📚 IB Science: Common Mistake Questions Explained | IB 科学：易错题精讲
Many IB Science students lose marks not from lack of knowledge, but from misinterpretation, careless errors, or misconceptions that examiners specifically target. This article analyses common pitfall questions across Physics, Chemistry and Biology, offering clear explanations and practical strategies to avoid these mistakes.
许多IB科学考生丢分并非因为知识不足，而是由于阅卷者特意设计的陷阱题，容易在理解偏差、粗心或错误观念上失分。本文分析物理、化学、生物中常见的易错题，提供清晰的解释和实用的避免策略。
1. Graph Interpretation and the Origin | 图表解读与原点陷阱
A classic mistake is to assume every graph must start at (0,0). In many IB data-based questions, the y-axis may be broken or the independent variable naturally starts above zero. For instance, a velocity–time graph might show an initial velocity v₀ > 0, yet students incorrectly calculate displacement by assuming a triangle from the origin. Always check axis scales and starting points, and never extrapolate without justification.
一个经典错误是假设所有图表都必须从(0,0)开始。在许多IB基于数据的题目中，纵轴可能被截断，或者自变量天然地从零以上开始。例如，速度–时间图可能显示初速度v₀ > 0，但学生却错误地假设从原点出发的三角形来计算位移。务必检查坐标轴刻度和起点，不要在没有依据的情况下外推。
Students often confuse the slope of a line with the ratio of coordinates at a point. In a resistance (V–I) graph, if current I is on the x-axis, the slope gives the reciprocal of resistance. Always identify what the gradient physically represents, rather than just reading a value. Additionally, ignore outliers only with valid reasons; arbitrarily removing data points to fit a trend is a common error.
学生常将直线上某点的纵坐标比值与斜率混淆。在电阻(V–I)图中，如果电流I在x轴，斜率表示电阻的倒数。要始终识别梯度在物理上代表什么，而不是仅仅读取数值。此外，只有在有正当理由时才剔除异常值；为拟合趋势而随意删除数据点是常见错误。
2. Unit Conversions and Significant Figures | 单位换算与有效数字
. Also, quoting results to the correct number of significant figures is crucial: if given data has two significant figures, an answer like 2.458 is penalised.
计算中常见错误是忽略单位的一致性转换。例如，求密度时，学生可能用克表示质量、立方厘米表示体积，但最终答案单位要求是kg/m³。记住1 cm³ = 1 × 10⁻⁶ m³。此外，用正确有效数字位数表示结果至关重要：如果给出的数据是两位有效数字，答案写2.458会被扣分。
the conversion of linear dimensions with area or volume is a typical slip that leads to answers out by factors of 10ⁿ.
在转换平方或立方单位时，换算因子也要平方或立方。1 m² = 10⁴ cm²，而不是100 cm²。将线性维度转换与面积或体积混淆是典型失误，会导致答案相差10的幂次倍。
3. Chemical Equilibrium and Le Chatelier's Principle | 化学平衡与勒夏特列原理
Many students wrongly believe that adding a catalyst shifts the equilibrium to the product side. A catalyst speeds up both the forward and reverse reactions equally, so the equilibrium composition remains unchanged. It only helps the system to reach equilibrium faster.
许多学生误以为加入催化剂会使平衡向产物方向移动。催化剂同等程度地加快正反应和逆反应速率，因此平衡组成保持不变。它只是帮助体系更快达到平衡。
For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), an increase in pressure shifts the equilibrium to the side with fewer gas molecules – the right. However, some students think the shift goes towards the side with higher molar mass. Always compare the total number of gaseous moles on each side.
对于反应 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，增加压力会使平衡向气体分子数较少的一侧移动，即右侧。但有些学生认为会向摩尔质量较大的一侧移动。始终比较两侧气体总物质的量。
4. Internal Resistance in Electrical Circuits | 电路中的内阻
oltage as equal to its EMF even when current flows. In reality, the terminal voltage V = ε – result will be too large. Always check whether internal resistance is negligible or given.
常见陷阱是当电流流动时仍将电池端电压视为等于其电动势EMF。实际上，端电压V = ε – Ir，其中r为内阻。如果学生直接用ε计算外电路电流而不考虑Ir，结果会偏大。始终判断内阻是否可忽略或题目是否给出。
Additionally, the power delivered to the load is maximised when the external resistance equals the internal resistance. Mistaking this for the condition of maximum efficiency is another typical error; efficiency actually decreases as more power is dissipated internally.
此外，当外电阻等于内阻时，输送给负载的功率最大。将此条件误以为是效率最高的条件是另一个典型错误；实际上，效率会随着内部耗散功率增大而降低。
5. Pedigree Analysis and Genetic Crosses | 系谱分析与遗传杂交
When analysing a pedigree, students often jump to autosomal dominant upon seeing affected individuals in every generation. However, X-linked recessive traits can also appear in every generation if female carriers are present. Always check for affected males who inherit the trait from carrier mothers, and note the absence of male-to-male transmission, which rules out X-linked recessive.
分析系谱时，学生看到每代都有患者就轻易断定为常染色体显性遗传。然而，如果存在女性携带者，X连锁隐性也可能每代出现。务必检查患病男性是否从携带者母亲获得性状，并注意是否不存在男传男现象，这可以排除X连锁隐性。
Another pitfall is forgetting that in autosomal recessive crosses, unaffected parents can have affected children if both are carriers. Students sometimes wrongly assume parents must show the trait for it to appear in offspring.
