Test for Gases

Question 1

Hydrogen Gas (H₂)

Observation & Explanation

Answer 1

Observation:

1. Effervescence occurs.
2. Colourless, odourless gas evolved.
3. Gas extinguishes a lighted splint with a ‘pop’ sound.
4. Gas is hydrogen.

Explanation:

• 2H₂(g) + O₂(g) → 2H₂O(l)
• The ‘pop’ sound is due to small explosions which occurs when hydrogen gas burns with oxygen in the air.

Question 2

Oxygen Gas (O₂)

Observation & Explanation

Answer 2

Observation:

1. Effervescence occurs.
2. Colour, odourless gas evolved.
3. Gas relights a glowing split.
4. Gas is oxygen.

Explanation:

• The glowing splint starts to burn again as oxygen gas supports combustion.

Question 3

Carbon Dioxide Gas (CO₂)

Observation & Explanation

Answer 3

Observation:

1. Effervescence occurs.
2. Colourless, odourless gas evolved.
3. Gas forms white precipitate with limewater.
4. Gas is carbon dioxide.

Explanation:

• CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
• Carbon dioxide gas, an acidic oxide, reacts with aqueous calcium hydroxide (limewater), an alkali, to form insoluble calcium carbonate (white precipitate) and water.

Question 4

Ammonia Gas (NH₃)

Observation & Explanation

Answer 4

Observation:

1. Colourless, pungent gas evolved.
2. Gas turns damp red litmus paper blue.
3. Gas is ammonia.

Explanation:

• NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH^-(aq)
• In the presence of water, ammonia partially dissociates to give hydroxde ions, which are responsible for the alkaline property.
• No effervescence as ammonia is highly soluble in water

Question 5

Chlorine Gas (Cl₂)

Observation & Explanation

Answer 5

Observation:

1. Effervescence occurs.
2. Greenish-yellow, pungent gas evolved.
3. Gas turns damp blue litmus paper red, then bleaches it.
4. Gas is chlorine.

Explanation:

• Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq)
• Chlorine reacts with water to form a mixture of hydrochloric acid and hypochlorous acid. In the presence of water, the acids dissociate to give hydrogen ions which turn damp blue litmus paper red. The hypochlorous acid formed will bleach the litmus paper.

Question 6

Sulfur Dioxide Gas (SO₂)

Observation & Explanation

Answer 6

Observation:

1. Effervescence occurs.
2. Colourless, pungent gas evolved.
3. Gas turns acidified potassium manganate(VII) from purple to colourless.
4. Gas is sulfur dioxide.

Explanation:

• Sulfur dioxide is a reducing agent. It reduces purple manganate(VII) ion, MnO₄^-, to colourless manganese(II) ion, Mn²⁺.