CIE iGCSE Co-Ordinated Science C10.1 Water Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Coordinated Sciences 0654):
• Topic C11.4 — Alkanes (Part (a))
• Topic C11.2 — Naming organic compounds (Part (b))
• Topic C3.2 — Relative masses of atoms and molecules (Part (c))
• Topic C10.1 — Water (Part (d)(i))
• Topic C10.1 — Water (Part (d)(ii))
• Topic C10.2 — Air quality and climate (Part (d)(iii))
▶️ Answer/Explanation
(a) single and double
Alkanes have only single C-C bonds, so they are saturated (cannot add more atoms). Alkenes have at least one C=C double bond, making them unsaturated and able to undergo addition reactions.
(b) Displayed formula of ethane with C-C single bond and all C-H bonds correct
Ethane (C₂H₆) has two carbon atoms joined by a single bond, with each carbon bonded to three hydrogen atoms. The displayed formula shows all atoms and bonds.
(c) 24 + 6 or 2 × 12 + 6 × 1; 30
Mᵣ of C₂H₆ = (2 × 12) + (6 × 1) = 24 + 6 = 30. Relative molecular mass is the sum of the relative atomic masses of all atoms in the molecule.
(d)(i) blue; (to) pink
Anhydrous cobalt(II) chloride is blue. When water is present, it turns pink as it forms hydrated cobalt(II) chloride. This is a positive test for water.
(d)(ii) pure water boils at 100°C; impurities increase boiling point
Pure water has a boiling point of 100°C at standard atmospheric pressure. The observed boiling point of 102°C is higher than 100°C, indicating the presence of impurities that elevate the boiling point.
(d)(iii) planting trees / planting plants; CO₂ removed by trees / CO₂ removed by plants
OR decreasing use of fossil fuels; less CO₂ produced (during combustion)
OR increasing use of renewables; no CO₂ produced
Plants absorb CO₂ during photosynthesis, reducing atmospheric CO₂. Reducing fossil fuel use decreases CO₂ emissions at source.
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).