📚 IB and WJEC Science: Mastering Common Mistakes | IB WJEC 科学：易错题精讲
In IB and WJEC science assessments, certain concepts trip up students again and again, causing them to lose marks on otherwise straightforward questions. This article unpacks some of the most frequent errors seen in physics, chemistry and biology topics, and provides clear explanations so you can avoid them. Each section highlights a typical misconception, shows why it is wrong, and presents the correct scientific thinking.
在 IB 和 WJEC 科学考试中，某些概念总是在同一个地方绊倒学生，导致他们在原本可以轻松得分的题目上白白丢分。本文拆解物理、化学和生物主题中最常出现的错误，并提供清晰的解释，帮助你彻底避开这些陷阱。每一个小节都指出了一个典型的误解，说明它为什么是错的，并展示正确的科学思维方式。
1. Mass vs. Weight Confusion | 混淆质量和重量
Many students treat mass and weight as the same quantity, writing “the weight of the object is 50 kg”. In physics, mass is the amount of matter in an object, measured in kilograms (kg), while weight is the force of gravity acting on that mass, measured in newtons (N). This mistake appears frequently in both IB and WJEC exam questions, especially when calculating forces or interpreting data.
许多学生把质量和重量当成相同的量，经常写出“物体的重量是50公斤”这样的表述。在物理学中，质量是物体所含物质的多少，单位是千克(kg)，而重量是作用在该质量上的重力，单位是牛顿(N)。这种错误在IB和WJEC的试卷中频繁出现，尤其是涉及力的计算或数据分析时。
The relationship between them is given by the equation:
它们之间的关系由以下公式给出：
W = m × g
On Earth, g ≈ 9.8 N/kg, so an object with a mass of 50 kg has a weight of about 490 N. A common exam trap: a question asks for weight but provides mass, and students simply copy the mass value. Always check the units required. If a balance reads “50 kg”, it is displaying mass; weight must be calculated.
在地球上，g ≈ 9.8 N/kg，因此一个质量为50 kg的物体的重量大约是490 N。常见的考试陷阱：题目要求写出重量却给出质量，学生往往直接抄下质量的数值。一定要检查题目要求的单位。如果秤上显示“50 kg”，那表示的是质量；重量必须进行计算。
In WJEC practical tasks and IB data-based questions, you may be asked to determine g from a graph of weight against mass. The slope of the line equals g. If you mistakenly label the axes the wrong way round, you will obtain an incorrect value. Remember: weight is the dependent variable, mass is the independent variable.
在WJEC的实验任务和IB数据题中，你可能会被要求根据重量-质量图像求g。图像的斜率等于g。如果你错误地把坐标轴标反了，就会得到错误的数值。请记住：重量是因变量，质量是自变量。
2. Unit Conversion Errors: cm³ to m³ | 单位换算错误：立方厘米和立方米
One of the most stubborn mistakes in density and volume calculations is treating 1 m³ as equal to 100 cm³. Because 1 m = 100 cm, students often assume the conversion for volume follows the same factor. In reality, volume scales with the cube of the length, so 1 m³ = (100 cm)³ = 1,000,000 cm³ (10⁶ cm³). This error leads to answers being off by a factor of one million.
密度和体积计算中最顽固的错误之一，就是把1 m³当成100 cm³。由于1 m = 100 cm，学生常常想当然地认为体积换算也遵循同样的倍数。实际上，体积随长度立方变化，所以1 m³ = (100 cm)³ = 1,000,000 cm³ (10⁶ cm³)。这个错误会导致答案相差一百万倍。
When converting a density from g/cm³ to kg/m³, students often incorrectly apply a factor of 1000 instead of the correct factor. Since 1 g/cm³ = 1000 kg/m³, the numerical value increases by a thousand, not decreases. A typical exam question provides a mass in grams and dimensions in centimetres, requiring the density in kg/m³. Many candidates divide by 1000 instead of multiplying, or mix up the steps altogether.
当把密度从g/cm³转换为kg/m³时，学生经常错误地应用1000的倍数，而不是正确的转换因子。实际上 1 g/cm³ = 1000 kg/m³，数值会增大一千倍，而不是减小。一道典型的考题会给出以克为单位的质量和以厘米为单位的尺寸，要求以kg/m³表示密度。许多考生会用除以1000代替乘以1000，或是把步骤完全搞混。
Worked example: A cube of side 2.0 cm has a mass of 12.0 g. Its density in g/cm³ is 12.0 / (2.0)³ = 1.5 g/cm³. In kg/m³, this becomes 1.5 × 1000 = 1500 kg/m³. Always write down the conversion factor explicitly before calculating.
