📚 Edexcel Combined Science 220: Core Practical Skills and Data Analysis | 爱德思综合科学 220：核心实验技能与数据分析
In Edexcel Combined Science, core practicals are not just demonstrations; they test your ability to plan, measure, process and evaluate scientific evidence. This article focuses on the skills assessed across Biology, Chemistry and Physics, using the specification code 220 as a reference for combined science practical work.
在爱德思综合科学中，核心实验不仅仅是演示，而是考查你计划、测量、处理与评价科学证据的能力。本文聚焦于生物、化学和物理共同评估的实验技能，以综合科学 220 中的核心实验内容为参考。
1. Variables and Hypotheses | 变量与假设
A hypothesis must be a clear, testable statement linking an independent variable (IV) to a dependent variable (DV). For combined science practicals, state how changing the IV is expected to change the DV, and keep all control variables constant.
假设必须是清晰、可检验的陈述，将自变量（IV）与因变量（DV）联系起来。在综合科学实验中，要说明改变自变量预期如何改变因变量，并保持所有控制变量不变。
| Variable type 变量类型 | Definition 定义 | Example: enzyme pH practical 示例：酶 pH 实验 |
|---|---|---|
| Independent 自变量 | The variable you change 你改变的变量 | pH of buffer 缓冲液 pH |
| Dependent 因变量 | The variable you measure 你测量的变量 | Time for starch to disappear 淀粉消失的时间 |
| Control 控制变量 | Variables kept constant 保持不变的变量 | Temperature, enzyme concentration 温度、酶浓度 |
If a control variable is not managed, the experiment becomes invalid because you cannot attribute the change in the dependent variable only to the independent variable.
如果没有控制变量得到管理，实验就会失效，因为你不能将因变量的变化仅归因于自变量。
2. Apparatus and Measurement | 仪器与测量
Choose apparatus with an appropriate resolution and range. For example, a 25 cm³ measuring cylinder has a resolution of 1 cm³, while a 50 cm³ burette has a resolution of 0.1 cm³; use the burette for titration.
选择具有合适分辨率和量程的仪器。例如，25 cm³ 量筒的分辨率为 1 cm³，而 50 cm³ 滴定管的分辨率为 0.1 cm³；滴定时应使用滴定管。
Record all measurements to the full precision shown by the instrument, including trailing zeros, to avoid losing resolution. For example, write 12.0 cm³ instead of 12 cm³ if the instrument measures to 0.1 cm³.
记录所有测量值时要保留仪器显示的全部精度，包括末尾的零，以免丢失分辨率。例如，如果仪器能测到 0.1 cm³，就应记录为 12.0 cm³，而不是 12 cm³。
3. Accuracy, Precision and Uncertainty | 准确度、精密度与不确定度
Accuracy describes how close a result is to the true value, while precision describes how close repeated measurements are to each other. A systematic error reduces accuracy; random error reduces precision.
准确度描述结果与真实值的接近程度，精密度描述重复测量值之间的接近程度。系统误差降低准确度，随机误差降低精密度。
For a single measurement, absolute uncertainty is usually half the smallest scale division. Percentage uncertainty is calculated as:
对于单次测量，绝对不确定度通常取最小分度值的一半。百分不确定度的计算公式为：
percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%
For example, if a thermometer has a scale division of 1 °C and reads 25 °C, the absolute uncertainty is ±0.5 °C and the percentage uncertainty is ±(0.5 ÷ 25) × 100% = ±2%.
例如，如果温度计的最小分度为 1 °C，读数为 25 °C，则绝对不确定度为 ±0.5 °C，百分不确定度为 ±(0.5 ÷ 25) × 100% = ±2%。
4. Recording Data and Tables | 记录数据与表格
Use a ruled table with clear headings, units and a column for the mean. Circle anomalies only after repeating, and do not include them in the mean.
使用带有清晰标题、单位和平均值栏的表格。只有在重复实验后才可以圈出异常值，且计算平均值时不包含异常值。
- Place the independent variable in the first column 将自变量放在第一列
- Place the dependent variable in later columns 将因变量放在后面的列
- Put units in the header, not next to every value 单位写在表头中，不要写在
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