IGCSE Science: Common Mistake Analysis | IGCSE 科学：易错题精讲

📚 IGCSE Science: Common Mistake Analysis | IGCSE 科学：易错题精讲

IGCSE Science, whether you are studying Combined or Coordinated Sciences, often features certain questions that repeatedly catch students out. These mistakes are not due to a lack of effort, but rather to common misconceptions or a tendency to memorise exam phrasing without truly understanding the underlying principles. This article brings together ten such tricky areas from physics, chemistry, and biology, unpacks the typical errors, and gives you clear, correct explanations. Mastering these will sharpen your exam technique and help you avoid losing valuable marks.

IGCSE 科学（无论是综合科学还是协调科学）经常出现一些让学生反复出错的题目。这些错误并非因为不够努力，而是源于常见的误解，或者只是机械记忆了考题的措辞，却没有真正理解背后的原理。本文汇集了来自物理、化学和生物的十个容易踩坑的知识点，剖析典型错误，并给出清晰、正确的解释。吃透这些内容能打磨你的应试技巧，帮助你避免丢掉宝贵的分数。

1. Why Graphite Conducts Electricity | 石墨为什么能导电

A very common exam answer claims that graphite conducts electricity because it has ‘free ions’ or ‘mobile ions’ that carry charge. This is incorrect and stems from confusing the structure of graphite with that of ionic compounds when molten or dissolved.

一个非常常见的考试答案是，石墨因含有“自由离子”或“可移动的离子”而导电。这个说法是错误的，源于学生把石墨的结构与熔融或溶解状态下的离子化合物混淆了。

Graphite is a giant covalent structure made entirely of carbon atoms. Each carbon atom forms three strong covalent bonds with other carbons in a layered arrangement. This leaves one delocalised electron per carbon atom, which is not tied to any particular bond. These delocalised electrons can move freely parallel to the layers, allowing graphite to conduct electricity. Ions are not present.

石墨是一种巨型共价结构，完全由碳原子构成。每个碳原子与其他碳原子形成三个强共价键，排列成层状。这样每个碳原子剩下一个离域电子，不属于任何特定的键。这些离域电子可以沿着层间自由移动，从而使石墨能够导电。这里根本不存在离子。

Remember: graphite conducts electricity due to delocalised electrons, whereas molten ionic compounds (like sodium chloride) conduct because their ions are free to move. Even diamond, another giant covalent structure of carbon, cannot conduct electricity because all its outer electrons are used in covalent bonds, leaving no free charge carriers.

请记住：石墨导电靠的是离域电子，而熔融的离子化合物（如氯化钠）导电是因为离子可以自由移动。即便是另一种碳的巨型共价结构——金刚石，也无法导电，因为它所有的外层电子都用于形成共价键，没有留下自由电荷载体。

2. Catalysts and Chemical Equilibrium | 催化剂与化学平衡

Many students learn that a catalyst ‘increases the yield’ of a reaction or ‘shifts the equilibrium to the right’. This is a dangerous misconception, especially in the context of reversible reactions like the Haber process.

许多学生学到，催化剂能“提高产率”或“使平衡向右移动”。这是一个危险的理解误区，特别是在哈伯法等可逆反应的题目中。

A catalyst speeds up both the forward and backward reactions equally. It provides an alternative reaction pathway with a lower activation energy, but it does not affect the position of equilibrium. The equilibrium yield of products remains exactly the same. Adding a catalyst only helps the system reach equilibrium faster, which is why it is used industrially to save time and energy, not to improve the percentage yield.

催化剂会同等程度地加快正反应和逆反应的速率。它提供了一个活化能更低的替代反应途径，但不会影响平衡的位置。生成物的平衡产率完全不变。加入催化剂只是帮助体系更快地达到平衡，这也是工业上用它来节省时间和能源的原因，而不是为了提高百分产率。

If a question asks about increasing product yield, you must look at conditions affecting equilibrium (temperature, pressure, concentration). A catalyst is only correct if the question asks about increasing the rate of reaction.

如果题目问如何提高产物产率，你必须考虑影响平衡的条件（温度、压强、浓度）。催化剂只有在题目问到如何提高反应速率时才可能是正确答案。

3. The Source of Oxygen in Photosynthesis | 光合作用中氧气的来源

It is tempting to think that the oxygen released during photosynthesis comes from the splitting of carbon dioxide molecules. After all, the overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, and it seems logical that O₂ comes from CO₂. This is wrong.

