Gas Identification Tests

Ammonia is the only common laboratory gas that is alkaline. The standard test is to hold a piece of damp red litmus paper at the mouth of the test tube. Ammonia dissolves in the water on the paper to form ammonium hydroxide (NH₄OH), making the solution alkaline and turning the red litmus blue. A secondary confirmatory test is to hold a glass rod dipped in concentrated hydrochloric acid near the gas: ammonia reacts with HCl vapour to produce dense white fumes of ammonium chloride (NH₄Cl). Ammonia has a pungent, characteristic odour, but smell alone is never sufficient evidence in a formal test.

Carbon dioxide is identified by bubbling the gas through limewater (a dilute aqueous solution of calcium hydroxide, Ca(OH)₂). Carbon dioxide reacts with calcium hydroxide to produce insoluble calcium carbonate (CaCO₃), which appears as a milky white precipitate, causing the limewater to turn cloudy. If excess CO₂ is passed through, the precipitate redissolves as soluble calcium hydrogencarbonate (Ca(HCO₃)₂) forms, and the solution clears again. This reversibility is a useful additional observation. CO₂ also turns damp blue litmus paper red, confirming its acidic nature.

Chlorine is a yellow-green toxic gas with a sharp, bleach-like odour. It is identified by holding damp blue litmus paper — or damp universal indicator paper — at the mouth of the container. Chlorine dissolves in water to form a mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl). The acidic solution first turns the litmus red, but HOCl is a powerful bleaching agent (oxidising agent) that rapidly decolourises the paper to white. The two-stage observation — red then bleached — is the definitive positive result. This test must always be performed in a fume cupboard.

Hydrogen is identified by the 'squeaky pop' test. A burning splint is held at the mouth of the test tube containing the gas. Hydrogen ignites and burns rapidly with a small pale-blue flame, and if the tube contains a mixture of hydrogen and air, the ignition produces a characteristic high-pitched 'squeaky pop' sound. The sound arises from the rapid, near-explosive combustion of the hydrogen–air mixture. The product of combustion is water vapour (H₂O), which may be seen as condensation on the inside of the tube.

Oxygen is identified using a glowing splint test. A wooden splint is lit, allowed to flame, then blown out so it continues to glow (smoulder). When the glowing splint is inserted into a tube of oxygen, it relights vigorously with a bright flame. This happens because oxygen is the oxidising gas required to support combustion; a higher concentration accelerates the oxidation of the carbon in the wood. A fully lit splint simply continues burning in air but burns more brightly in pure oxygen. The test confirms the presence of an oxidising gas.

Sulfur dioxide is a toxic, colourless gas with a sharp, suffocating smell reminiscent of burning matches. It is identified by bubbling or exposing it to acidified potassium manganate(VII) solution (acidified KMnO₄, which is purple). Sulfur dioxide acts as a reducing agent, reducing the Mn⁷⁺ ions in permanganate to Mn²⁺, causing the purple solution to decolourise to a pale pink or colourless solution. A secondary test involves damp blue litmus paper, which turns red because SO₂ dissolves to form sulfurous acid (H₂SO₃). This test must be performed in a fume cupboard.

Water vapour is identified using anhydrous cobalt(II) chloride paper (CoCl₂). Anhydrous CoCl₂ is blue; when water molecules coordinate to the cobalt ion, it forms the hydrated complex [Co(H₂O)₆]²⁺, which is pink. Therefore, blue cobalt chloride paper turns pink in the presence of water vapour. Alternatively, anhydrous copper(II) sulfate (CuSO₄) can be used: the white anhydrous powder turns blue when it absorbs water to form the blue hydrated salt CuSO₄·5H₂O. Both tests confirm the presence of water; to confirm that the water is pure, its boiling point (100 °C) can subsequently be measured.

• \ce{Ca(OH)_2} — Calcium hydroxide (limewater) (aqueous): The reagent used in the test; a dilute alkaline solution.
• \ce{CO_2} — Carbon dioxide (gas): The gas being tested; produced in many acid–carbonate and combustion reactions.
• \ce{CaCO_3} — Calcium carbonate (solid): Insoluble white precipitate; its formation causes the cloudiness observed.
• \ce{H_2O} — Water (liquid): Co-product of the neutralisation reaction.

Problem. A student heats a mixture of zinc metal and dilute sulfuric acid. A colourless, odourless gas is collected in a test tube. The student applies a burning splint to the mouth of the tube and hears a high-pitched squeaky pop. (a) Identify the gas. (b) Write the ionic equation for its formation. (c) Write the equation for its combustion.

Answer. The gas is hydrogen (H₂). It is formed by the displacement of hydrogen ions from sulfuric acid by zinc metal, and it burns in oxygen to form water vapour.

• Acid–Base Indicators — Several gas tests (ammonia, chlorine, carbon dioxide, sulfur dioxide) rely on litmus or universal indicator paper, making an understanding of how pH indicators work essential for interpreting results correctly.
• Precipitation Reactions — The limewater test for CO₂ and the formation of CaCO₃ are classic precipitation reactions; understanding ionic equations and solubility rules underpins interpretation of this test.
• Redox Reactions — The tests for hydrogen (combustion), oxygen (supporting combustion), chlorine (bleaching via oxidation), and sulfur dioxide (reducing KMnO₄) all involve electron transfer and are best understood through oxidation states and redox principles.
• Collection of Gases in the Laboratory — Before a gas can be tested, it must be collected using upward or downward displacement of air, or over water. The collection method depends on the gas's density and solubility, directly linking to the properties studied in gas tests.