📚 Year 9 WJEC Chemistry: Unit Test Mock Paper Walkthrough | WJEC 9年级化学单元测试模拟卷解析
Preparing for your Year 9 WJEC Chemistry unit test? This walkthrough analyzes a mock paper covering key topics such as atomic structure, chemical bonding, equations, acids and bases, reactivity, energy changes, and practical techniques. Each section presents a typical exam-style question followed by a detailed step-by-step explanation in both English and Chinese to reinforce your understanding.
正在为你的 Year 9 WJEC 化学单元测试做准备吗？本文解析一份模拟试卷，涵盖原子结构、化学键、化学方程式、酸和碱、金属活动性、能量变化以及实验技术等核心主题。每个部分呈现一道典型考题，并附带逐步详细解答，英汉对照以加深你的理解。
1. Atomic Structure and Subatomic Particles | 原子结构与亚原子粒子
Question: The element magnesium has an atomic number of 12 and a mass number of 24. Calculate the number of protons, neutrons and electrons in a neutral magnesium atom.
问题：镁元素的原子序数为12，质量数为24。计算中性镁原子中的质子数、中子数和电子数。
The atomic number always equals the number of protons, so a magnesium atom has 12 protons.
原子序数始终等于质子数，因此一个镁原子有12个质子。
om, the negative electrons balance the positive protons, meaning electrons = protons = 12.
在中性原子中，带负电的电子数与带正电的质子数相等，因此电子数 = 质子数 = 12。
l of protons and neutrons. Neutrons = mass number - atomic number = 24 - 12 = 12 neutrons.
质量数是质子数与中子数之和。中子数 = 质量数 - 原子序数 = 24 - 12 = 12 个中子。
Always remember that the atomic number defines the element, and the mass number only affects the isotope. A neutral atom must have equal numbers of protons and electrons.
请牢记，原子序数决定了元素种类，质量数只影响同位素。中性原子必须具有相等的质子数和电子数。
2. Electron Configuration and the Periodic Table | 电子排布与周期表
Question: An atom has the electron arrangement 2,8,2. State its period number, group number and name the element.
问题：某原子的电子排布为 2,8,2。请说出它所在的周期、主族以及元素名称。
The number of occupied electron shells equals the period. Here three shells are occupied (2,8,2), so the element is in Period 3.
占据的电子层数等于周期数。此处共有三层电子（2,8,2），因此该元素位于第3周期。
The number of electrons in the outermost shell gives the group for main-group elements. With 2 outer electrons, it belongs to Group 2.
最外层电子数决定了主族元素的族。最外层有2个电子，因此属于第2族。
8+2 = 12, so the atomic number is 12. The element with atomic number 12 is magnesium (Mg).
电子总数 = 2+8+2 = 12，因此原子序数为12。原子序数为12的元素是镁（Mg）。
to the periodic table: period = number of shells, group = outer electrons for Groups 1-2.
电子排布与周期表直接相关：周期 = 电子层数，主族（1-2族）= 最外层电子数。
3. Ionic Bonding and Formulae of Ionic Compounds | 离子键与离子化合物化学式
Question: Magnesium ions are Mg²⁺ and chloride ions are Cl⁻. Determine the correct chemical formula of magnesium chloride and explain why it has a high melting point.
问题：镁离子为 Mg²⁺，氯离子为 Cl⁻。确定氯化镁的化学式，并解释为什么它具有高熔点。
Magnesium loses two electrons to form Mg²⁺; each chlorine atom gains one electron to form Cl⁻. To balance the charges, two Cl⁻ ions are needed for every Mg²⁺, giving the formula MgCl₂.
镁失去两个电子形成 Mg²⁺；每个氯原子获得一个电子形成 Cl⁻。为了平衡电荷，每个 Mg²⁺ 需要两个 Cl⁻，因此化学式为 MgCl₂。
Magnesium chloride is an ionic compound held together by strong electrostatic forces between oppositely charged ions in a giant lattice. A large amount of energy is needed to overcome these forces, so it has a high melting point.
氯化镁是离子化合物，由带相反电荷的离子通过强静电引力构成的巨型晶格。需要大量能量来克服这些引力，因此熔点很高。
When writing ionic formulae, always make sure the total positive charge equals the total negative charge. The charges can be found from the group number.
书写离子式时，务必确保正负电荷总数相等。离子电荷可从元素所在族推断。
4. Covalent Bonding and Simple Molecules | 共价键与简单分子
Question: Describe the bonding in a water molecule, H₂O, using the idea of shared pairs of electrons. Also explain why water has a relatively low boiling point.
