Qualitative Tests for Gases

 General Observation  

- Bubbles often indicate the presence of a gas during a reaction.  

Safety Note: Always wear gloves and work in a fume cupboard when testing gases.  

Properties of Common Gases  

- Less dense than air: Hydrogen, Ammonia  

- More dense than air: Carbon dioxide, Chlorine, Oxygen (slightly)  

- Solubility in water:  

  - Hydrogen → Very low  

  - Oxygen → Low  

  - Carbon dioxide → Low  

  - Ammonia → Very high  

  - Chlorine → Moderate  

Collection Tip:  

- Less dense gases rise → collect in an inverted test tube.  

- Denser gases sink → collect in an upright test tube.  

Specific Gas Tests  

- Hydrogen:  

  - Test: Light a wooden splint → produces a squeaky pop sound.  

- Oxygen:  

  - Test: A glowing splint relights in its presence.  

- Carbon Dioxide:  

  - Test: Bubble through limewater (Ca(OH)₂ solution) → turns cloudy white due to precipitation of calcium carbonate.  

- Ammonia:  

  - Properties: Strong choking smell.  

  - Test: Turns damp red litmus paper blue.  

  - Additional: Forms white smoke of ammonium chloride when exposed to hydrogen chloride gas.  

  - Confirmatory: Heating NaOH with aluminium metal liberates ammonia if nitrate ions are present.  

- Chlorine:  

  - Properties: Strong choking smell.  

  - Test: Turns damp blue litmus paper red, then bleaches white.  

  - Additional: Turns damp starch iodide paper blue‑black.  

- Sulfur Dioxide (SO₂):  

  - Properties: Smell of rotten eggs.  

  - Test: Potassium dichromate paper changes from orange to green.  

  - Also: Turns damp blue litmus paper red.  

- Nitrogen Dioxide (NO₂):  

  - Properties: Pale brown gas visible against a white background.  

  - Test: Build‑up in a sealed test tube causes sides to turn pale brown; releasing the bung shows the gas clearly.  

Practical Notes  

- Always dampen litmus paper with distilled water before use.  

- Ensure litmus paper does not touch the test tube walls directly.