Tests for gases

Both of these gases are tested with a wooden splint — but the splint is in a different state for each, and that is the detail examiners check.

Test for OXYGEN (O₂).

• Test: insert a glowing wooden splint (one that has just been blown out and is still smouldering) into a test tube of the gas.
• Positive result: the splint relights — the smouldering ember bursts back into flame.
• Why: oxygen supports combustion. The hot wood already has fuel and heat; only oxygen is missing, so adding oxygen reignites it.

Test for HYDROGEN (H₂).

• Test: insert a lighted (burning) wooden splint at the mouth of the test tube.
• Positive result: the hydrogen burns with a squeaky pop.
• Why: hydrogen burns explosively in air: 2H₂ + O₂ → 2H₂O. The tiny explosion makes the 'pop' sound — no other common gas does this.

The trap: swapping the two splints. Oxygen → glowing splint; hydrogen → lighted splint. Mixing them up loses the mark.

Carbon dioxide is the gas tested with limewater — a solution of calcium hydroxide, Ca(OH)₂(aq).

• Test: bubble the gas through limewater.

• Positive result: the limewater turns milky / cloudy / white.

• Why: carbon dioxide reacts with the calcium hydroxide to form an insoluble white precipitate of calcium carbonate, which makes the limewater cloudy:

  Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Key wording. Examiners accept milky, cloudy or a white precipitate forms. Vague answers such as 'it changes colour' score nothing — you must say milky/cloudy. Do not say it 'turns blue' (that confuses it with a different test).

These two gases are both tested with damp litmus paper held at the mouth of the test tube — but they give opposite-looking results, and that is how you tell them apart.

Test for AMMONIA (NH₃).

• Test: hold a piece of damp red litmus paper at the mouth of the tube. ('Damp' is essential — the gas must dissolve in the water on the paper.)
• Positive result: the red litmus turns blue.
• Why: ammonia is alkaline — it dissolves to give an alkaline solution, which turns red litmus blue. It also has a sharp, pungent smell (waft gently; never sniff directly).

Test for CHLORINE (Cl₂).

• Test: hold a piece of damp litmus paper (or universal indicator paper) at the mouth of the tube.
• Positive result: the litmus is bleached — it turns white. (It may turn red first, because chlorine is acidic, then go white.)
• Why: chlorine forms an acidic, bleaching solution that removes the colour from the paper.

Telling them apart. Ammonia is alkaline → red litmus goes blue (and stays blue). Chlorine is acidic and bleaching → litmus may go red, then is bleached white. The bleaching (loss of colour) is unique to chlorine.

Set out clearly: the gas, the test and the exact positive result. Reproduce this table from memory.

Gas	Test	Positive result
Oxygen (O₂)	Glowing splint	Splint relights
Hydrogen (H₂)	Lighted splint	Squeaky pop
Carbon dioxide (CO₂)	Bubble through limewater	Limewater turns milky
Ammonia (NH₃)	Damp red litmus paper	Turns blue
Chlorine (Cl₂)	Damp litmus paper	Bleached white (red first)

How to revise this fast. For every gas, store two things in your head: the test and the result. In the exam, a question gives you one and asks for the other:

• "What is the test for hydrogen?" → lighted splint → squeaky pop.
• "A gas turns limewater milky. Name it." → carbon dioxide.
• "Which gas turns damp red litmus blue?" → ammonia.

Distinguishing pairs is a favourite question — pick the test that gives different results for the two gases:

• O₂ vs CO₂: glowing splint relights in O₂; in CO₂ the splint goes out (CO₂ does not support combustion).
• H₂ vs CO₂: lighted splint gives a pop with H₂; CO₂ puts the splint out.