另一个易错点是忘记在常染色体隐性杂交中，表型正常的父母若都是携带者，可以生出患病子女。学生有时错误地认为父母必须表现出性状才会遗传给后代。
6. Vector Components in Mechanics | 力学中的矢量分解
On an inclined plane, the weight mg is resolved into components parallel and perpendicular to the slope. If the angle of incline is θ, the component down the slope is mg sin θ, and the normal reaction balances mg cos θ. A frequent mistake is using cos for the parallel component simply because the angle is ‘adjacent’ in the drawn triangle; recall that the angle between the weight vector and the perpendicular to the slope is also θ, making the parallel side opposite to that angle.
在斜面上，重力mg分解为平行于斜面和垂直于斜面的分量。若倾角为θ，沿斜面向下的分量为mg sin θ，支持力与mg cos θ平衡。常见错误是仅仅因为在绘制的三角形中该角“邻边”就使用余弦表达平行分量；要记住重力矢量与斜面垂线之间的夹角也是θ，因此平行边是那个角的对边。
When resolving any vector, clearly draw the right triangle with the angle correctly placed relative to the component directions. Labelling angles and sides before writing trigonometric expressions prevents swapping sin and cos.
分解任何矢量时，清晰地画出直角三角形，并正确标注角度相对于各分量的位置。在写出三角函数式之前标记角度和边可以防止正弦与余弦的混淆。
7. Uncertainties and Error Analysis | 不确定度与误差分析
length L = (10.0 ± 0.2) cm and width W = (5.0 ± 0.1) cm, first convert each to percentage: %U(L) = 2%, %U(W) = 2%. The percentage uncertainty in area is 2% + 2% = 4%, so area = 50 cm² ± 4% (approx ± 2 cm²).
当加减测量值时，绝对不确定度相加；对于乘除，百分比不确定度相加。错误地混合绝对与百分比不确定度是常见陷阱。例如，求长L = (10.0 ± 0.2) cm、宽W = (5.0 ± 0.1) cm的矩形面积，先将各自转为百分比：%U(L) = 2%，%U(W) = 2%。面积百分比不确定度为2% + 2% = 4%，因此面积 = 50 cm² ± 4%（约 ± 2 cm²）。
| Operation | Uncertainty Rule |
| Addition / Subtraction (A ± a) + (B ± b) | Absolute uncertainties add: a + b |
| Multiplication / Division | Percentage uncertainties add |
| Power (e.g., xⁿ) | Multiply % uncertainty by n |
Another frequent mistake is recording calculated values with far more digits than justified by the uncertainty. Always match the last significant digit to the uncertainty’s position.
另一个常见错误是记录的计算值位数远超不确定度所容许的范围。始终让最后一个有效数字与不确定度的数量级对齐。
8. Bond Energy Calculations in Energetics | 化学键能计算
Enthalpy change ΔH = sum of bond energies of bonds broken – product bonds to reactant bonds. For the reaction H₂ + Cl₂ → 2HCl, using bond energies (H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol), ΔH = (436 + 243) – (2 × 432) = 679 – 864 = –185 kJ (exothermic).
焓变ΔH = 断裂键的键能总和 – 形成键的键能总和。常见错误是弄反顺序或者把生成物键能加和到反应物。对于反应 H₂ + Cl₂ → 2HCl，使用键能 (H–H 436，Cl–Cl 243，H–Cl 432 kJ/mol)，ΔH = (436 + 243) – (2 × 432) = 679 – 864 = –185 kJ（放热）。
Students also forget that bond energies are averages from many compounds, so calculations are only approximate. Using them for reactions involving giant covalent structures or where bond environment differences are significant can yield misleading results.
学生也常忘记键能是从许多化合物中取得的平均值，因此计算仅是近似。将它们用于涉及巨型共价结构或键环境差异显著的反应时，可能得出误导性结果。
9. Enzyme Activity and Limiting Factors | 酶活性与限制因子
Under constant enzyme concentration, increasing substrate concentration only raises the reaction rate up to the saturation point (Vmax). Many students continue the graph as a straight line, ignoring the plateau. This plateau occurs because all enzyme active sites are occupied; adding more enzyme can raise it. Mistaking substrate concentration as the sole limiting factor beyond saturation is a common graph-drawing error.
在酶浓度恒定时，增加底物浓度仅能在达到饱和点(Vmax)之前提高反应速率。许多学生将图继续画成直线，忽略了平台期。平台出现是因为所有酶活性位点都被占据；增加酶浓度可使平台升高。在饱和之后仍将底物浓度视为唯一限制因子是常见的作图错误。
Similarly, for temperature, rate increases up to an optimum; denaturation then causes a sharp drop, not a gradual decline. Plotting a symmetric bell-shaped curve is a frequent mistake – the descent must be steeper.
类似地，对于温度，速率上升到最适值；变性随后导致急剧下降，而不是缓慢下降。画一条对称钟形曲线是常见错误——下降段必须更陡。
10. Newton's Third Law Misconceptions | 牛顿第三定律误解
The weight of a book (Earth pulls book) and the normal force from the table are not an action–reaction pair. The true third-law pair for the book’s weight is the book pulling Earth upward with the same magnitude. The pair for the normal force on the book is the book pushing down on the table. Misidentifying these pairs leads to incorrect free-body diagrams and confusion about equilibrium.
书的重力（地球拉书）与桌面的支持力不是一对作用力与反作用力。书的重力的真正第三定律对是书以等大反向的力向上拉地球。桌面支持力的对是书向下压桌子。错误识别这些力对会导致错误的受力分析和平衡理解混乱。
Remember: action and reaction forces always act on two different bodies and are of the same type. If you think two forces balance on a single object, they cannot be a third-law pair.
记住：作用力和反作用力总是作用在两个不同物体上且属同一类型。如果你认为两个力在一个物体上平衡，那它们不可能是第三定律的力对。
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