Tests for Common Gases

Hydrogen is the least dense gas and is flammable, so its test uses a small flame. Hold a lit (burning) splint at the open end of a test tube containing the gas.

If hydrogen is present, it burns rapidly with a squeaky pop. The pop is the sound of the hydrogen reacting quickly with oxygen in the air to make water.

$2H_2(g)+O_2(g)\to 2H_{2}O(l)$

Feature of the test	Detail
Reagent / tool	Lit (burning) splint
Where	At the open mouth of the test tube
Positive result	Squeaky pop

The word "pop" is what the mark scheme looks for. Do not just say "the splint goes out" or "it burns" — the specific squeaky pop is the identifying observation for hydrogen.

The pop happens because a small volume of hydrogen mixed with air ignites almost instantly when the flame reaches it, releasing energy as a sharp sound.

Oxygen does not burn itself, but it supports combustion, so its test uses a splint that is already glowing rather than fully alight. Light a splint, blow it out so it still glows red, then insert the glowing splint into the test tube.

If oxygen is present, the extra oxygen lets the smouldering wood burn again, so the glowing splint relights (bursts back into flame).

Feature of the test	Detail
Reagent / tool	Glowing splint
Positive result	The splint relights

The contrast with the hydrogen test is important and often examined:

Gas	Splint used	Result
Hydrogen	Lit (burning) splint	Squeaky pop
Oxygen	Glowing splint	Splint relights

Hydrogen is itself the fuel that burns; oxygen is not flammable but makes other things burn better. That is why hydrogen needs a lit splint and oxygen needs a glowing one.

Carbon dioxide is tested with limewater, which is calcium hydroxide solution. Bubble the gas through the limewater, usually by shaking the gas with limewater in a tube or passing it through with a delivery tube.

If carbon dioxide is present, the limewater turns milky (cloudy). The cloudiness is caused by tiny particles of insoluble calcium carbonate forming.

$Ca(OH{)}_2(aq)+C{O}_2(g)\to CaC{O}_3(s)+H_{2}O(l)$

The white calcium carbonate is a fine, insoluble solid (a suspension), and it is these particles that make the clear limewater look milky.

Feature of the test	Detail
Reagent	Limewater (calcium hydroxide solution)
Positive result	Limewater turns milky / cloudy
Cause of the change	Insoluble calcium carbonate forms

"Milky" and "cloudy" both gain the mark. Saying the limewater "turns white" is weaker; describe it as going milky or cloudy.

Chlorine is a toxic, pale green gas that acts as a bleach. Its test uses damp litmus paper held in the gas. The paper must be damp so the chlorine can dissolve into the moisture and act on the dye.

Chlorine bleaches the damp litmus paper and turns it white. Blue litmus may first turn red because chlorine solution is acidic, but the identifying result is that the colour is removed and the paper goes white.

Feature of the test	Detail
Reagent / tool	Damp litmus paper
Positive result	Litmus is bleached and turns white
Why damp	Chlorine must dissolve in the moisture to react with the dye

The key word is bleached. If you only write "turns red", you have described an acid, not the chlorine test. The full observation is that the litmus is bleached white.

Bringing the four tests together makes them easy to revise and compare. Each has its own reagent and its own distinctive result.

Gas	Test	Positive result
Hydrogen (H₂)	Lit splint at the mouth of the tube	Squeaky pop
Oxygen (O₂)	Glowing splint into the tube	Splint relights
Carbon dioxide (CO₂)	Bubble through limewater	Limewater turns milky (cloudy)
Chlorine (Cl₂)	Damp litmus paper	Bleached, turns white

A useful way to sort them: hydrogen and oxygen both use a splint (the difference is lit vs glowing), while carbon dioxide and chlorine both use a chemical indicator (limewater vs damp litmus). Learning them in these pairs helps avoid mixing up the results under pressure.

1. Swapping the lit and glowing splints

Hydrogen uses a lit splint and gives a pop; oxygen uses a glowing splint and relights it. Swapping which splint goes with which gas is one of the most common errors.

2. Describing hydrogen as "the splint goes out"

A lit splint dipped into a gas that puts flames out (like carbon dioxide) goes out, but that is not the hydrogen test. Hydrogen gives a distinct squeaky pop.

3. Saying limewater "goes white" instead of milky

The mark-scheme wording for carbon dioxide is that limewater turns milky or cloudy. Describe the change as milky/cloudy, caused by insoluble calcium carbonate.

4. Forgetting the litmus must be damp for chlorine

Chlorine only bleaches litmus when the paper is damp, because it needs to dissolve in the moisture first. Dry paper will not give the test.

5. Writing "turns red" for chlorine and stopping there

Chlorine can turn litmus red first because it is acidic, but the identifying result is that the litmus is bleached white. Leaving out "bleached" loses the mark.