📚 IB & Edexcel Science: Common Mistakes and How to Avoid Them | IB 与 Edexcel 科学：易错题精讲
Many high-achieving science students lose valuable marks not because they lack understanding, but because they fall into predictable traps set by examiners. This article analyses the most frequent errors across Physics, Chemistry and Biology in IB and Edexcel specifications, offering clear explanations and strategies to avoid repeating these mistakes.
许多高分段学生失分并不是因为知识欠缺，而是掉入了出题人设计的常见陷阱。本文梳理了 IB 与 Edexcel 科学（物理、化学、生物）考试中最常出现的错误，提供清晰的解释和避免再犯的策略。
1. Units and Dimensional Analysis | 单位与量纲分析
A classic pitfall is substituting quantities with incorrect or inconsistent units. In F = ma, if mass is given in grams and acceleration in cm/s², converting them to kg and m/s² before calculation is essential. Many students skip this step and produce numerically incorrect answers that still look plausible.
典型的陷阱是代入单位错误或不一致的物理量。在 F = ma 中，如果质量以克给出、加速度以 cm/s² 给出，必须在计算前把它们转换成 kg 和 m/s²。很多学生跳过此步骤，得出数值错误但貌似合理的答案。
Similarly, in energy calculations (E = ½mv²), using km/h for velocity without conversion to m/s leads to catastrophic errors. Always check that all quantities are expressed in base SI units unless the question explicitly uses a derived unit like kJ.
同样，在能量计算（E = ½mv²）中，用 km/h 表示速度而不转换为 m/s 会导致严重的错误。除非题目明确使用如 kJ 这样的导出单位，务必确保所有物理量都用基本 SI 单位表示。
2. Graph Interpretation Errors | 图表分析错误
Examiners often ask for the gradient of a curve at a point or the area under a velocity-time graph. A common mistake is calculating Δy/Δx using two data points far apart instead of drawing a tangent. Students end up with an average gradient over a range, not the instantaneous rate.
考官常要求计算曲线上某一点的斜率，或速度-时间图下的面积。常见的错误是取两个相隔很远的数据点求 Δy/Δx 而不是画切线。这样得到的是区间内的平均斜率，而非瞬时变化率。
For area under a graph, miscounting squares or ignoring the units of the axes (e.g., cm vs. m) is frequent. In chemistry rate graphs, drawing a tangent for initial rate requires a ruler to be placed at time zero and carefully aligned with the curve; any sloppiness results in inaccurate rate values.
在计算图下面积时，数错格子或忽略坐标轴单位（如 cm 与 m）很常见。在化学速率图中，画初始速率切线需要把直尺对准时间零点并小心对齐曲线；稍有马虎就会得到错误的速率值。
3. Stoichiometry and Mole Calculations | 化学计量学与摩尔计算
Confusing the mole ratio from a balanced equation with the masses directly is a recurrent issue. Students often assume 2 g of H₂ reacts with 1 g of O₂ because the equation says 2H₂ + O₂ → 2H₂O. The correct approach is to convert masses to moles, apply the mole ratio, then convert back to mass.
反复出现的问题是混淆方程式中的摩尔比与直接质量比。学生常认为 2 g H₂ 与 1 g O₂ 反应，因为方程式是 2H₂ + O₂ → 2H₂O。正确的做法是将质量转换为摩尔，使用摩尔比，再转换回质量。
Another trap involves limiting reactants: calculating the amount of product based on the reactant in excess rather than identifying the limiting reagent first. Always determine which reactant runs out first, and base your product yield on that substance.
另一个陷阱与限量反应物有关：根据过量的反应物计算产物量，而不是先确定哪种反应物是限量的。务必先找出先消耗完的物质，然后再以此为基础计算产物产率。
4. Chemical Equilibrium Misconceptions | 化学平衡误解
A deep misunderstanding of Le Chatelier’s principle leads students to claim that a catalyst increases yield at equilibrium. A catalyst only speeds up the rate at which equilibrium is reached; it does not shift the position of equilibrium. This is often tested in multiple-choice questions.
对勒夏特列原理的深层误解导致学生声称催化剂能增加平衡时的产率。催化剂只加快到达平衡的速率，并不会移动平衡位置。这类概念常在选择题中考查。
Similarly, adding an inert gas at constant volume does not change the partial pressures of reacting gases, so the equilibrium position remains unchanged. Many assume any addition of gas disturbs the equilibrium. Explain using the concept of partial pressure or concentration.
