Testing for Gases

Aim

To demonstrate and practise the standard chemical tests for five gases commonly produced in IGCSE Chemistry reactions: hydrogen, oxygen, carbon dioxide, ammonia, and chlorine.

Apparatus

• Test tubes
• Test tube rack
• Bunsen burner and splints (wooden)
• Limewater (calcium hydroxide solution)
• Red litmus paper and blue litmus paper
• Wash bottle with distilled water (for dampening litmus paper)
• Delivery tube (for directing gas into limewater)
• Reagents to generate gases:
  • Zinc and dilute hydrochloric acid (for hydrogen)
  • Manganese dioxide and hydrogen peroxide solution (for oxygen)
  • Calcium carbonate and dilute hydrochloric acid (for carbon dioxide)
  • Ammonium chloride and sodium hydroxide solution, warmed (for ammonia)
  • Manganese dioxide and concentrated hydrochloric acid (for chlorine — teacher demonstration only due to toxicity)

Method

Test 1: Hydrogen (H2)

1. Place a few pieces of zinc into a test tube and add dilute hydrochloric acid. Effervescence occurs as hydrogen is produced.
2. Allow the gas to collect for 30 seconds (hydrogen is lighter than air and collects at the top of the tube if held upside down, or escapes from an open tube).
3. Hold a burning splint at the mouth of the test tube.
4. Observation: the hydrogen burns with a squeaky pop sound.

The reaction producing the gas: Zn + 2HCl → ZnCl2 + H2

Test 2: Oxygen (O2)

1. Pour approximately 5 cm3 of hydrogen peroxide solution into a test tube.
2. Add a small amount of manganese dioxide (catalyst) to the hydrogen peroxide. Vigorous effervescence occurs.
3. Light a splint, blow it out so it is glowing (not flaming), and insert the glowing splint into the test tube.
4. Observation: the glowing splint relights, bursting back into flame.

The reaction: 2H2O2 → 2H2O + O2 (MnO2 is the catalyst).

Test 3: Carbon dioxide (CO2)

1. Place a few small pieces of calcium carbonate (marble chips) into a test tube.
2. Add dilute hydrochloric acid. Effervescence occurs.
3. Use a delivery tube to pass the gas into a separate test tube containing limewater.
4. Observation: the limewater turns milky (cloudy/white).

The reaction: CaCO3 + 2HCl → CaCl2 + H2O + CO2 The limewater test: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l). The white precipitate of calcium carbonate causes the milkiness.

Test 4: Ammonia (NH3)

1. Place a small amount of ammonium chloride in a test tube. Add sodium hydroxide solution.
2. Warm the mixture gently.
3. Hold a piece of damp red litmus paper at the mouth of the test tube (do not insert it into the liquid).
4. Observation: the damp red litmus paper turns blue.

The reaction: NH4Cl + NaOH → NaCl + H2O + NH3 Ammonia is the only common alkaline gas, which is why it turns red litmus blue.

Test 5: Chlorine (Cl2) — teacher demonstration

1. This test is carried out as a teacher demonstration because chlorine is toxic.
2. Chlorine gas is produced and directed towards damp litmus paper (can be either red or blue).
3. Observation: the damp litmus paper is bleached (turns white). Blue litmus may first turn red (because chlorine is acidic in water) before being bleached.

Chlorine reacts with water to form hydrochloric acid and hypochlorous acid (a bleaching agent): Cl2 + H2O → HCl + HOCl.

Expected results

Gas	Test	Positive result
Hydrogen	Burning splint	Burns with a squeaky pop
Oxygen	Glowing splint	Splint relights
Carbon dioxide	Limewater	Turns milky/cloudy
Ammonia	Damp red litmus paper	Turns blue
Chlorine	Damp litmus paper	Bleaches (turns white)

Analysis

Each test exploits a unique chemical property of the gas:

• Hydrogen is flammable and burns explosively in small quantities (the pop is a micro-explosion of H2 with O2 in the air).
• Oxygen supports combustion, so it allows a glowing splint (which has hot carbon) to reignite.
• Carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate — the white solid suspended in the liquid appears milky.
• Ammonia is alkaline in water (forming NH4OH/NH3.H2O), and alkalis turn red litmus blue.
• Chlorine is a powerful oxidising agent that destroys the dye molecules in litmus paper (bleaching).

Sources of error

• Testing too early: If you test before enough gas has collected, the concentration may be too low for a clear result. Allow the reaction to proceed for at least 30 seconds.
• Wet splint: A wet splint does not glow properly and may not relight in oxygen. Ensure the splint is dry.
• Excess CO2 through limewater: If too much CO2 is passed through limewater, the milkiness disappears as soluble calcium hydrogen carbonate forms: CaCO3 + CO2 + H2O → Ca(HCO3)2. This can confuse results — stop once milkiness is confirmed.
• Dry litmus paper for ammonia and chlorine: The litmus paper must be damp. Ammonia and chlorine need water to form the ions that interact with the indicator. Dry litmus paper gives no result.

Exam tips

These five gas tests appear on virtually every IGCSE Chemistry paper. Memorise them as five distinct pairs: gas name plus its unique observation. The most common error is confusing the hydrogen test (burning splint, squeaky pop) with the oxygen test (glowing splint, relights). Remember: hydrogen is flammable so you use a flame; oxygen supports combustion so you use a glow.

When describing a test in an exam, state exactly what you do and what you observe. “Test with a splint” alone is insufficient — specify whether the splint is burning or glowing. “Limewater goes white” is insufficient — say “turns milky” or “turns cloudy.”

For ammonia, emphasise that it is the only common alkaline gas. If a gas turns damp red litmus blue, it must be ammonia. For chlorine, emphasise the bleaching — turning litmus white, not just changing its colour.