人们很容易认为光合作用释放的氧气来自二氧化碳分子的分解。毕竟，总方程式是 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂，看起来氧气来自二氧化碳也很合理。但这是错误的。

Using radioactive oxygen-18 tracers, scientists proved that all the oxygen gas produced in photosynthesis originates from water molecules, not from carbon dioxide. During the light-dependent reactions, water is split (photolysis) into hydrogen ions, electrons, and oxygen gas. The hydrogen and electrons are then used to reduce carbon dioxide in the Calvin cycle to form glucose, but the oxygen by‑product is solely from H₂O.

科学家利用氧-18 放射性同位素示踪剂证明，光合作用产生的所有氧气都来自水分子，而非二氧化碳。在光反应阶段，水被裂解（光解）成氢离子、电子和氧气。接着，氢和电子用于在卡尔文循环中还原二氧化碳以形成葡萄糖，但副产物氧气完全来自水。

This distinction is frequently tested with a simple question: ‘Where does the oxygen released in photosynthesis come from?’ The correct answer is ‘from water’ or ‘from the photolysis of water’. Mentioning carbon dioxide will be marked incorrect.

这个区别常常以一个简单的问题来考查：“光合作用释放的氧气来自哪里？”正确的答案是“来自水”或“来自水的光解”。提到二氧化碳将被判为错误。

4. Current, Voltage and Resistance Misunderstandings | 电流、电压和电阻的误解

A common exam error is to say that ‘if current increases, resistance decreases’ or to treat resistance as a variable that automatically adjusts to voltage. This confuses cause and effect, and often leads to misapplying Ohm’s law.

一个常见的考试错误是说“如果电流增大，电阻就减小”，或者把电阻当作一个能随电压自动调节的变量。这混淆了因果关系，并且常常导致欧姆定律被误用。

For a fixed ohmic conductor at constant temperature, resistance is a property of the component itself and does not change simply because you apply a different voltage. According to Ohm’s law, V = I × R, the current through the resistor is directly proportional to the voltage across it, provided the temperature stays the same. If you double the voltage, the current doubles, but the resistance remains constant.

对于一个温度恒定的固定欧姆导体，电阻是元件本身的性质，不会仅仅因为施加了不同的电压就改变。根据欧姆定律，V = I × R，只要温度不变，通过电阻器的电流与其两端的电压成正比。如果把电压加倍，电流也会加倍，但电阻保持不变。

Resistance only changes if the component itself is altered (e.g., by changing its length in a wire) or if the temperature changes significantly, as in a filament bulb or thermistor. Always remember: R = V / I is a definition; it does not mean that R is inversely proportional to I for a given component.

电阻只有在元件本身被改变（例如改变导线的长度）或温度显著变化时（如灯丝灯泡或热敏电阻）才会改变。请始终记住：R = V / I 是定义式；对于给定的元件，它并不表示 R 与 I 成反比。

5. pH After Neutralisation: Strong vs Weak Acids | 中和后的 pH：强酸与弱酸

After neutralising an acid with an alkali, many students automatically write that the final pH of the solution is exactly 7. This is only true for certain combinations, and failing to recognise exceptions loses marks on titration questions.

用碱中和酸之后，许多学生不假思索地写出溶液的最终 pH 恰好为 7。这只对某些组合成立，未能识别例外情况会在滴定题目中丢分。

When a strong acid (e.g., hydrochloric acid) is completely neutralised by a strong alkali (e.g., sodium hydroxide), the resulting salt solution contains ions that do not hydrolyse to affect pH, so the pH is 7 at the equivalence point. However, if a weak acid such as ethanoic acid is neutralised by a strong alkali, the salt formed (sodium ethanoate) is slightly basic, and the pH at equivalence will be greater than 7. Conversely, a weak base with a strong acid gives a slightly acidic salt and a pH less than 7 at equivalence.

当强酸（如盐酸）被强碱（如氢氧化钠）完全中和时，生成的盐溶液中的离子不会水解影响 pH，因此在等当点时 pH 为 7。然而，如果是弱酸（如乙酸）被强碱中和，生成的盐（乙酸钠）呈弱碱性，等当点的 pH 会大于 7。反过来，弱碱与强酸生成的盐呈弱酸性，等当点 pH 小于 7。

Do not state ‘pH 7’ as a universal truth for all neutralisations. The term ‘neutralisation’ simply means the acid and base have reacted to form a salt and water; it does not automatically guarantee a neutral solution of exactly pH 7.

