Properties and uses of common non-metals including oxygen, nitrogen, chlorine, sulfur, carbon, and hydrogen; laboratory preparation, collection, drying, and testing of oxygen, carbon dioxide, and ammonia.
Non-metals occupy the right-hand side of the periodic table. Many are gases at room temperature; their chemistry is dominated by covalent bonding. The ability to prepare, collect, dry, and identify specific gases is a core laboratory skill tested in both Paper 02 and the SBA.
Unlike metals, non-metals typically:
| Property | Non-metals |
|---|---|
| Electrical conductivity | Poor conductors (except graphite) |
| Thermal conductivity | Poor |
| Physical state | Many are gases at room temperature; some are solids (C, S, P); Br₂ is a liquid |
| Melting and boiling points | Generally low (molecular structures) |
| Lustre | Dull (not shiny) |
| Malleability | Brittle if solid |
| Bonding | Covalent (in molecules or giant covalent structures) |
Two main allotropes: diamond (hardest natural substance, insulator) and graphite (soft, conductor). Uses of carbon: graphite for electrodes and pencils, diamond for cutting tools, carbon black as a reinforcing filler in tyres. Also exists as fullerenes (e.g. buckyballs) and carbon nanotubes.
Several of the non-metals above are prepared and tested in standard lab practicals. For each gas, the syllabus expects you to know the preparation reaction, collection method, drying agent, and identification test.
Preparation: decomposition of hydrogen peroxide using manganese(IV) oxide as a catalyst.
Collection: upward displacement of air (oxygen is denser than air), or over water since oxygen is only slightly soluble.
Drying: pass through anhydrous calcium chloride or silica gel (NOT concentrated H₂SO₄ with excess H₂O₂).
Test: glowing splint relights.
Preparation: dilute hydrochloric acid reacted with marble chips (calcium carbonate). Dilute HCl is preferred because CaSO₄ forms an insoluble layer that stops the reaction if H₂SO₄ is used.
Collection: upward displacement of air (CO₂ is denser than air). Cannot be collected over water — CO₂ dissolves.
Drying: concentrated sulfuric acid (NOT anhydrous CaCl₂, which reacts with CO₂).
Test: turns limewater milky:
Preparation: heating an ammonium salt with a base (e.g. ammonium chloride with calcium hydroxide).
Collection: downward displacement of air (ammonia is less dense than air). Cannot be collected over water — ammonia is extremely soluble.
Drying: through calcium oxide (CaO) — NOT concentrated H₂SO₄ (it would react with NH₃) and NOT anhydrous CaCl₂ (forms a complex with NH₃).
Test: turns damp red litmus paper blue.
The three drying agents and why they cannot be used for certain gases are a favourite exam question. H₂SO₄ reacts with NH₃; anhydrous CaCl₂ reacts with both NH₃ and CO₂. CaO is the only safe drying agent for ammonia.