CIE iGCSE Co-Ordinated Science C7.1 The characteristic properties of acids and bases Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Coordinated Sciences 0654):
• Topic C2.4 — Ions and ionic bonds (Part (a)(i))
• Topic C2.4 — Ions and ionic bonds (Part (a)(ii))
• Topic C8.3 — Group VII properties (Part (b))
• Topic C7.1 — Characteristic properties of acids and bases (Part (c))
▶️ Answer/Explanation
(a)(i) eight electrons added to outer shell; – sign shown
A chlorine atom has 7 electrons in its outer shell. It gains one electron to achieve a stable octet, becoming a chloride ion (Cl⁻) with a single negative charge.
(a)(ii) generally soluble in water; good electrical conductivity; high melting point
Ionic compounds have high melting points due to strong electrostatic attractions. They conduct electricity when dissolved in water (but not when solid) because ions are free to move. They are generally soluble in water.
(b) (pale) yellow-green; gas
Chlorine is a pale yellow-green gas at room temperature and pressure. It is a diatomic molecule (Cl₂) with a distinctive colour.
(c) hydrochloric acid; sodium hydroxide
Neutralisation of hydrochloric acid with sodium hydroxide produces sodium chloride and water. The acid provides chloride ions, and the alkali provides sodium ions.
Question
Most-appropriate topic codes (Cambridge IGCSE Coordinated Sciences 0654):
• Topic C10.2 — Air quality and climate (Part (a)(i)-(iv))
• Topic C9.5 — Corrosion of metals (Part (a)(v))
• Topic C2.5 — Simple molecules and covalent bonds (Part (b))
• Topic C7.1 — Characteristic properties of acids (Part (c))
• Topic C4.2 — Hydrogen-oxygen fuel cells (Part (d))
▶️ Answer/Explanation
(a)(i) He
Helium (He) is a monatomic gas because it consists of single atoms rather than molecules. It is a noble gas with a full outer electron shell.
(a)(ii) CO
Carbon monoxide (CO) is a toxic gas produced by incomplete combustion. It binds to haemoglobin in red blood cells, preventing oxygen transport.
(a)(iii) N₂
Nitrogen (N₂) makes up approximately 78% of clean, dry air. It is an inert gas at normal temperatures.
(a)(iv) He or N₂ or O₂
Elements are substances that contain only one type of atom. He, N₂, and O₂ are all elements, while CO, CO₂, and H₂O are compounds.
(a)(v) O₂ and H₂O
Rusting of iron requires both oxygen (O₂) and water (H₂O). These react with iron to form hydrated iron(III) oxide.
(b) Shared pair correct; all else correct
In HCl, hydrogen shares one electron with chlorine to complete both their outer shells. The dot-and-cross diagram should show one shared pair of electrons.
(c) turns red
Dilute hydrochloric acid is acidic. Acids turn blue litmus paper red, indicating a pH below 7.
(d) hydrogen; oxygen
A hydrogen-oxygen fuel cell uses hydrogen and oxygen as reactants. The chemical energy is converted directly into electrical energy with water as the only product.