不要把“pH 为 7”当作所有中和反应的普遍真理。“中和”一词只表示酸和碱反应生成了盐和水；它并不自动保证溶液是恰好 pH 7 的中性溶液。

6. Confusion Between Mass and Weight | 质量与重量的混淆

In everyday language we often use ‘weight’ when we mean ‘mass’, but in IGCSE Science they are distinct concepts. A typical mistake is stating that the mass of an astronaut changes when they go to the Moon, or that weight is measured in kilograms.

在日常用语中，我们常常在本该用“质量”的地方说“重量”，但在 IGCSE 科学中它们是不同的概念。一个典型的错误是说宇航员到月球后质量改变了，或者说重量的单位是千克。

Mass is the amount of matter in an object and is a scalar quantity measured in kilograms (kg). It does not depend on location. Weight is the force acting on an object due to gravity, calculated by W = m × g, where g is the gravitational field strength. Weight is a vector and is measured in newtons (N). On the Moon, g is about 1.6 N/kg, so the astronaut’s weight is much less than on Earth, but their mass remains unchanged.

质量是物体所含物质的多少，是标量，单位为千克（kg），与位置无关。重量是由于重力作用在物体上的力，计算公式为 W = m × g，其中 g 是引力场强度。重量是矢量，单位为牛顿（N）。在月球上，g 大约为 1.6 N/kg，因此宇航员的重量远小于在地球上，但其质量保持不变。

When asked ‘What does a balance measure?’, the correct answer is mass, not weight. Many balances are calibrated to give a reading in kilograms by using the local g value, but fundamentally they compare masses.

当被问到“天平测量什么？”，正确的答案是质量，而不是重量。许多天平通过使用当地 g 值进行校准，从而给出以千克为单位的读数，但它们本质上是比较质量的。

7. Density Unit Conversions and Calculation Pitfalls | 密度的单位换算与计算陷阱

Questions on density are often straightforward, but a heavy mark penalty comes from incorrect unit conversions, especially between g/cm³ and kg/m³. Students frequently multiply by 1000 instead of using the correct factor, or they forget to convert units for mass and volume before dividing.

密度的题目通常很直接，但严重的扣分来自错误的单位换算，特别是在 g/cm³ 和 kg/m³ 之间转换时。学生常常错误地乘以 1000 而不是使用正确的系数，或者在相除之前忘记换算质量和体积的单位。

1 g/cm³ is equivalent to 1000 kg/m³, not 1 kg/m³. This is because 1 m³ = (100 cm)³ = 1 000 000 cm³, and 1 kg = 1000 g. Therefore, 1 g/cm³ = 0.001 kg / 0.000 001 m³ = 1000 kg/m³. To convert from g/cm³ to kg/m³, you multiply by 1000, not by 1/1000. Also, when calculating density using ρ = m / V, ensure that mass is in grams or kilograms and volume is in the matching cubic units; never mix cm³ with kg unless you convert.

1 g/cm³ 等于 1000 kg/m³，而不是 1 kg/m³。这是因为 1 m³ = (100 cm)³ = 1 000 000 cm³，而 1 kg = 1000 g。所以，1 g/cm³ = 0.001 kg / 0.000 001 m³ = 1000 kg/m³。要从 g/cm³ 转换成 kg/m³，需要乘以 1000，而不是除以 1000。此外，在使用 ρ = m / V 计算密度时，要确保质量用克或千克，体积用对应的立方单位；绝不能将 cm³ 和 kg 混用，除非经过换算。

A classic exam task: an object has a mass of 500 g and a volume of 200 cm³. Find its density in kg/m³. Step 1: density in g/cm³ = 500 / 200 = 2.5 g/cm³. Step 2: convert to kg/m³: 2.5 × 1000 = 2500 kg/m³. Missing the ×1000 step loses the marks.

一个经典的考题：一个物体的质量为 500 g，体积为 200 cm³，求其密度（以 kg/m³ 表示）。第一步：密度以 g/cm³ 计 = 500 / 200 = 2.5 g/cm³。第二步：转换为 kg/m³：2.5 × 1000 = 2500 kg/m³。漏掉乘以 1000 这一步就会丢分。

8. Osmosis Misconceptions in Biology | 生物中的渗透作用误解

Osmosis is frequently described incorrectly as ‘the movement of water and solutes across a membrane’ or ‘the movement of water from a low concentration to a high concentration’. These imprecise phrasings are penalised because they miss the essential concepts of a partially permeable membrane and water potential.

