Testing for Gases | Chemistry

📖 In-Depth Theory

Tests for Hydrogen and Oxygen

HYDROGEN (H₂):

Test: hold a LIT SPLINT near the mouth of a test tube containing the gas.

Positive result: SQUEAKY POP sound — the hydrogen ignites and burns rapidly with the oxygen in the air.

Why: H₂ + O₂ → H₂O (explosive combustion).

Hydrogen is the lightest gas — produces a very fast, audible pop.

OXYGEN (O₂):

Test: hold a GLOWING SPLINT near the mouth of a test tube containing the gas.

Positive result: the glowing splint RELIGHTS — bursts back into flame.

Why: oxygen supports combustion — a glowing ember in oxygen reignites and burns vigorously.

Note: a lit splint would simply burn faster in oxygen — the glowing splint test is more distinctive.

Remember the difference: LIT splint for H₂ (squeaky pop). GLOWING splint for O₂ (relights).

Tests for Carbon Dioxide and Chlorine

CARBON DIOXIDE (CO₂):

Test: bubble the gas through LIMEWATER (calcium hydroxide solution, Ca(OH)₂).

Positive result: limewater turns MILKY (cloudy/white precipitate forms).

Why: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Calcium carbonate (CaCO₃) is insoluble — it forms as a white precipitate, making the solution appear milky.

If excess CO₂ is bubbled through: the precipitate dissolves again → solution clears:

CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble calcium hydrogencarbonate).

CHLORINE (Cl₂):

Test: hold DAMP LITMUS PAPER (or damp universal indicator paper) near the gas.

Positive result: damp litmus paper is BLEACHED — turns white (loses its colour).

Why: chlorine dissolves in water to form hydrochloric acid and hypochlorous acid:

Cl₂ + H₂O → HCl + HClO

Hypochlorous acid (HClO) is a powerful bleaching agent — it destroys the dye molecules in litmus.

Note: the paper must be DAMP — chlorine needs water to react and bleach.

Summary and Common Confusions

QUICK REFERENCE TABLE:

HYDROGEN (H₂):

Test: lit splint.

Result: squeaky pop.

OXYGEN (O₂):

Test: glowing splint.

Result: relights.

CARBON DIOXIDE (CO₂):

Test: bubble through limewater.

Result: turns milky (white precipitate of CaCO₃).

CHLORINE (Cl₂):

Test: damp litmus paper.

Result: bleaches (turns white).

COMMON CONFUSIONS:

H₂ uses a LIT splint — if you use a glowing splint, it might not ignite the hydrogen reliably.

O₂ uses a GLOWING splint — a lit splint would just burn faster (less distinctive test).

CO₂ test requires LIMEWATER specifically — water doesn't turn milky with CO₂.

Cl₂ requires DAMP litmus — dry litmus paper does NOT bleach (chlorine needs water to form the bleaching agent).

⚠️ Common Mistake

Students confuse the splint tests. LIT splint → H₂ (squeaky pop). GLOWING splint → O₂ (relights). The litmus paper for chlorine MUST be DAMP — dry litmus paper will NOT bleach. Also: limewater turns milky with CO₂, not just any acidic gas.

📐 Key Equations

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (CO₂ test — limewater turns milky)

Cl₂ + H₂O → HCl + HClO (chlorine dissolves to form bleaching acid)

📌 Key Note

H₂: lit splint → squeaky pop. O₂: glowing splint → relights. CO₂: bubble through limewater → turns milky (CaCO₃ precipitate). Cl₂: damp litmus paper → bleaches white. All are simple, reliable tests for these four common gases.

🎯 Matching Activity — Gas Test Match

Match each gas to its test and expected result. — drag the symbols on the right to match the component names on the left.

Hydrogen (H₂)

Drop here

Oxygen (O₂)

Drop here

Carbon dioxide (CO₂)

Drop here

Chlorine (Cl₂)

Drop here

Glowing splint → relights as oxygen supports combustion

Bubble through limewater → turns milky (white CaCO₃ precipitate)

Damp litmus paper → bleaches white

Lit splint → squeaky pop as hydrogen ignites in air

🎯 Test Yourself

Question 1 of 2

1. A student collects a gas and holds a glowing splint near it. The splint relights. What gas is present?

2. Why must the litmus paper be damp when testing for chlorine?

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🧪 Testing for Gases

Tests for Hydrogen and Oxygen

Tests for Carbon Dioxide and Chlorine

Summary and Common Confusions

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