类似地，在恒容条件下加入惰性气体不会改变反应气体的分压，因此平衡位置不变。很多人以为任何气体加入都会扰动平衡。要用分压或浓度的概念来解释。
5. Newton’s Laws and Free-Body Diagrams | 牛顿定律与受力分析
When drawing free-body diagrams, students often include forces such as ‘ma’ or ‘centripetal force’ as separate arrows, which reveals a fundamental misinterpretation. The net force equals ma, but ma is not a force acting on the object; it is the result of all real forces.
画受力分析图时，学生常常把 “ma” 或 “向心力” 当作独立的力画成箭头，这暴露出根本性的误解。合力等于 ma，但 ma 并非作用在物体上的力，而是所有实际力的结果。
Another common slip: forgetting that action-reaction pairs act on different bodies. In an elevator problem, the normal force on the person and the person’s weight are not an action-reaction pair because they act on the same body. Pair the normal force with the force the person exerts on the floor.
另一个常见疏漏：忘记了作用力与反作用力作用在不同物体上。在电梯问题中，人受到的支持力和人的重力并不是一对作用与反作用力，因为它们作用在同一物体上。应把支持力与人对地板的力配对。
6. Energy Conservation and Work | 能量守恒与功
A mistake that appears regularly in exams is equating the work done by a force with the change in kinetic energy when other forces (such as friction or an applied force at an angle) are present. The work-energy theorem states that the net work done by all forces equals the change in kinetic energy.
考试中经常出现的错误是，当存在其他力（如摩擦力或斜向拉力）时，将某个力做的功等同于动能的变化。动能定理指出，所有力做的总功等于动能的变化量。
In gravitational potential energy questions, students forget that ΔGPE = mgΔh uses the vertical height change, not the distance along a slope. If a block slides down a frictionless incline of length L at angle θ, the height change is L sin θ, not L.
在重力势能问题中，学生忘记了 ΔGPE = mgΔh 使用的是垂直高度的变化，而不是沿斜面的距离。如果一个物体沿长 L、倾角 θ 的光滑斜面下滑，高度变化是 L sin θ，而非 L。
7. Electricity and Circuit Analysis | 电路与电路分析
Misapplying Ohm’s law (V = IR) to non-ohmic components like filament lamps or diodes is a frequent error. For a filament lamp, resistance increases with temperature, so the V-I graph is a curve. Calculating resistance using a single pair of V and I gives the resistance at that point, but stating that the lamp obeys Ohm’s law is incorrect.
对灯丝灯泡或二极管等非欧姆元件误用欧姆定律 (V = IR) 是常见的错误。灯丝灯泡的电阻随温度升高而增大，因此 V-I 图是曲线。用一组 V 和 I 计算电阻只能得到该点的电阻，但声称灯泡遵循欧姆定律就是错误的。
In series and parallel circuits, many learners confuse the rules for current and voltage. A common trap question: when an extra resistor is added in parallel, the total resistance decreases, so the current from the battery increases. Students often assume total resistance always increases when more components are added.
在串联和并联电路中，许多学习者混淆了电流和电压的规则。常见的陷阱题：当并联一个额外的电阻时，总电阻减小，电池输出电流增大。学生往往以为增加元件总电阻一定增大。
8. Cell Structure and Membrane Transport | 细胞结构与膜运输
In Biology, a classic mistake is confusing the terms ‘cell wall’ and ‘cell membrane’. A plant cell has both, but animal cells lack a cell wall. When labelling diagrams, students must specify precisely; writing ‘wall’ without ‘cell’ can be ambiguous.
在生物学中，一个经典的错误是混淆 “细胞壁” 与 “细胞膜”。植物细胞两者都有，但动物细胞没有细胞壁。标注示意图时必须精确说明；只写 “壁” 而不加 “细胞” 可能产生歧义。
Osmosis is often described incorrectly as ‘the movement of water molecules towards a higher solute concentration’. The correct definition involves the movement of water from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) through a partially permeable membrane. Mentioning the membrane is crucial.
渗透常被错误描述为 “水分子向较高溶质浓度方向移动”。正确的定义是水分子通过半透膜从较低溶质浓度（较高水势）区域向较高溶质浓度（较低水势）区域移动。提及半透膜至关重要。
9. Genetics and Pedigree Analysis | 遗传学与系谱分析
When determining inheritance patterns, pupils often jump to conclusions without testing for both dominant and recessive possibilities. A common error: seeing affected individuals in every generation automatically indicates dominant inheritance, but autosomal recessive traits can also appear in multiple generations if carriers are common.