渗透作用经常被错误地描述为“水和溶质穿过膜的运动”或“水从低浓度向高浓度运动”。这些不严谨的表述会被扣分，因为它们遗漏了半透膜和水势的关键概念。

Osmosis is the net diffusion of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane. Only water moves; dissolved solutes cannot pass freely through the membrane. The direction of movement is determined by the water potential gradient, not by solute concentration alone.

渗透作用是水分子通过半透膜，从水势较高的区域（稀溶液）到水势较低的区域（浓溶液）的净扩散。只有水分子移动；溶解的溶质不能自由穿过膜。移动的方向取决于水势梯度，而不仅仅是溶质浓度。

To describe osmosis accurately in an exam, you must mention all three key elements: (1) water molecules, (2) partialy permeable membrane, and (3) the water potential gradient (higher to lower). Saying ‘from a low solute concentration to a high solute concentration’ can be accepted by some mark schemes, but explicitly using ‘water potential’ is the safest and most precise approach.

要在考试中准确描述渗透作用，你必须提到三个关键要素：（1）水分子，（2）半透膜，以及（3）水势梯度（从高到低）。说“从低溶质浓度到高溶质浓度”可能被某些评分方案接受，但明确使用“水势”是最安全、最精确的方式。

9. Reactivity Series and Displacement Reactions | 金属活动性顺序与置换反应

When predicting whether a displacement reaction will occur, students often rely on memorised pairs rather than applying the reactivity series logically. A common error is to claim that copper can displace zinc from a solution, or that silver reacts with dilute acids to liberate hydrogen.

在预测是否会发生置换反应时，学生常依赖于记忆的成对例子，而不是逻辑地应用金属活动性顺序。一个常见错误是声称铜能从锌的盐溶液中置换出锌，或者银能与稀酸反应放出氢气。

A more reactive metal can displace a less reactive metal from an aqueous solution of its salt. For instance, if you place a piece of zinc metal in copper(II) sulfate solution, zinc will displace copper ions because zinc is above copper in the reactivity series: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). However, if you put copper into zinc sulfate solution, no reaction occurs. The rule also applies to metal–acid reactions: only metals above hydrogen in the reactivity series react with dilute acids to produce hydrogen gas.

更活泼的金属能够从其盐溶液中置换出较不活泼的金属。例如，把锌片放入硫酸铜(II)溶液中，锌会置换出铜离子，因为锌在金属活动性顺序中排在铜之上：Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)。然而，如果把铜放入硫酸锌溶液中，则无反应发生。此规律也适用于金属与酸的反应：只有活动性排在氢以上的金属才能与稀酸反应产生氢气。

Always check the position in the reactivity series. A mistake could be thinking that iron can displace aluminium, but aluminium is more reactive. Aluminium can displace iron from iron oxide in the thermite reaction, iron cannot displace aluminium from its oxide. Be precise.

一定要检查在活动性顺序中的位置。一个错误可能是认为铁能置换出铝，但铝更活泼。铝可以在铝热反应中从氧化铁中置换出铁，铁却不能从氧化铝中置换出铝。务必准确。

10. Calculating Energy Efficiency Correctly | 正确计算能量效率

Energy efficiency calculations appear easy, yet many candidates lose marks by mixing up useful output and total input, or by leaving the answer as a decimal instead of a percentage when required. Additionally, the concept of ‘efficiency can be greater than 100%’ is sometimes mistakenly believed.

能量效率的计算貌似简单，但很多考生会混淆有用输出和总输入而丢分，或者在要求百分比时把答案留成了小数。此外，有时还会错误地相信“效率可以超过 100%”。

The formula for efficiency is: Efficiency = (Useful energy output / Total energy input) × 100%. The useful output is always smaller than the input for any real device, so efficiency is always less than 100% due to energy dissipated as thermal energy to the surroundings. If a question gives a Sankey diagram, identify the useful output arrow width along with the total input width; do not accidentally use the wasted energy in the numerator.

效率公式为：效率 =（有用能量输出 / 总能量输入）× 100%。对于任何真实设备，有用输出总是小于输入，因此效率总是低于 100%，因为有一部分能量会以热能的形式散失到周围环境中。如果题目给出桑基图，要认准有用输出箭头的宽度以及总输入宽度；不要误把浪费的能量放在分子上。

A common exam blunder: a motor uses 200 J of electrical energy and delivers 150 J of useful kinetic energy. Some students mistakenly write (200/150)×100 = 133%, which is physically impossible. The correct calculation is (150/200)×100 = 75%. Also ensure the answer is clearly written with units as required – typically either a decimal (0.75) or as a percentage (75%).

