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?
Gas present if when a glowing splint is put inside a test tube of gas it relight?
Gas present if it turns milky/cloudy?
Gas present is damp, blue litmus paper turns bleached
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