计算示例：一个边长为2.0 cm的立方体，质量为12.0 g。其在g/cm³下的密度为 12.0 / (2.0)³ = 1.5 g/cm³。用kg/m³表示则为 1.5 × 1000 = 1500 kg/m³。计算前最好明确写出换算因子。
3. Balancing Equations and State Symbols | 配平化学方程式和状态符号
In both IB and WJEC chemistry papers, students lose marks for forgetting to balance equations or for using incorrect state symbols. A classic example: Na + Cl₂ → NaCl. The correct balanced equation is 2Na + Cl₂ → 2NaCl. Leaving it unbalanced suggests atoms are not conserved, which violates the fundamental principle of chemical reactions.
在IB和WJEC化学试卷中，学生经常因忘记配平方程式或使用了错误的状态符号而丢分。一个经典例子：Na + Cl₂ → NaCl。正确的配平方程式应写作 2Na + Cl₂ → 2NaCl。如果不配平，暗示原子没有被守恒，这违背了化学反应的基本原则。
State symbols are equally important: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution. A precipitate formed in a double displacement reaction must be marked (s). For example, AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Missing the (s) on AgCl will cost the mark. Similarly, in combustion reactions, H₂O is often produced as a gas, not a liquid, at high temperatures, so H₂O(g) may be required.
状态符号同样重要：(s)是固体，(l)是液体，(g)是气体，(aq)是水溶液。复分解反应中生成的沉淀必须标示(s)。例如：AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)。漏写AgCl的(s)就会丢分。同样，在燃烧反应中，高温下生成的水通常是气态而非液态，因此可能需要写H₂O(g)。
IB often asks students to write ionic equations, where state symbols are crucial for identifying spectator ions. WJEC unit 1 specifications also require correct use of state symbols in symbol equations. Practice by predicting products and checking both atoms and charges balance.
IB考试常要求学生书写离子方程式，此时状态符号对于识别旁观离子至关重要。WJEC第一单元的考纲也要求在符号方程中正确使用状态符号。可通过预测产物并检查原子和电荷是否守恒来进行练习。
4. Significant Figures and Decimal Places | 有效数字与小数位数
Calculation results must be expressed with the appropriate number of significant figures (sf) or decimal places (dp) as specified in the question. A frequent mistake is to copy all the digits from the calculator display without considering the precision of the given data. For instance, if a length is measured as 5.0 cm (2 sf) and a width as 2.00 cm (3 sf), the area should be reported to 2 sf, not as 10.00 cm² from the raw multiplication.
计算结果必须按照题目规定，以恰当的有效数字(sf)或小数位数(dp)表示。常见错误是直接照抄计算器上的所有数字，而不考虑原始数据的精度。例如，长度测量值为5.0 cm（2位有效数字），宽度为2.00 cm（3位有效数字），面积就应该保留2位有效数字，汇报成10 cm²，而不是把乘法算出的10.00 cm²直接抄上去。
Rules for significant figures include: leading zeros are not significant (0.0056 has 2 sf); trailing zeros in a decimal number are significant (0.5600 has 4 sf). In WJEC practical examinations, students may be penalized for giving too many decimal places on a calculated density or titration result. IB data analysis questions explicitly test this skill, often requiring uncertainties to be expressed to the same number of decimal places as the measurement.
有效数字的规则包括：前导零不算有效数字（0.0056有2位有效数字）；小数点后末尾的零则算（0.5600有4位有效数字）。在WJEC实验考试中，若计算出的密度或滴定结果给出了过多的小数位数，可能会被扣分。IB数据分析题明确考察这项技能，常要求不确定度与测量值保留相同的小数位数。
When multiplying or dividing, the answer should have the same number of sf as the measurement with the fewest sf. When adding or subtracting, the answer should have the same number of dp as the measurement with the fewest dp. Always round at the very end of the calculation, not during intermediate steps.
在乘除运算中，答案应保留与有效数字最少的测量值相同的有效数字位数；在加减运算中，答案应保留与小数位数最少的测量值相同的小数位数。务必在计算的最后一步才进行修约，不要在中间步骤提前取整。
5. Misinterpreting Velocity–Time Graphs | 速度–时间图的误解
Students often confuse velocity–time graphs with displacement–time graphs, leading to errors when describing motion. On a velocity–time graph, the gradient represents acceleration, not velocity, and the area under the graph gives the displacement, not the distance travelled (unless the motion is in a straight line without change of direction).
