📚 A-Level WJEC Science: Common Mistake Questions Explained | A-Level WJEC 科学：易错题精讲
WJEC A-Level Science examinations challenge students not only on knowledge but also on the ability to avoid common pitfalls. This article highlights frequent mistakes across physics, chemistry, and biology, providing clear explanations and strategies to boost exam performance.
WJEC A-Level 科学考试不仅考查知识，还考验学生避开常见误区的能力。本文重点剖析物理、化学和生物中反复出现的易错题，给出清晰讲解和提升考试成绩的策略。
1. Understanding Significant Figures and Rounding | 理解有效数字与舍入
A common mistake in WJEC practical-based questions is rounding intermediate values too early. For example, when calculating density from mass and volume, students might round the volume before dividing, leading to an inaccurate final answer.
在 WJEC 实验题中常见错误是过早舍入中间值。例如，根据质量和体积计算密度时，学生可能在除法前就舍入体积，导致最终答案不准确。
The correct approach is to carry all digits through intermediate steps and only round the final answer to the required number of significant figures. If mass = 5.12 g and volume = 2.3 cm³, the density is 5.12 / 2.3 = 2.226… g cm⁻³, which should be given as 2.2 g cm⁻³ (2 sig. fig., matching the least precise measurement).
正确方法是将中间步骤的所有数字保留，只对最终答案按要求有效数字舍入。若质量 = 5.12 g，体积 = 2.3 cm³，密度为 5.12 / 2.3 = 2.226… g cm⁻³，最终应写作 2.2 g cm⁻³（2位有效数字，与最不精确的测量值一致）。
Also, when adding or subtracting, the answer’s decimal places should match the least number of decimal places in the data, not significant figures. Misapplying this rule is a frequent error.
此外，加减运算时，答案的小数位数应与数据中小数位数最少的一致，而不是有效数字。误用此规则是常见错误。
For instance, 12.11 cm + 0.3 cm = 12.41 cm, but it must be reported as 12.4 cm because 0.3 cm has only one decimal place.
例如，12.11 cm + 0.3 cm = 12.41 cm，但必须报告为 12.4 cm，因为 0.3 cm 只有一位小数。
2. Balancing Chemical Equations – Charge and Mass Conservation | 配平化学方程式——电荷与质量守恒
Many students balance atoms successfully but overlook the conservation of charge in ionic equations. This leads to equations that appear balanced but are chemically impossible.
许多学生能成功配平原子，却忽略了离子方程式中的电荷守恒，导致看似配平却在化学上不可能成立的方程式。
For example, the reaction between iron(III) ions and iodide ions: Fe³⁺ + I⁻ → Fe²⁺ + I₂ is often written incorrectly. The charge on the left is (+3) + (-1) = +2, while on the right it is +2 only. This suggests balance, but the atoms are not conserved because iodine appears as I₂ on the right and as I⁻ on the left. The correct balanced equation is 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
例如，铁(III)离子与碘离子的反应：Fe³⁺ + I⁻ → Fe²⁺ + I₂ 常被错误书写。左边电荷为（+3）+（-1）= +2，右边也为 +2，看似平衡，但原子未守恒，因为右边出现 I₂，而左边是 I⁻。正确配平的方程式为 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂。
In half-equations, always check that both atoms and total charge are equal on each side. Adding electrons to the more positive side neutralises the charge difference.
在半方程式中，务必检查两侧原子和总电荷均相等。将电子添加到正电荷较多的一侧以平衡电荷差。
A helpful trick: after adding coefficients, sum the charges on both sides; they must match. For redox in acidic solution, add H⁺ and H₂O correctly.
实用技巧：添加系数后，将两侧电荷相加，它们必须相等。对于酸性条件下的氧化还原反应，正确添加 H⁺ 和 H₂O。
3. Graphs: Gradient and Unit Conversion | 图表：斜率和单位换算
Questions requiring gradient calculations often see students forgetting to convert plotted units into SI before finding the gradient, or misreading the axis scales.
在要求计算斜率的题目中，学生常常在求斜率之前忘记将坐标轴单位转换为国际单位制（SI），或者误读坐标轴刻度。
For example, if a graph plots temperature rise (in °C) against time (in minutes), the gradient has units °C/min. But if the question asks for rate in K/s, a conversion is necessary: 1 °C min⁻¹ = (1 K) / (60 s) = 0.0167 K s⁻¹. Students frequently omit this step and give the wrong units.
例如，一张图描绘了温度升高（°C）随时间（分钟）的变化，其斜率单位为 °C/min。但若题目要求以 K/s 为单位表示速率，则需转换：1 °C min⁻¹ = (1 K) / (60 s) = 0.0167 K s⁻¹。学生常忽略这一步，给出错误单位。
When using a large triangle to calculate gradient, it is critical to read coordinates accurately from the line of best fit, not from data points, and to show the working clearly on the graph.
