Chemical Tests
Inorganic Chemistry
Tests for Gases
Why these five gases matter
- Many cation, anion and water tests release a gas that then has to be identified
- You are expected 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 |
Things to be careful about
- 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"
Common exam question
Describing the test for chlorine
Question: Describe a test for chlorine gas, or name the gas that bleaches damp litmus paper (1–2 marks).
Set in 3 of the 23 papers, twice as a two-mark test to describe and once as a one-mark name-the-gas. One mark is for holding damp blue litmus paper in the gas (universal indicator paper is accepted too) and one for the paper being bleached, or turning white; saying it turns red first and is then bleached is also credited. In one scheme the second mark depends on the first, so name the paper.
Do not start from red litmus paper. One mark scheme scored an answer built on red litmus (turning blue, then bleaching) at one mark only. Blue paper, then bleached: that is the whole answer.
Common exam question
Giving the test for hydrogen or for oxygen
Question: Give the test for hydrogen, or give the test for oxygen (1 mark).
One of these one-mark tests appears in 9 of the 23 papers (hydrogen in 5, oxygen in 5, with one paper asking both), usually tacked onto an electrolysis, a metal-and-acid reaction or the decomposition of hydrogen peroxide. Hydrogen: a lighted splint gives a squeaky pop. Oxygen: a glowing splint relights.
The single mark needs both the splint and the result, and the state of the splint is what the examiners check. Three mark schemes reject a glowing splint for hydrogen outright, and two ignore "burns with a squeaky pop" on its own because it never mentions the lighted splint. Lit for hydrogen, glowing for oxygen.