在确定遗传模式时，学生常不经过检验显性和隐性两种可能就匆忙下结论。常见错误：看到每一代都有患病者就自动认为是显性遗传，但若携带者普遍，常染色体隐性性状也可以在多代中出现。
In Punnett square calculations, forgetting that the probability of having a child with a certain genotype is independent for each birth leads to mistakes. For a couple both heterozygous for a recessive disease, the chance of having two affected children is (¼)² = 1/16, not ¼. Examiners frequently test this understanding of independent events.
在旁氏表计算中，忘记每一胎孩子具有某种基因型的概率是独立的会导致错误。对于一个双方均为隐性遗传病杂合子的夫妇，生育两个患病孩子的概率是 (¼)² = 1/16，而不是 ¼。考官经常考查对独立事件的理解。
10. Experimental Design and Variable Control | 实验设计与变量控制
In the IA (Internal Assessment) for IB or core practicals for Edexcel, students lose marks by stating the independent and dependent variables incorrectly. The independent variable is the one deliberately changed; the dependent variable is what is measured. Confusing the two, or failing to specify how they will be measured, is a common shortcoming.
在 IB 的内部评估或 Edexcel 的核心实验中，学生因错误陈述自变量和因变量而丢分。自变量是有意改变的变量，因变量是被测量的变量。混淆两者，或未能说明如何测量，是常见的不足。
Control variables must be items kept constant that would otherwise affect the dependent variable. A vague statement like ‘keep the temperature the same’ without specifying how (e.g., ‘using a thermostatically controlled water bath at 25 °C’) does not demonstrate precise scientific thinking. Always quantify or specify instrumentation.
控制变量必须是那些需保持不变、否则会影响因变量的因素。像 “保持温度相同” 这样的模糊表述而没有说明如何实现（如 “使用恒温水浴控制在 25 °C”），无法体现严谨的科学思维。始终要量化或说明使用的仪器。
11. Rate of Reaction and Collision Theory | 反应速率与碰撞理论
Explaining why increasing temperature increases reaction rate requires reference to both collision frequency and the proportion of particles with energy exceeding activation energy. Many answers only mention ‘particles move faster’, which is insufficient for full marks. Examiners look for the link to the Maxwell-Boltzmann distribution and the area under the curve beyond activation energy.
解释为何升高温度会增加反应速率，需要同时提到碰撞频率和能量超过活化能的粒子比例两个方面。许多答案只写 “粒子运动更快”，这不足以拿到满分。考官希望看到与麦克斯韦-玻尔兹曼分布以及超过活化能的曲线下面积相联系。
Catalysts work by providing an alternative pathway with lower activation energy, increasing the proportion of successful collisions without being used up. Drawing a labelled energy profile diagram with the catalyst pathway clearly lower than the uncatalysed peak is often required and must include the correct labels (reactants, products, Eₐ with and without catalyst).
催化剂通过提供一条活化能较低的替代路径，提高有效碰撞的比例，且自身不被消耗。考试常要求绘制带有标注的能级图，催化剂路径的峰值必须明显低于无催化剂路径，并正确标注（反应物、生成物、有和无催化剂的 Eₐ）。
12. Data Analysis and Significant Figures | 数据分析与有效数字
Mishandling significant figures (s.f.) is a routine source of lost marks. When multiplying or dividing, the final answer should have the same number of s.f. as the least precise measurement used. If a mass is given as 2.0 g (2 s.f.) and a volume as 25.00 cm³ (4 s.f.), the calculated density should be given to 2 s.f.
有效数字处理不当是常失分的地方。在乘除运算中，最终答案的有效数字位数应与所用数据中精度最低的一致。若质量为 2.0 g（2位有效数字），体积为 25.00 cm³（4位有效数字），计算出的密度应保留 2 位有效数字。
In tables and graphs, recording all raw data with the same number of decimal places appropriate to the measuring instrument is essential. A digital balance reading 0.500 g must be recorded as 0.500, not 0.5. Dropping trailing zeros changes the precision implied.
在表格和图表中，所有原始数据必须按照测量仪器的精度保留相同的小数位数。读数为 0.500 g 的电子天平必须记录为 0.500，而不是 0.5。省去末尾的零会改变所暗示的精度。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导