gases (QA)

• Three scenarios where we need to test for a gas in a practical setting:

  • When effervescence is observed, it indicates gas production
  • When a smell is detected, suggesting the presence of a gas
  • When instructed in the practical exam to test for a gas
• There are six gases in the syllabus that need to be learned for qualitative analysis
• Ammonia:

  • Colorless but pungent
  • Test: Place a piece of moist red litmus paper at the mouth of the test tube, turning it blue
• Carbon dioxide:

  • Colorless and odorless
  • Test: Bubble the gas into lime water, resulting in a white precipitate of calcium carbonate
• Chlorine:

  • Greenish-yellow with a swimming pool smell
  • Test: Place a moist blue litmus paper at the mouth of the test tube, turning it red and then bleached
• Hydrogen:

  • Colorless and odorless
  • Test: Place a lighter splint at the mouth of the test tube, resulting in a pop sound and extinguished flame
• Oxygen:

  • Colorless and odorless
  • Test: Insert a glowing splint into the test tube, relighting it if oxygen is present
• Sulfur dioxide:

  • Colorless and pungent with a rotten egg smell
  • Test: Place a filter paper soaked with acidified potassium permanganate, turning colorless in the presence of sulfur dioxide
• Deducing which gas to test for based on reagents added:

  • Gray or silvery solid with acid indicates testing for hydrogen gas
  • White solid with acid indicates testing for carbon dioxide gas
• Test for water:

  • Use cobalt chloride paper (blue turning pink) or anhydrous copper sulfate (white turning blue) to confirm the presence of water
• Writing ionic equations for the formation of precipitates:

  • Write the formula of the precipitate
  • List the ions present in the precipitate in their aqueous state