Identification of Gases (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note
Identification of gases
Several tests for anions and cations produce gases which then need to be tested
The gases included in the syllabus are:
Ammonia
Carbon dioxide
Chlorine
Hydrogen
Oxygen
Sulfur dioxide
Test for ammonia
Ammonia is a gas with a characteristic sharp choking smell that turns damp red litmus paper blue
Hold the litmus paper near the mouth of the test tube, but be careful to avoid touching the sides of the test tube
If you are testing for ammonia produced from ammonium ions and sodium hydroxide, avoiding touching the sides to prevent traces of sodium hydroxide from also turning the red litmus paper blue
Testing for ammonia gas
Damp red litmus paper turns blue in the presence of ammonia
Examiner Tips and Tricks
Make sure you understand the difference between ammonium and ammonia.
Ammonium refers to the aqueous cation, NH4+
Ammonia refers to the gas, NH3.
Test for carbon dioxide
The test for carbon dioxide involves bubbling the gas through an aqueous solution of limewater (calcium hydoxide)
If the gas is carbon dioxide, the limewater turns cloudy white
Testing for carbon dioxide gas
Limewater turns cloudy white in the presence of carbon dioxide
Examiner Tips and Tricks
Sometimes students think that extinguishing a burning splint indicates carbon dioxide gas.
However, while it is a property of carbon dioxide, other gases, such as nitrogen, will also do this.
So, the test is not definitive and should not be given as an exam answer.
Test for chlorine gas
The test for chlorine makes use of litmus paper
If chlorine gas is present, damp blue litmus paper will turn red and then be bleached white
It turns red initially as acids are produced when chlorine comes into contact with water
Chlorine also has a characteristic sharp, choking smell
Chlorine should always be handled in a fume cupboard due to its toxicity
Testing for chlorine gas
Chlorine bleaches damp blue litmus paper white
Examiner Tips and Tricks
You should distinguish between properties of gases and tests for gases. Chlorine 'smells like swimming pools' is a characteristic, but it is not an acceptable means of identification.
Test for hydrogen gas
The test for hydrogen consists of holding a burning splint at the open end of a test tube of gas
If the gas is hydrogen it burns with a loud “squeaky pop” which is the result of the rapid combustion of hydrogen with oxygen to produce water
Be sure not to insert the splint right into the tube, just at the mouth, as the gas needs air to burn
Testing for hydrogen gas
A burning splint gives a 'squeaky pop' sound
Examiner Tips and Tricks
It is easy to confuse the tests for hydrogen and oxygen.
Try to remember that a ligHted splint has an H for Hydrogen, while a glOwing splint has an O for Oxygen.
Test for oxygen
The test for oxygen consists of placing a glowing splint inside a test tube of gas
If the gas is oxygen, the splint will relight
Testing for oxygen gas
A glowing splint will relight in the presence of oxygen
Examiner Tips and Tricks
Sometimes the splint does not relight, but it glows very brightly, which is also a positive result. In an exam, however, it is best to state it relights the glowing splint.
Test for sulfur dioxide
Sulfur dioxide is a gas with a characteristic sharp choking smell
The test for sulfur dioxide involves bubbling the gas through an acidified solution of potassium manganate(VII)
If the gas is sulfur dioxide, the potassium manganate(VII) changes from purple to colourless
This is similar to the test for the sulfite ion
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