CIE iGCSE Co-Ordinated Science C2.5 Simple molecules and covalent bonds Exam Style Questions Paper 3
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.
Question
Topic codes:
• Topic C11.2 — Naming organic compounds (Part (a)(iii))
• Topic C11.6 — Alcohols (Part (a)(i), (a)(ii) & (b))
• Topic C3.2 — Relative masses of atoms and molecules (Part (a)(iv))
• Topic C10.2 — Air quality and climate (Part (b)(i))
• Topic C10.1 — Water (Part (b)(ii))
• Topic C2.5 — Simple molecules and covalent bonds (Part (b)(iii))
▶️ Answer/Explanation
(a)(i) Hydrocarbons only contain carbon and hydrogen.
Ethanol contains carbon, hydrogen, and oxygen atoms. The presence of oxygen means it is not a hydrocarbon. Hydrocarbons are compounds that contain only carbon and hydrogen atoms.
(a)(ii) Solvent or fuel
Ethanol is commonly used as a solvent in manufacturing and as a fuel (e.g., in alcoholic drinks, as a fuel additive in petrol, or as a biofuel).
(a)(iii) Displayed formula of ethanol:
The displayed formula shows all the atoms and all the bonds in the molecule. Each line represents a covalent bond (shared pair of electrons).
(a)(iv) \(M_r = 46\)
Calculation: \((2 \times 12) + (6 \times 1) + (1 \times 16) = 24 + 6 + 16 = 46\)
Ethanol has 2 carbon atoms, 6 hydrogen atoms (5 from C₂H₅ and 1 from OH), and 1 oxygen atom.
(b)(i) Increased global warming / climate change
Carbon dioxide is a greenhouse gas that traps heat in the atmosphere. Higher levels of CO₂ lead to enhanced greenhouse effect, causing global warming and climate change.
(b)(ii) Anhydrous copper(II) sulfate: white to blue
Test: Add anhydrous copper(II) sulfate (white powder) to the sample. If water is present, it turns blue because copper(II) sulfate forms hydrated copper(II) sulfate (CuSO₄·5H₂O).
Alternative test: Anhydrous cobalt(II) chloride (blue) turns pink in the presence of water.
(b)(iii) Dot-and-cross diagram for water (\(\text{H}_2\text{O}\)):
In a water molecule, the oxygen atom shares one electron with each of the two hydrogen atoms, forming two single covalent bonds. Oxygen has 6 outer-shell electrons, and each hydrogen has 1 outer-shell electron. Each shared pair of electrons is drawn as a cross (×) for one atom and a dot (•) for the other. Oxygen also has two lone pairs of electrons (not involved in bonding).