使用大三角形计算斜率时，必须从最佳拟合线上准确读取坐标，而非数据点，并在图上清楚展示计算过程。
Another subtle error occurs in curved graphs: drawing a tangent to find instantaneous rate. The tangent must touch the curve at only the point of interest and should not cross it.
另一个易错点出现在曲线图中：为求瞬时速率而画切线。切线必须仅在所关注的点接触曲线，且不应穿过曲线。
4. Misinterpreting Half-Life in Radioactivity | 误解放射性衰变中的半衰期
A persistent error is believing that radioactivity ‘disappears’ after two half-lives. In reality, each half-life reduces the remaining undecayed nuclei by half, never reaching zero.
一个长期存在的错误是认为经过两个半衰期后放射性会“消失”。实际上，每个半衰期使剩余未衰变的原子核减少一半，永远不会达到零。
The decay follows N = N₀ (½)^(t / t₁/₂). After one half-life, N = N₀/2; after two, N = N₀/4; after three, N = N₀/8, and so on. The fraction remaining after n half-lives is (½)ⁿ.
衰变遵循 N = N₀ (½)^(t / t₁/₂)。一个半衰期后，N = N₀/2；两个后，N = N₀/4；三个后，N = N₀/8，以此类推。n 个半衰期后剩余的分数为 (½)ⁿ。
In WJEC questions, this concept is often tested by asking for the mass of a radioactive isotope remaining after a given time, or the time required for activity to drop to a certain count rate. Students must read the half-life from a decay curve accurately.
在 WJEC 试题中，常通过询问给定时间后剩余同位素的质量，或活度降至某一计数率所需的时间来考查该概念。学生必须从衰变曲线上准确读取半衰期。
Note: background radiation must be subtracted before plotting or calculating. Failing to do so distorts the half-life estimation.
注意：在绘图或计算之前必须扣减本底辐射。未扣减会导致半衰期的估算失真。
5. Free-Body Diagrams: Common Errors | 受力图中的常见错误
Free-body diagrams are a crucial part of mechanics problems, yet they are often drawn incorrectly. The most typical mistake is failing to include all forces or including forces that do not act on the body.
受力图是力学问题的重要组成部分，却常被画错。最典型的错误是遗漏某些力，或包含实际不作用在该物体上的力。
For an object sliding down an inclined plane, the correct forces are weight (acting vertically downwards), the normal reaction (perpendicular to the plane), and friction (acting up the plane, opposing motion). Students often mistakenly replace weight with its components, drawing ‘mg sin θ’ and ‘mg cos θ’ directly on the diagram, which should only be shown as resolved components beside the diagram.
对于一个从斜面滑下的物体，正确的力包括重力（竖直向下）、法向反力（垂直于斜面）和摩擦力（沿斜面向上，阻碍运动）。学生常错误地用重力的分量取代重力，直接在图中画出 ‘mg sin θ’ 和 ‘mg cos θ’，这些分量只应在图旁作为分解示意。
Another error is confusing action–reaction pairs. If a book rests on a table, the normal force on the book by the table and the downward force on the table by the book are an action–reaction pair. They do not cancel out on a single free-body diagram because they act on different objects.
另一个错误是混淆作用力与反作用力对。若一本书放在桌上，桌子给书的法向力和书给桌子的向下力是一对作用力与反作用力。它们在单一物体的受力图中并不抵消，因为它们作用在不同对象上。
Always verify: are all forces real (gravity, normal, friction, tension, etc.)? Are they drawn on the correct object? Is there a net force in the expected direction of acceleration?
务必检查：所有力是否真实（重力、法向力、摩擦力、张力等）？是否画在正确的物体上？合力方向是否与预期的加速度方向一致？
6. Osmosis vs. Diffusion Terminology Confusion | 渗透与扩散术语混淆
In biology exams, using ‘osmosis’ and ‘diffusion’ interchangeably is a marking pitfall. Diffusion is the net movement of particles from a region of higher concentration to lower concentration, and it applies to any substance. Osmosis is specifically the diffusion of water molecules through a partially permeable membrane.
在生物考试中，互换使用“渗透”和“扩散”是扣分点。扩散是指粒子从高浓度区域向低浓度区域的净移动，适用于任何物质。渗透则是特指水分子通过部分透性膜的扩散。
When describing the movement of oxygen into a respiring cell, the correct term is diffusion, not osmosis. Osmosis only describes water movement. Saying ‘oxygen molecules move by osmosis’ loses marks immediately.
描述氧气进入进行呼吸作用的细胞时，正确术语是扩散，而非渗透。渗透只描述水分子运动。说“氧分子通过渗透移动”会直接失分。
Another mistake is forgetting to mention the partially permeable membrane when explaining osmosis. Water moves from a high water potential (low solute concentration) region to a low water potential region across the membrane.
另一个错误是在解释渗透时忘记提及部分透性膜。水分子从水势高的区域（低溶质浓度）通过膜移动到水势低的区域。
In plant cell diagrams, a turgid cell has a high pressure potential; a flaccid cell is in an isotonic solution. Correct terminology ensures full marks.
