Tests for Gases — Chemical Tests | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Ion tests, flame tests and gas tests all appear regularly; expect to recall observations.

Many cation, anion and water tests release a gas that then has to be identified
The IGCSE syllabus expects students to recognise the test, the positive result, and the name of the gas in each case
The five gases assessed are ammonia, carbon dioxide, chlorine, hydrogen and oxygen

Gas	Test	Positive result
Ammonia, NH₃	Hold a piece of damp red litmus paper at the mouth of the tube	The paper turns blue
Carbon dioxide, CO₂	Bubble the gas through limewater (Ca(OH)₂(aq))	The limewater turns milky (cloudy white)
Chlorine, Cl₂	Hold a piece of damp blue litmus paper at the mouth of the tube	The paper turns red and is then bleached white
Hydrogen, H₂	Hold a lit splint at the mouth of the tube	A loud squeaky pop sound (hydrogen burns rapidly with the oxygen in the air)
Oxygen, O₂	Hold a glowing splint inside the mouth of the tube	The splint relights

For ammonia: keep the litmus paper just at the mouth of the tube and clear of the inner walls, so any alkali residue on the glass does not give a false positive
For carbon dioxide: a burning splint does go out in CO₂, but it also goes out in nitrogen and many other gases, so the splint test is not a valid identification — only the limewater test is accepted
For chlorine: the paper goes red first because chlorine reacts with the moisture to form an acid, then bleaches as the chlorine attacks the dye; do the test in a fume cupboard because chlorine is toxic
For hydrogen vs oxygen: a ligHted splint identifies Hydrogen; a glOwing splint identifies Oxygen — the H and O hidden inside each word avoid the common mix-up
For oxygen: a "much brighter glow" usually counts as a positive result, but in an exam stick to the textbook answer "relights the glowing splint"