📚 IGCSE WJEC Science: Common Mistakes Explained | IGCSE WJEC 科学：易错题精讲
Mastering IGCSE WJEC Science requires more than just remembering facts — it demands a clear understanding of concepts and the ability to avoid the common errors that repeatedly trap students. This article analyses the most frequent mistakes made in Physics, Chemistry and Biology, and shows you exactly how to correct them, helping you to build confidence and secure higher marks.
掌握 IGCSE WJEC 科学不仅需要记忆事实，还需要对概念有清晰透彻的理解，并能避开那些屡屡让学生失分的常见错误。本文剖析了物理、化学和生物中最常出现的错误，并告诉你如何逐一纠正，从而建立信心、拿到更高分数。
1. Mass vs. Weight: A Fundamental Distinction | 质量与重量：基本区分
One of the most persistent errors is treating mass and weight as the same thing. A student might be asked to calculate the weight of a 5 kg object and simply write “5 kg” or “5 N”. Weight is not mass in newtons — it is a completely different quantity.
一个最顽固的错误是把质量和重量当作一回事。学生可能被要求计算一个 5 kg 物体的重量，然后简单地写上 “5 kg” 或 “5 N”。重量并不是以牛顿为单位的质量——它是一个完全不同的物理量。
Mass is the amount of matter in an object, measured in kilograms (kg). Weight is the gravitational force acting on that mass, calculated as W = m × g, where g ≈ 10 N/kg on Earth. For a 5 kg mass, weight = 5 × 10 = 50 N.
质量是物体所含物质的多少，单位是千克 (kg)。重量是作用在该质量上的引力，计算公式为 W = m × g，地球上 g ≈ 10 N/kg。对于 5 kg 的质量，重量 = 5 × 10 = 50 N。
Common mistake: stating “weight = 5 kg”. Correct: weight = 50 N. Never record weight in kilograms.
常见错误：写出 “重量 = 5 kg”。正确：重量 = 50 N。千万不要用千克来记录重量。
In WJEC questions, you must also state the correct unit. If a question uses g = 10 N/kg, always calculate the force and attach the newton unit.
在 WJEC 题目中，你还必须写出正确的单位。如果题目用 g = 10 N/kg，一定要算出力并加上牛顿单位。
2. Area Under a Velocity–Time Graph | 速度-时间图下的面积
Students often try to find distance from a velocity–time graph by using the formula speed = distance/time. This only works when the velocity is constant. In WJEC exams, graphs often show changing velocities — acceleration and deceleration — and the distance travelled is the area under the graph.
学生经常试图用 速度 = 距离/时间 从速度-时间图上求距离。这只适用于速度恒定不变的情况。WJEC 考试中，图像经常呈现变化的速度——加速和减速——而行驶的距离是图形下方的面积。
For a trapezium-shaped section, you must split the area into rectangles and triangles. A common error is counting only the area of the rectangular part and ignoring the triangle, or miscalculating the triangle’s area as base × height instead of ½ × base × height.
对于梯形区域，你必须将面积拆分为矩形和三角形。一个常见错误是只算了矩形部分的面积而忽略了三角形，或者在计算三角形面积时误用了底×高，而正确是 ½ × 底 × 高。
Remember: distance = area under v–t graph. If the graph drops below the x-axis, treat that area as a separate magnitude for distance, but for displacement you consider direction.
牢记：距离 = v-t 图下的面积。如果图像落到 x 轴下方，计算距离时要把那段面积的大小单独加上，但计算位移时需要考虑方向。
distance = area under velocity–time graph
距离 = 速度-时间图下的面积
Always label your working area on the graph and show the sum of areas clearly.
解题时一定要在图上标注你的工作面积，并清楚地写出面积之和。
3. Action–Reaction Pairs Act on Different Objects | 作用力与反作用力作用在不同物体上
Newton’s third law states that when object A exerts a force on object B, object B exerts an equal and opposite force on object A. These pairs never act on the same object, yet a typical mistake is identifying the weight of a book and the normal force from the table as an action–reaction pair.
牛顿第三定律指出，当物体 A 对物体 B 施加一个力时，物体 B 会同时对物体 A 施加一个大小相等、方向相反的力。这一对力永远不会作用在同一个物体上，但典型的错误是把书的重量和桌子的支持力当作作用力与反作用力对。
The weight of the book is the gravitational pull by the Earth on the book. Its reaction pair is the gravitational pull by the book on the Earth. The normal force is the contact force from the table on the book; its pair is the contact force from the book pushing down on the table.
书的重量是地球对书的引力。它的反作用力是书对地球的引力。支持力是桌子对书的接触力；它的反作用力是书向下压桌子的接触力。
In the exam, always check: do the two forces act on different objects? If they both act on the same body, they are not a third-law pair.
考试中，请始终检查：这两个力是不是作用在不同的物体上？如果它们都作用在同一个物体上，它们就不是第三定律力对。
4. Mole Calculations: Watch Your Units | 摩尔计算：注意单位
Many students lose marks in Chemistry by mixing units. The mole formula n = m / Mr works when mass is in grams. A common blunder is inserting mass in kilograms directly, giving a value that is 1000 times too small.
很多学生在化学中因混合单位而丢分。摩尔公式 n = m / Mᵣ 只有在质量以克为单位时才能直接使用。常见的大错是把以千克为单位的质量直接代入，得出一个小了 1000 倍的值。
If a question gives mass as 0.5 kg of CaCO₃, you must convert to 500 g before dividing by the Mᵣ of 100. Correct moles = 500 / 100 = 5 mol, not 0.5 / 100 = 0.005 mol.
如果题目给出 0.5 kg 的 CaCO₃，你必须先转换成 500 g，再除以相对分子质量 100。正确的摩尔数 = 500 / 100 = 5 mol，而不是 0.5 / 100 = 0.005 mol。
Similarly, when using molar gas volume at rtp (24 dm³/mol), volume must be in dm³. Convert cm³ to dm³ by dividing by 1000. A frequent slip is using 24 cm³/mol — a unit that doesn’t exist.
同样，当使用室温下气体的摩尔体积 (24 dm³/mol) 时，体积必须用 dm³。把 cm³ 转换成 dm³ 要除以 1000。经常出现的失误是使用 24 cm³/mol —— 这个单位并不存在。
moles = mass (g) / Mᵣ
moles = volume (dm³) / 24
摩尔数 = 质量 (g) / Mᵣ
摩尔数 = 体积 (dm³) / 24
5. Energy Level Diagrams for Reactions | 化学反应的能级图
Exothermic reactions release energy, so the products have less energy than the reactants. Yet many students draw the arrow pointing upwards and label the energy change as positive, which actually describes an endothermic process.
放热反应释放能量，因此产物的能量低于反应物。但很多学生把箭头画得朝上，并把能量变化标为正值
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