学生常常混淆速度–时间图和位移–时间图，导致在描述运动时出错。在速度–时间图上，斜率表示的是加速度，而不是速度；曲线下的面积给出的是位移，而不一定是路程（除非是直线运动且方向不变）。
A flat horizontal line on a velocity–time graph means constant velocity, not the object being at rest. Many candidates incorrectly state the object is stationary. Stationary is represented by the line lying on the time axis (v = 0). An upward sloping line indicates positive acceleration; a downward sloping line indicates negative acceleration (deceleration).
速度–时间图上的一条水平直线意味着速度恒定，而不是物体静止。许多考生错误地认为这表示物体处于静止状态。静止是由位于时间轴上的线段（v = 0）来表示。向上倾斜的线段表示正加速度，向下倾斜的线段表示负加速度（减速）。
In both IB and WJEC questions, you may be asked to calculate distance from a graph with both positive and negative velocity. Remember: the total distance is the sum of the magnitudes of the areas, whereas displacement is the net area. If the line crosses the time axis, separate the area into positive and negative parts and add their absolute values for distance.
在IB和WJEC的题目中，可能会让你从同时包含正速度和负速度的图像中计算路程。记住：总路程是各面积绝对值的总和，而位移是净面积（带符号）。如果线段穿过时间轴，就要把面积分成正负两部分，再将它们的绝对值相加得到路程。
6. Current and Voltage in Series and Parallel | 串联与并联电路中的电流和电压
A classic mix-up is swapping the rules for current and voltage in series and parallel circuits. In a series circuit, the current is the same at all points, but the voltage across each component adds up to the supply voltage. In a parallel circuit, the voltage across each branch is the same as the supply voltage, while the currents in the branches add up to the total current from the source.
一个经典的混淆点就是把串联和并联电路中的电流与电压规则弄反。在串联电路中，各处电流相等，但每个元件两端的电压之和等于电源电压。在并联电路中，各支路两端的电压与电源电压相等，而各支路的电流之和等于干路总电流。
Students often write “current is used up by components” or “voltage stays the same throughout a series circuit”. Both are incorrect. Current is the rate of flow of charge and is conserved; it is never ‘used up’. In a series circuit, identical bulbs will share the supply voltage equally, but if the bulbs have different resistances, they will have different voltages across them.
学生经常写出“电流被元件消耗掉了”或“串联电路中各处电压相等”之类的结论。这两种说法都是错误的。电流是电荷流动的速率，它是守恒的，永远不会被“消耗”。在串联电路中，相同的灯泡会平分电源电压，但如果灯泡电阻不同，它们两端的电压也会不同。
Circuit calculation questions in IB and WJEC require applying V = IR correctly in combination with these rules. A common pitfall: using total voltage with a branch resistance to find branch current in a parallel circuit without remembering that the branch voltage is the full supply voltage. Draw arrows to indicate current paths and label known voltages to avoid mistakes.
IB和WJEC的电路计算题要求结合这些规则正确使用V = IR。一个常见陷阱：在并联电路中，用总电压去除以支路电阻求支路电流的时候，忘了支路电压就是完整的电源电压。可以画箭头表示电流路径并标出已知电压，以避免出错。
7. Photosynthesis vs. Respiration Equations | 光合作用与呼吸作用方程式混淆
The word equations and symbol equations for photosynthesis and aerobic respiration are often written in reverse or with missing reactants. Photosynthesis uses carbon dioxide and water to produce glucose and oxygen, driven by light energy. Aerobic respiration uses glucose and oxygen to release energy, producing carbon dioxide and water.
光合作用和有氧呼吸的文字方程式与符号方程式经常被写反，或者漏掉反应物。光合作用利用二氧化碳和水，在光能驱动下生成葡萄糖和氧气。有氧呼吸则利用葡萄糖和氧气释放能量，产生二氧化碳和水。
The correct balanced symbol equations are:
正确的配平符号方程式为：
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
In WJEC Biology units, students may be asked to state that light energy is converted to chemical energy in photosynthesis, while respiration releases energy for cellular activities. IB questions often ask for the source of oxygen released during photosynthesis; the oxygen atoms come from water, not carbon dioxide. This subtlety is easily overlooked.
在WJEC生物单元的考题中，学生可能需要答出光能在光合作用中被转化为化学能，而呼吸作用为细胞活动释放能量。IB考题常问光合作用释放的氧气的来源，氧气原子来自水，而不是二氧化碳。这种细节很容易被忽略。
Another common error is forgetting that respiration occurs in all living cells, including plant cells. Some students believe plants only photosynthesise and do not respire. In reality, plants respire continuously; photosynthesis happens only in the presence of light, and the net gas exchange depends on light intensity.
