Chapter 25: Tests for Gases, Anions and Cations

25.1-Tests for Gases and Anions

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

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

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

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

Tests for anions

Method:

• dissolve a small sample of the solid salt you are testing in water.
• place approximately 10cm3 of the solution into a test tube.
• add four drops of nitric acid.
• add silver nitrate solution, dropwise.
• if a precipitate is produced, observe the colour.

25.2-Tests for Cations

Tests for Cations

• Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide and ammonia
• If only a small amount of NaOH is used then normally the metal hydroxide precipitates

Analysing results

• If a precipitate is formed from NaOH then the hydroxide is insoluble in water

Method:

• Add a few drops of dilute sodium hydroxide (NaOH()aq) into an unknown solution and observe any changes.
  • We can test further by adding excess NaOH()aq and recording any changes.
• Add a few drops of dilute ammonium hydroxide (NHOH()4aq) into an unknown solution and observe any changes.
  • We can test further by adding excess NHOH()4aq and recording any changes

Practice questions

• Gas present if there’s a pop sound?

  • Oxygen

• Gas present if when a glowing splint is put inside a test tube of gas it relight?

  • Hydrogen

• Gas present if it turns milky/cloudy?

  • Carbon dioxide

• Gas present is damp, blue litmus paper turns bleached

  • Chlorine