Testing for Common Gases (OCR GCSE Chemistry A (Gateway)): Revision Note
Carbon Dioxide
The test for carbon dioxide involves bubbling the gas through an aqueous solution of calcium hydroxide (limewater)
If the gas is carbon dioxide, the limewater turns milky or cloudy
Diagram showing the test for carbon dioxide gas
Examiner Tips and Tricks
Sometimes students write 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 quoted in an exam answer. All is shows is that there is a lack of sufficient oxygen present to keep the flame going!
Hydrogen
The test for hydrogen is often referred to as the 'squeaky pop' test
The test for hydrogen consists of holding a burning splint held at the open end of a test tube of gas
If the gas is hydrogen it burns with a loud “pop” sound 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
Diagram showing the test for hydrogen gas
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!
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
Diagram showing the test for oxygen gas
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 that the oxygen gas relights the glowing splint.
Chlorine
The test for chlorine makes use of litmus paper
If chlorine gas is present, damp blue litmus paper will be bleached white
It may turn red briefly before bleaching, as acids are produced when chlorine comes into contact with water
Chlorine should always be handled in a fume cupboard due to its toxicity
Diagram showing the test for chlorine gas
Examiner Tips and Tricks
You should distinguish between properties of gases and tests for gases. Chlorine 'smells like swimming pools' is a characteristic, but not an acceptable means of identification. You can use blue litmus, red litmus or universal indicator paper to show the bleaching effect of chlorine gas.
The best response to write is still that 'chlorine gas bleaches damp litmus paper'.
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?