CIE iGCSE Co-Ordinated Science C10.1 Water Exam Style Questions Paper 4
Question
Classify magnesium oxide.
Explain your answer.
Determine the electronic configuration of a magnesium atom.
Describe the similarity and the difference between the three isotopes.
The boiling point is 102 °C.
Describe how the student knows that the sea water is not pure water.
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C2.4 — Ions and ionic bonds (Part (a))
• Topic C7.2 — Oxides (Part (b))
• Topic C2.2 — Atomic structure and the Periodic Table (Part (c))
• Topic C2.3 — Isotopes (Part (d))
• Topic C3.2 — Relative masses of atoms and molecules / C3.3 — The mole and the Avogadro constant (Part (e))
• Topic C10.1 — Water (Part (f))
▶️ Answer/Explanation
(a) Formula = MgO.
The diagram shows equal numbers of Mg²⁺ and O²⁻ ions in a 1:1 ratio. The charges balance (+2 and -2), so the formula is MgO.
(b) Classification: Basic oxide.
Explanation: Magnesium is a metal. Metal oxides react with acids to form salts and water. Magnesium oxide is a basic oxide because it neutralises acids.
(c) Electronic configuration = 2.8.2.
Magnesium has 12 protons/electrons. The first shell holds 2, the second holds 8, and the remaining 2 electrons go into the third shell.
(d) Similarity: Same number of protons (atomic number = 12).
Difference: Different number of neutrons / different mass numbers (24, 25, 26).
(e) Calculation:
\(M_r\) of MgSO₄ = 26 + 32 + (4 × 16) = 122
\(\text{moles} = \frac{\text{mass}}{M_r} = \frac{3.05}{122} = 0.025\text{ mol}\)
Note: The \(^{26}\text{Mg}\) isotope has mass number 26, so the relative atomic mass used for Mg is 26 in this calculation.
(f) Pure water boils at 100°C at standard atmospheric pressure.
The sea water sample boils at 102°C, which is higher than 100°C. This elevation in boiling point indicates the presence of dissolved impurities (salts), so the water is not pure.
Question
Topic codes:
• Topic C1.1 — Solids, liquids and gases (Part (a) & (b))
• Topic C10.1 — Water (Part (c) & (d))
• Topic C2.5 — Simple molecules and covalent bonds (Part (e))
▶️ Answer/Explanation
(a) Gas — water particles are furthest apart in the gaseous state.
(b) During melting, the movement of water particles changes from vibrating about fixed positions (in the solid state) to moving around each other (in the liquid state). The particles gain kinetic energy and move faster as the solid melts.
(c)(i) The ice is melting during the horizontal part of the graph where the temperature remains constant at 0°C (the plateau). The letter X should be placed on this horizontal section.
(c)(ii) Fig. 5.1 shows that the ice is pure because there is a horizontal part to the graph at the melting point. A pure substance melts at a single, specific temperature, whereas a mixture melts over a range of temperatures.
(d) The correct matching is:
(e)(i) The completed dot-and-cross diagram for water should show:
- Oxygen atom with 6 outer-shell electrons (2 pairs and 2 unpaired)
- Two hydrogen atoms each with 1 electron
- Two shared pairs of electrons (covalent bonds) between oxygen and each hydrogen
- The remaining 4 electrons on oxygen shown as two lone pairs
(e)(ii) Pure water is a poor conductor of electricity because it does not contain any free (moving) electrons or ions. For electrical conduction, charged particles (ions or electrons) must be free to move. In pure water, the molecules are neutral and there are no mobile charge carriers.