Why do we need tests for gases?
Most gases are colourless. Hydrogen, oxygen and carbon dioxide all look like plain air. So we cannot tell them apart by looking.
A gas test is a quick reaction that gives a clear sign: a sound, a flame, a colour change or a cloudy liquid. Each common gas has its own test.
A good test is specific: only that gas gives the result. We collect the gas in a test tube, then do the test at once, before the gas escapes.
Test for hydrogen and test for oxygen
Hydrogen: the squeaky pop
Hold a lit splint (a burning wooden stick) at the mouth of the tube. Hydrogen burns very fast with the oxygen in air and you hear a squeaky pop. The product is water: 2H₂ + O₂ → 2H₂O.
Oxygen: the glowing splint
Light a splint, then blow it out so only a red glowing tip is left. Put it into the tube. If the gas is oxygen, the splint relights (bursts into flame). Oxygen supports burning.
Do not mix them up: lit splint for hydrogen, glowing splint for oxygen.
Test for carbon dioxide
Bubble the gas through limewater (calcium hydroxide solution). If it is carbon dioxide, the clear limewater turns milky (cloudy).
Why? Tiny bits of white, insoluble calcium carbonate form: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
A lit splint goes out in carbon dioxide, but that is not a good test: nitrogen and many other gases also put out flames. Only limewater is the proper test.
Test for chlorine, ammonia and other gases
Chlorine
Chlorine is a pale green-yellow, toxic gas with a strong smell. Hold damp blue litmus paper in it. The paper turns red (chlorine is acidic) and then white: chlorine bleaches it. The paper must be damp. Always work in a fume cupboard.
Ammonia
Ammonia has a sharp smell. It turns damp red litmus paper blue, because it is an alkaline gas. Damp universal indicator paper goes blue/purple.
More gases you may meet
- Water vapour: turns white anhydrous copper(II) sulfate blue, or blue cobalt chloride paper pink.
- Sulfur dioxide: turns acidified potassium manganate(VII) solution from purple to colourless.
Try it: safe tests at home and in the 3D
In the 3D: go to the last step. Choose a gas and a test. Say your prediction out loud, then press Test. Try the wrong test on purpose, for example a lit splint in CO₂, and see why it does not prove anything.
At home (with an adult): put a spoon of baking soda in a glass and add vinegar. The fizz is carbon dioxide. Hold a lit candle stub just above the glass: it goes out, because CO₂ is heavier than air and pushes the air away.
Key formulas and definitions
- Hydrogen: lit splint → squeaky pop (2H₂ + O₂ → 2H₂O)
- Oxygen: glowing splint → relights
- Carbon dioxide: limewater → milky (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O)
- Chlorine: damp blue litmus → red → white (bleached)
- Ammonia: damp red litmus → blue
- Water vapour: anhydrous CuSO₄ white → blue
- Sulfur dioxide: acidified KMnO₄ purple → colourless
Worked examples
1. Zinc is added to dilute hydrochloric acid and a gas bubbles off. Name the gas and give its test and result.
Metal + acid → salt + hydrogen. Test: hold a lit splint at the mouth of the tube. Result: a squeaky pop.
2. Marble chips (calcium carbonate) react with acid. A student holds a lit splint in the gas and it goes out. She says the gas is carbon dioxide. Is her test good enough?
No. Many gases (for example nitrogen) put out a flame. She must bubble the gas through limewater: it turns milky only with carbon dioxide.
3. Hydrogen peroxide breaks down with a catalyst and gives a gas that relights a glowing splint. Name the gas and write the word equation.
Oxygen. Hydrogen peroxide → water + oxygen (2H₂O₂ → 2H₂O + O₂).
4. A gas turns damp blue litmus red and then white. A second gas turns damp red litmus blue. Name both.
The first bleaches litmus, so it is chlorine. The second is alkaline, so it is ammonia.
Common mistakes
- Using a glowing splint for hydrogen or a lit splint for oxygen. Hydrogen: lit splint, pop. Oxygen: glowing splint, relights.
- Saying "a lit splint goes out, so it is CO₂". Many gases put out flames; only limewater turning milky proves CO₂.
- Using dry litmus paper for chlorine or ammonia. The paper must be damp, because the gas must dissolve in water to act.
- Writing "chlorine turns litmus red" and stopping. The key result is that it is bleached white.