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explain / predict
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Calcium bromide is an ionic compound made from calcium and bromine. (a) A pupil writes: "A bromine atom gains one electron, so the bromide ion is Br+." Explain why the pupil is incorrect. (b) A second pupil writes: "The ionic bond in calcium bromide is the two electrons that moved from the calcium atom to the bromine atoms." Explain why this pupil is incorrect. (c) Predict whether calcium bromide has high or low melting and boiling points, and whether it conducts electricity as a solid, when molten and when aqueous. (d) Supplement: explain, in terms of structure and bonding, your predictions for the melting point and for the boiling point.
explain: give reasons and connect them to evidence. predict: use the information to say what you expect.
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Scope and sources
Cambridge IGCSE Chemistry 0620 · 2026–2028 v2 · Original learning materials.
Ions and ionic bonds (2.4 Core 1–4, Supplement 5–7): cations and anions, the ionic bond as a strong electrostatic attraction between oppositely charged ions, dot-and-cross diagrams for Group I with Group VII compounds and, in the Supplement, for other metallic and non-metallic elements, the high melting and boiling points of ionic compounds and their conduction when molten or aqueous but not when solid, the giant lattice of alternating ions, and how that structure explains the properties. Covalent bonds, giant covalent and metallic structures, writing formulae and naming compounds in general, 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