OCR Gateway GCSE Chemistry

Revision Notes

4.2.1 Testing for Common Gases

Test Yourself

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

Test for Carbon Dioxide, IGCSE & GCSE Chemistry revision notes

Diagram showing the test for carbon dioxide gas

Exam Tip

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

& GCSE Chemistry revision notes

Diagram showing the test for hydrogen gas

Exam 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!

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

Test for Oxygen, IGCSE & GCSE Chemistry revision notes

Diagram showing the test for oxygen gas

Exam 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 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

Test for Chlorine, IGCSE & GCSE Chemistry revision notes

Diagram showing the test for chlorine gas

Exam Tip

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'.

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