另一个常见错误是忘记呼吸作用发生在所有活细胞中，包括植物细胞。一些学生认为植物只进行光合作用而不呼吸。但实际上植物一直在进行呼吸作用；光合作用只在有光的条件下发生，净气体交换取决于光照强度。
8. Endothermic and Exothermic Mix-up | 吸热与放热反应的混淆
Endothermic and exothermic are frequently reversed in explanations. An exothermic reaction releases energy to the surroundings, causing the temperature of the surroundings to rise. An endothermic reaction absorbs energy from the surroundings, causing the temperature to drop. Students often write that an endothermic reaction “makes things hot” because they focus on the reaction vessel feeling warm, forgetting that the system is absorbing heat.
吸热和放热在解释时经常被弄反。放热反应向周围环境释放能量，使得周围温度上升；吸热反应则从周围环境吸收能量，导致周围温度下降。学生常常因为感觉反应容器变热而把吸热写作“使东西变热”，却忘了系统是在吸热。
Classic examples: combustion of fuels, respiration and neutralisation are exothermic; photosynthesis, thermal decomposition of calcium carbonate, and the reaction between citric acid and sodium hydrogencarbonate are endothermic. In IB, you need to interpret energy profile diagrams correctly: exothermic reactions have products lower in energy than reactants (ΔH negative); endothermic reactions have products higher (ΔH positive).
典型的例子：燃料燃烧、呼吸作用和中和反应是放热的；光合作用、碳酸钙的热分解以及柠檬酸与碳酸氢钠的反应是吸热的。在IB中，你需要正确解读能量分布图：放热反应的生成物能量低于反应物（ΔH为负）；吸热反应的生成物能量则高于反应物（ΔH为正）。
WJEC often examines energy changes through simple calorimetry questions. If the temperature of water increases, the reaction being investigated is exothermic. A common error is to assign the sign of the enthalpy change incorrectly when calculating ΔH in J/mol. Make sure you know that a temperature rise corresponds to a negative ΔH in the context of the system.
WJEC经常通过简单的量热法考查能量变化。如果水温升高，说明被研究的反应是放热的。常见错误是在计算ΔH（J/mol）时给焓变配错正负号。务必明确：温度升高，对于反应体系而言，ΔH为负。
9. Misunderstanding the Role of Catalysts | 催化剂作用的误解
A very persistent myth is that a catalyst “does not take part in the reaction” or is “unchanged after the reaction, so it does not interact”. While it is true that a catalyst remains chemically unchanged at the end, it does participate by providing an alternative reaction pathway with a lower activation energy. It forms intermediate compounds which then break down to regenerate the catalyst.
一个很顽固的误区是：催化剂“不参与反应”，或者“反应前后不变，所以它不与反应物作用”。虽然催化剂在反应结束后化学性质确实不变，但它的确参与了反应，通过提供一条活化能更低的替代反应路径而起作用。它会形成中间产物，这些中间产物随后分解再生成催化剂。
In both IB Chemistry and WJEC specifications, students must know that a catalyst increases the rate of reaction without affecting the equilibrium position or the yield. It does this by lowering the activation energy so that a greater proportion of particles have energy equal to or greater than the activation energy. It does not provide energy or become a reactant that is used up.
在IB化学和WJEC考纲中，学生必须知道催化剂能提高反应速率，但不影响平衡位置或产率。它通过降低活化能，使得更大比例的粒子具有大于或等于活化能的能量。它不是能量的提供者，也不会成为被消耗的反应物。
A common exam trap: a graph showing a Maxwell–Boltzmann distribution with a new activation energy line drawn to the left of the original. Students may incorrectly state that the catalyst increases the number of particles with higher energy. The correct interpretation is that the catalyst lowers the energy threshold, so a larger fraction of the existing particles can react.
常见的考试陷阱：给出麦克斯韦–玻尔兹曼分布图，并在原活化能线的左侧画了一条新的活化能线。学生可能错误地声称催化剂增加了高能量粒子的数量。正确的解读是：催化剂降低了能量门槛，因此现有粒子中有更大比例的能够发生反应。
When asked to explain why a catalyst does not increase yield in a reversible reaction, remember that it speeds up both the forward and reverse reactions equally, so the equilibrium composition remains unchanged. This point is often tested in questions about the Haber process or contact process.
被问到为什么催化剂不会增加可逆反应的产率时，要记住它同等程度地加速了正反应和逆反应的速率，因此平衡组成保持不变。这一点经常在有关哈柏法或接触法的考题中出现。
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Cancel reply