Qualitative analysis

Question 1

Describe test and observations for H2 gas

Answer 1

• Test gas with a lighted splint
• Colourless, odourless gas evolved extinguishes lighted splint with ‘pop’ sound
• Gas is hydrogen

Question 2

Describe test and observations for O2 gas

Answer 2

• Test gas with a glowing splint
• Colourless, odourless gas evolved relights glowing splint
• Gas is oxygen

Question 3

Describe test and observations for CO2 gas

Answer 3

• Test gas by bubbling it into limewater (Calcium hydroxide)
• Colourless, odourless gas evolved forms white precipitate in limewater
• Gas is carbon dioxide

Question 4

Describe test and observations for NH3 gas

Answer 4

• Test gas with damp red litmus paper
• Colourless, pungent gas evolved turns damp red litmus paper blue
• Gas is ammonia

Question 5

Describe test and observations for Cl2 gas

Answer 5

• Test gas with damp blue litmus paper
• Greenish-yellow, pungent gas evolved turns damp blue litmus paper red, then bleaches it
• Gas is chlorine

Question 6

Describe test and observations for SO2 gas

Answer 6

• Test gas with filter paper soaked in acidified KMnO4
• Colourless, pungent gas evolved turns acidified KMnO4 from purple to colourless
• Gas is sulfur dioxide

Question 7

Why does precipitate form during tests for cations

Answer 7

• Cations in test solution react with hydroxide ions in aqueous sodium hydroxide or aqueous ammonia to form an insoluble metal hydroxide, which appears as precipitate

Question 8

Why do some precipitates dissolve in excess aqueous sodium hydroxide or aqueous ammonia

Answer 8

• Precipitates can react further with excess alkali to form a soluble complex salt

Question 9

How to differentiate between solutions containing lead or aluminium

Answer 9

Add aqueous solution containing iodide/chloride/sulfate ions
- If aluminium present, no visible change
- If lead present, precipitate seen

Question 10

Why is aqueous ammonia not used to test for ammonium ions

Answer 10

• Upon heating, aqueous ammonia will produce ammonia gas
• Presence of ammonia gas cannot be used to confirm presence of ammonium

Question 11

Describe test and observations for carbonate ions

Answer 11

• Add dilute acids
• Effervescence occurs
• Colourless, odourless gas evolved forms white precipitate in limewater

Question 12

Describe test and observations for nitrate ions

Answer 12

• Add aqueous sodium hydroxide
• Add aluminium foil
• Warm
• Colourless, pungent gas evolved turns damp red litmus paper blue

Question 12

Describe test and observations for sulfate ions

Answer 12

• Add dilute nitric acid
• Add aqueous barium nitrate
• White precipitate seen

Question 13

Describe test and observations for chloride ions

Answer 13

• Add dilute nitric acid
• Add aqueous silver nitrate
• White precipitate seen

Question 13

Describe test and observations for iodide ions

Answer 13

• Add dilute nitric acid
• Add aqueous silver nitrate
• Pale yellow precipitate seen

Question 14

Why is there initial effervescence in test for nitrate ions

Answer 14

• H2 gas produced in a side reaction between aqueous sodium hydroxide and aluminium

Question 15

Why must dilute nitric acid be added before test reagent in tests for sulfate/chloride/iodide ions

Answer 15

• Removes any carbonate ion
• Carbonate ions will also react with test reagent to form precipitate

Question 16

Define thermal decomposition

Answer 16

Thermal decomposition is a chemical process where heat is used to break down a single compound into 2 or more substances

Question 17

What forms when nitrates decompose

Answer 17

• NO2 and O2 formed
  Except: K and Na form metal nitrate and O2

Question 18

What forms when carbonates decompose

Answer 18

• Metal oxide and CO2 gas formed
  Except: K and Na do not decompose