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
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 ammoniaExaminer Tip
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 dioxideExaminer Tip
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 whiteExaminer Tip
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' soundExaminer Tip
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 oxygenExaminer Tip
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.
