Qualitative Analysis (Very High Yield): Gas Tests (H₂, O₂, CO₂, NH₃, Cl₂) (Copy)
Qualitative Analysis (Very High Yield)
Gas Tests (H₂, O₂, CO₂, NH₃, Cl₂)
Importance of Gas Tests in ATP
- Gas tests are among the highest-yield ATP questions
- Examiners assess:
- Correct test selection
- Correct observation wording
- Correct sequence
- Gas identification questions are often:
- Easy full marks
- Lost due to wrong wording
Core Examiner Rule for Gas Tests
- Always write answers in this order:
- Test used
- Observation
- Inference (only if asked)
- Never write the name of the gas as an observation
- Observation must be:
- Visible
- Audible
- Measurable
General Safety Rule (Implicit)
- Gas tests are assumed to be:
- Carried out carefully
- Using small quantities
- Safety explanations are not required unless asked
Test for Hydrogen Gas (H₂)
Test Used
- Lighted splint
Observation
- A squeaky pop sound is heard
Inference
- Hydrogen gas is present
Examiner Notes
- “Pop” alone is acceptable
- “Squeaky pop” is ideal
- Do not write:
- Hydrogen gas burns
- Hydrogen is produced (as observation)
Common Examiner Trap (H₂)
- Writing:
- “Hydrogen gas is produced”
- This is an inference, not an observation
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Test for Oxygen Gas (O₂)
Test Used
- Glowing splint
Observation
- The glowing splint relights
Inference
- Oxygen gas is present
Examiner Notes
- Must use:
- Glowing splint
- Writing:
- “Burns brighter” is not accepted
- Exact phrase preferred:
- “Relights”
Test for Carbon Dioxide Gas (CO₂)
Test Used
- Limewater
Observation
- Limewater turns milky
Inference
- Carbon dioxide gas is present
Examiner Notes
- Must state:
- “Turns milky”
- Writing:
- “Turns cloudy” is not accepted
- Do not write:
- Carbon dioxide reacts with limewater (observation error)
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Test for Ammonia Gas (NH₃)
Tests Used
- Damp red litmus paper
- Smell (supporting only)
Observations
- Damp red litmus paper turns blue
- Gas has a pungent smell
Inference
- Ammonia gas is present
Examiner Notes
- Litmus test is essential
- Smell alone:
- Not sufficient
- Do not write:
- “Ammonia gas is produced” as observation
Test for Chlorine Gas (Cl₂)
Tests Used
- Damp blue litmus paper
Observations
- Damp blue litmus paper turns red
- Then it is bleached white
Inference
- Chlorine gas is present
Examiner Notes
- Both steps must be stated:
- Turns red
- Then bleached
- Writing only “bleaches litmus”:
- Often loses a mark
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Summary Table: Gas Tests (Very High Yield)
| Gas | Test | Observation |
|---|---|---|
| H₂ | Lighted splint | Squeaky pop |
| O₂ | Glowing splint | Splint relights |
| CO₂ | Limewater | Turns milky |
| NH₃ | Damp red litmus | Turns blue |
| Cl₂ | Damp blue litmus | Turns red then bleached |
Acceptable Examiner Observation Vocabulary
- Squeaky pop
- Relights
- Turns milky
- Turns blue
- Pungent smell
- Turns red then bleached
Unacceptable Observation Vocabulary
- Hydrogen gas produced
- Oxygen released
- Carbon dioxide formed
- Ammonia detected
- Chlorine present
Writing Gas Tests in Tables
Correct Table Entry Example
| Test | Observation |
|---|---|
| Lighted splint | Squeaky pop |
Incorrect Table Entry Example
| Test | Observation |
|---|---|
| Lighted splint | Hydrogen gas produced |
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Common Examiner Traps in Gas Test Questions
- Using wrong splint (lighted vs glowing)
- Writing inference instead of observation
- Using vague terms like “burns”
- Missing second step in chlorine test
- Forgetting limewater for CO₂
ATP-Focused Strategy for Gas Test Questions
- Memorise:
- Gas → test → exact observation
- Always:
- Use examiner-approved wording
- Never:
- Name the gas as an observation
- If unsure:
- Describe exactly what happens physically
Core Scientific Principle
- Gas tests rely on:
- Characteristic reactions with simple reagents
- ATP tests:
- Recognition
- Precision of language
- Correct sequencing
- Mastery of gas tests:
- Secures guaranteed ATP marks