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describe / explain
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In an electric motor, two blocks of graphite called brushes press on a spinning part and carry the current into it. A dentist’s drill has a tip coated with tiny diamonds. (a) Describe how each carbon atom is bonded in graphite, and use it to explain why the brushes conduct electricity. (b) Describe how each carbon atom is bonded in diamond, and use it to explain why the diamond-coated tip can cut into a tooth. (c) Supplement: in some electric heaters, the glowing wire is sealed inside a tube made of silicon(IV) oxide, and the tube must not conduct electricity. Explain, in terms of structure and bonding, why silicon(IV) oxide does not conduct, just as diamond does not.
describe: give the features or what happens. explain: give reasons and connect them to evidence.
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Scope and sources
Cambridge IGCSE Chemistry 0620 · 2026–2028 v2 · Original learning materials.
Giant covalent and metallic structures (2.6 Core 1–2, Supplement 3–4; 2.7 Supplement 1–2): the giant covalent structures of diamond and graphite and their uses in cutting tools, as a lubricant and as an electrode, the giant covalent structure of silicon(IV) oxide and its similarity in properties to diamond, metallic bonding as the attraction between positive ions in a giant metallic lattice and a sea of delocalised electrons, and the electrical conductivity, malleability and ductility of metals. Simple molecules, ionic lattices, alloys, the uses of metals and electrolysis are left to other modules.
Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.
Official Cambridge syllabus