Testing for Hydrogen Gas

Section 1 of 7The test

1

Collect a sample of the gas in a test tube, held upside down if the gas is lighter than air, and keep your thumb or a bung over the opening.

2

Light a wooden splint.

3

Bring the burning splint to the mouth of the test tube, removing the bung or thumb.

4

Do not push the splint deep inside the tube, because hydrogen needs oxygen from the air to burn.

5

Listen for the result: hydrogen burns with a loud squeaky pop.

Section 2 of 7Why it pops

1Flame meets hydrogen

• Flame meets hydrogen Hydrogen is flammable and ignites at the open end of the tube.

2Mixes with air

• Mixes with air It combines rapidly with oxygen in the air in a small, quick combustion.

3Water is made

• Water is made 2H₂(g) + O₂(g) → 2H₂O(g)

4Sound heard

• Sound heard The rapid burning of the small volume of gas makes the pop.

Section 3 of 7Where the test is used

Alkali metals and water

• Alkali metals and water Sodium or potassium reacting with water gives fizzing.
  • Hydrogen is identified by the squeaky pop.

Metals with acids

• Metals with acids Magnesium with hydrochloric acid gives hydrogen and a salt. Collect the gas and test it.

Safety

• Safety Use small volumes, wear eye protection and keep flames away from the reaction vessel, as hydrogen is explosive with air.

Section 4 of 7Properties of hydrogen that explain the test

Very low density

• Very low density Hydrogen is the least dense gas, much lighter than air.
  • It escapes quickly from an open tube, so the splint should be applied immediately after the bung is removed.
• A tube is therefore collected upside down so that the gas stays trapped against the closed end.

Flammable

• Flammable Hydrogen reacts readily with oxygen and releases a lot of energy.
  • A pure sample in a tube burns more quietly than a mixture with air, which is why a mixture gives the sharper pop.
• A positive test needs both hydrogen and oxygen, so a tube full of hydrogen with no air at its mouth will not pop.

Interpreting a negative result

• Interpreting a negative result If the splint simply goes out with no pop, there is probably no hydrogen,
  • or too little gas was collected,
  • or the sample escaped before testing.
• A conclusion should always say what was seen: 'the lit splint made a squeaky pop, so the gas is hydrogen'.

Section 5 of 7Gas tests compared

Gas	Test	Positive result
Hydrogen	Lit splint at the mouth of the tube	Squeaky pop
Oxygen	Glowing splint in the gas	Splint relights
Carbon dioxide	Bubble through limewater	Limewater turns cloudy
Chlorine	Damp blue litmus paper	Paper bleached white

Section 6 of 7Key terms

Term	Meaning
Squeaky pop	The sound made when hydrogen burns rapidly with oxygen.
Lit splint	A burning wooden splint used to test for hydrogen.
Glowing splint	A splint with a glowing end but no flame, used to test for oxygen.
Combustion	Burning, a reaction with oxygen that releases energy.

Section 7 of 7Exam technique

1

Confusing the splint tests. A lit splint is for hydrogen, and a glowing splint is for oxygen.

2

Pushing the splint far into the tube so the flame is starved of air.

3

Describing the result as just an explosion instead of a squeaky pop.

4

Forgetting that the product of the burning is water.