问题：用共用电子对的概念描述水分子 H₂O 中的化学键，并解释为什么水的沸点相对较低。
In water, an oxygen atom shares two pairs of electrons — one pair with each of two hydrogen atoms. This forms two single covalent bonds, giving each atom a full outer shell (O has 8, each H has 2).
在水中，一个氧原子共用两对电子——分别与两个氢原子各共用一对。这就形成了两个单共价键，使每个原子都达到稳定外壳层（O 有8个电子，每个 H 有2个电子）。
Water exists as small H₂O molecules. The covalent bonds inside the molecule are strong, but the forces between molecules are weak. Only a small amount of energy is needed to overcome these intermolecular forces, so water has a low boiling point.
水以小 H₂O 分子形式存在。分子内部的共价键很强，但分子间作用力微弱。只需少量能量即可克服分子间力，因此水沸点较低。
Remember that simple molecular substances have low melting and boiling points because of weak intermolecular forces, not because of weak covalent bonds.
请记住，简单分子物质熔沸点低是由于分子间作用力弱，而非共价键弱。
5. Balancing Chemical Equations | 化学方程式配平
Question: Balance the equation for the combustion of methane: CH₄ + O₂ → CO₂ + H₂O
问题：配平甲烷燃烧的化学方程式：CH₄ + O₂ → CO₂ + H₂O
each side. Left: C=1, H=4, O=2. Right: C=1, H=2, O=3. Hydrogen and oxygen are unbalanced.
步骤1：数两侧原子数。左边：C=1，H=4，O=2。右边：C=1，H=2，O=3。氢和氧尚未配平。
₂ + 2 H₂O. Now right side has H=4, O=4 (from CO₂ + 2×1 from H₂O). Left side still has O=2.
步骤2：在 H₂O 前加系数 2 以配平氢：CH₄ + O₂ → CO₂ + 2 H₂O。此时右边 H=4，O=4（CO₂ 中的2个 + 2个 H₂O 中的2个）。左边 O=2 仍然不平衡。
+ 2 O₂ → CO₂ + 2 H₂O. Final check: C=1, H=4, O=4 on both sides. The equation is balanced.
步骤3：在左边 O₂ 前加系数 2 以配平氧：CH₄ + 2 O₂ → CO₂ + 2 H₂O。最后检查：两侧均有 C=1，H=4，O=4。方程式已配平。
Always balance atoms by adjusting only the coefficients in front of substances, never change subscripts inside a formula.
配平原子时，只能调整物质前的系数，绝不能改变化学式内部的下标数字。
6. Conservation of Mass and Reacting Masses | 质量守恒与反应物质量
Question: 24 g of magnesium reacts completely with oxygen to form 40 g of magnesium oxide. Calculate the mass of oxygen that reacted and state the law that explains your calculation.
问题：24 克镁与氧气完全反应，生成 40 克氧化镁。计算参与反应的氧气质量，并说明运用了哪条定律。
products in a closed system. Mass of magnesium + mass of oxygen = mass of magnesium oxide.
根据质量守恒定律，在封闭体系中，反应物的总质量等于生成物的总质量。镁的质量 + 氧气的质量 = 氧化镁的质量。
So, mass of oxygen = mass of magnesium oxide - mass of magnesium = 40 g - 24 g = 16 g.
因此，氧气的质量 = 氧化镁的质量 - 镁的质量 = 40 g - 24 g = 16 g。
This law applies because atoms are rearranged during a chemical reaction, not created or destroyed. The total number of each type of atom remains unchanged.
该定律适用是因为在化学反应中原子只是重新排列，既没有生成也没有消失。每种原子的总数保持不变。
7. Acids, Bases and the pH Scale | 酸、碱与pH标度
Question: A solution has a pH of 9. Is it acidic, alkaline or neutral? Name one common laboratory indicator and describe the colour change for this solution.
问题：某溶液的 pH 为 9。它是酸性、碱性还是中性？说出一种常见实验室指示剂，并描述它在该溶液中的颜色变化。
A pH less than 7 is acidic, pH 7 is neutral, and pH greater than 7 is alkaline. A pH of 9 is therefore alkaline (basic).
pH 小于 7 为酸性，pH 等于 7 为中性，pH 大于 7 为碱性。因此 pH 为 9 是碱性的。
Universal indicator can be used: in an alkaline solution of pH 9, it turns blue or greenish-blue, depending on the exact pH. Alternatively, red litmus paper turns blue in alkaline conditions.
可使用万用指示剂：在 pH 9 的碱性溶液中，它会显示蓝色或绿蓝色。也可用红色石蕊试纸，在碱性条件下变为
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