在植物细胞图示中，胀大的细胞具有高的压力势；松弛的细胞处于等渗溶液。正确使用术语可确保获得满分。
7. Rate of Reaction and Temperature: Activation Energy Misconception | 反应速率与温度：活化能误区
A widely held misconception is that increasing temperature lowers the activation energy (Eₐ) of a reaction. In fact, the activation energy remains unchanged. Heating merely increases the average kinetic energy of molecules, so a greater proportion possess energy equal to or above Eₐ.
一个普遍误解是升高温度会降低反应的活化能（Eₐ）。事实上，活化能保持不变。加热只是增加了分子的平均动能，因此更多比例分子具有大于或等于 Eₐ 的能量。
The Maxwell–Boltzmann distribution illustrates this: the curve flattens and shifts to the right, but the activation energy threshold stays fixed. The area beyond Eₐ increases, leading to more frequent successful collisions.
麦克斯韦-玻尔兹曼分布说明了这一点：曲线变平并右移，但活化能阈值不变。超过 Eₐ 的面积增大，导致有效碰撞频率增加。
Some students also confuse catalyst function. A catalyst provides an alternative pathway with a lower activation energy; it does not raise temperature or increase kinetic energy. Always distinguish between temperature effect and catalyst effect.
一些学生还会混淆催化剂的作用。催化剂提供一条活化能更低的替代路径；它不会升高温度或增加动能。务必区分温度效应和催化剂效应。
When answering WJEC analysis questions, state clearly: “Temperature increases the rate because more particles have E ≥ Eₐ, not because Eₐ is lowered.”
回答 WJEC 分析题时，明确陈述：“温度提高速率是因为更多粒子的能量 E ≥ Eₐ，而非因为 Eₐ 降低。”
8. Circuit Calculations: Series vs Parallel Resistance | 电路计算：串联与并联电阻
Ohm’s law and resistor combinations are frequent sources of arithmetic mistakes. In series, R_total = R₁ + R₂ + … ; in parallel, 1/R_total = 1/R₁ + 1/R₂ + … . Students sometimes forget to take the reciprocal after summing the parallel conductances.
欧姆定律和电阻组合是算术错误的频发点。串联时，R_total = R₁ + R₂ + …；并联时，1/R_total = 1/R₁ + 1/R₂ + …。学生有时在求出并联的电导和之后忘记取倒数。
Consider two resistors, 4 Ω and 6 Ω, in parallel. The incorrect answer R_total = 4 + 6 = 10 Ω is too common. The correct approach: 1/R_total = 1/4 + 1/6 = 5/12, so R_total = 12/5 = 2.4 Ω.
考虑两个电阻，4 Ω 和 6 Ω，并联。错误答案 R_total = 4 + 6 = 10 Ω 太常见了。正确做法：1/R_total = 1/4 + 1/6 = 5/12，因此 R_total = 12/5 = 2.4 Ω。
Another pitfall is mixing up voltage and current in series and parallel. In series, current is the same through all components, while voltage splits. In parallel, voltage across each branch is the same, while current divides. Misapplying these rules leads to lost marks in circuit analysis.
另一个易错点是混淆串联和并联中的电压与电流特性。串联电路中，通过每个元件的电流相同，而电压分配。并联电路中，各支路电压相等，而电流分配。错误应用这些规则会导致电路分析失分。
A helpful table for comparison:
| Property | Series | Parallel |
| Current | Same everywhere | Splits between branches |
| Voltage | Divides across components | Same across each branch |
| Resistance | R_total = R₁ + R₂ + … | 1/R_total = 1/R₁ + 1/R₂ + … |
9. Genetic Crosses: Probability and Independent Assortment | 遗传杂交：概率与自由组合
Dihybrid crosses test understanding of independent assortment, but probability errors are widespread. A typical mistake is adding probabilities for independent events instead of multiplying them.
双因子杂交考察对自由组合的理解，但概率错误非常普遍。典型错误是将独立事件的概率相加而非相乘。
For a cross AaBb × AaBb (both heterozygous for two unlinked genes), the probability of obtaining an offspring with the phenotype A_bb (dominant for A, recessive for b) is incorrectly calculated by some as 1/4 + 1/4 = 1/2. The correct product rule gives: probability of A_ = 3/4, probability of bb = 1/4, therefore combined = 3/4 × 1/4 = 3/16.
对于杂交 AaBb × AaBb（两对不连锁基因均为杂合），获得表型为 A_bb（A 显性、b 隐性）后代的概率，一些学生错误地算作 1/4 + 1/4 = 1/2。正确的乘积法则为：A_ 的概率 = 3/4，bb 的概率 = 1/4，因此合并概率 = 3/4 × 1/4 = 3/16。
Another common slip occurs in pedigree analysis, where students forget to consider all possible parental genotypes when calculating risk. They might assume a parent is homozygous dominant without checking the family history, leading to incorrect carrier probability.
另一个常见失误出现在系谱分析中，学生计算风险时忘记考虑所有可能的亲本基因型
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