A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Experiments and applications
CSCA Chemistry · Lesson 19 of 21Preparing and testing gases
Préparation et identification des gaz · تحضير الغازات والكشف عنها
▶ Watch the video lesson (A7LAM students)
For each common gas, know the reaction that makes it, how to collect it and the test that proves it.
Key ideas
- O₂: 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂↑ (heat); relights a glowing splint.
- CO₂: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑; turns limewater milky.
- H₂: Zn + H₂SO₄ → ZnSO₄ + H₂↑; burns with a squeaky pop.
- NH₃: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O (heat); turns damp red litmus blue.
Formulas to know
- Over water: gases that do not dissolve (O₂, H₂)
- Upward delivery: gases denser than air (CO₂, Cl₂)
- Downward delivery: gases lighter than air (H₂, NH₃)
Worked example
A gas turns limewater milky and does not burn. What is it, and how is it made in the lab?
- Test: milky limewater → CO₂
- Reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
- Collect: upward delivery, CO₂ is denser than air
Answer: CO₂, from marble and dilute HCl
Practice questions
1. Which gas relights a glowing splint?
H₂ · CO₂ · O₂ ✓ · N₂
Oxygen supports burning.
2. NH₃ is not collected over water because it:
dissolves very easily in water ✓ · is denser than air · is toxic · burns
It is extremely soluble.
3. Why not use dilute H₂SO₄ with marble to make CO₂?
It is too strong · CaSO₄ coats the marble and stops the reaction ✓ · It gives SO₂ · It is too expensive
CaSO₄ is only slightly soluble.