一个常见的考试失误：一台电动机消耗了 200 J 的电能，提供了 150 J 的有用动能。有些学生错误地写成 (200/150)×100 = 133%，这在物理学上是不可能的。正确的计算是 (150/200)×100 = 75%。还要确保答案按要求清晰写出单位——通常是小数（0.75）或百分比（75%）。

11. Confusing Respiration with Breathing | 混淆呼吸作用与呼吸

In biology, the terms ‘respiration’ and ‘breathing’ are often used interchangeably in everyday English, but they have distinct scientific meanings. Students frequently lose marks by stating that respiration is the same as breathing, or that it happens only in the lungs.

在生物学中，“呼吸作用”和“呼吸”（通气）在日常英语中常被混用，但它们有明确的科学定义。学生经常因声称呼吸作用就是呼吸，或者只发生在肺里而丢分。

Respiration is a chemical process that occurs in every living cell, in which glucose is oxidised to release energy. It is summarised by the equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy). Breathing, or ventilation, is the physical process of inhaling and exhaling air to supply oxygen for respiration and remove carbon dioxide. Breathing occurs at the lungs, but respiration takes place in all cells, particularly in the mitochondria.

呼吸作用是一个发生在每个活细胞中的化学过程，葡萄糖被氧化以释放能量。它可以用以下方程式概括：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O（+ 能量）。呼吸（通气）是指吸气和呼气以供应呼吸作用所需的氧气并排出二氧化碳的物理过程。呼吸发生在肺里，而呼吸作用则在所有细胞中进行，主要在线粒体中。

Never say ‘we respire through our lungs’; say ‘we breathe using our lungs’ and ‘our cells carry out respiration’. This clarity secures marks on definitions and comparison questions.

永远不要说“我们通过肺进行呼吸作用”；要说“我们用肺呼吸”和“我们的细胞进行呼吸作用”。这种清晰的表述能在定义和比较题上确保得分。

12. Electrolysis and Selective Discharge of Ions | 电解与离子的选择性放电

Predicting the products of electrolysis of aqueous solutions consistently trips students up, especially when the solution is not a simple melt. A classic mistake is assuming that the metal ion is always discharged at the cathode, ignoring the presence of hydrogen ions from water.

预测水溶液电解的产物常常让学生栽跟头，尤其是当溶液不是简单熔融状态时。一个经典的错误是假设金属离子总是在阴极放电，而忽略了来自水的氢离子。

During electrolysis of an aqueous solution such as sodium chloride, the cathode attracts positive ions. Both Na⁺ ions and H⁺ ions (from water) are present. Hydrogen is less reactive than sodium, so H⁺ ions are preferentially discharged to form hydrogen gas. Similarly, at the anode, chloride ions and hydroxide ions (from water) compete; in concentrated NaCl, Cl⁻ ions discharge to give chlorine gas, while in dilute solution, OH⁻ ions may discharge to give oxygen. The reactivity of the metal determines cathode selectivity: if the metal is more reactive than hydrogen (e.g., potassium, sodium), hydrogen gas is produced; if the metal is less reactive (e.g., copper), the metal itself plates out.

在像氯化钠这样的水溶液电解过程中，阴极吸引阳离子。溶液中同时存在 Na⁺ 离子和 H⁺ 离子（来自水）。氢不如钠活泼，所以 H⁺ 离子优先放电生成氢气。同样，在阳极，氯离子和氢氧根离子（来自水）会竞争放电；在浓氯化钠溶液中，Cl⁻ 离子放电产生氯气，而在稀溶液中，OH⁻ 离子可能放电产生氧气。金属的活泼性决定阴极的选择性：如果金属比氢更活泼（如钾、钠），则产生氢气；如果金属较不活泼（如铜），该金属本身就会析出。

Remember these rules: at the cathode, the less reactive cation is discharged; at the anode, a halide ion is usually discharged if present, otherwise hydroxide ions discharge to give oxygen. Always check the concentration and the reactivity series before giving your final products.

记住这些规则：在阴极，较不活泼的阳离子放电；在阳极，如果存在卤素离子，通常会放电，否则氢氧根离子放电生成氧气。在给出最终产物之前，一定要确认浓度和金属活动性顺序。

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IGCSE Science: Common Mistake Analysis | IGCSE 科学：易错题精讲