CO2)
Carbon dioxide gas is often produced when an acid reacts with a metal carbonate
For example:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
To test for hydrogen:
The gas is bubbled through limewater (an aqueous solution of calcium hydroxide).
If the gas is carbon dioxide, the limewater will turn cloudy or milky.
Examiner Tips and Tricks
Do not write "it extinguishes a lit splint" as the test for carbon dioxide
While CO2 will do this, so will many other non-flammable gases
This means that it is not a unique test
The only correct test is with bubbling through limewater
Unlock more, it's free!
Did this page help you?
