📚 GCSE WJEC Chemistry: Chromatography Exam Essentials | GCSE WJEC 化学：色谱 考点精讲
Chromatography is a powerful separation technique used to separate and identify components in a mixture. In WJEC GCSE Chemistry, understanding paper chromatography, thin-layer chromatography, and how to calculate Rf values is essential for both Unit 1 and Unit 2. This article covers key exam points to help you succeed.
色谱是一种强大的分离技术，用于分离和识别混合物中的成分。在 WJEC GCSE 化学中，理解纸色谱、薄层色谱以及如何计算 Rf 值对于 Unit 1 和 Unit 2 都很重要。本文涵盖关键考点，助你取得好成绩。
1. What Is Chromatography? | 什么是色谱？
Chromatography is a physical method of separating the substances in a mixture based on their different affinities for a stationary phase and a mobile phase. The word comes from Greek: ‘chroma’ meaning colour and ‘graphein’ meaning to write.
色谱是一种物理方法，根据混合物中各物质对固定相和流动相的不同亲和力将其分离。这个词源自希腊语：“chroma”意为颜色，“graphein”意为书写。
In a typical chromatography experiment, a sample mixture is placed on a stationary phase. A mobile phase then moves over or through the stationary phase, carrying the components with it. Because different substances have different strengths of attraction to the two phases, they travel at different speeds and separate.
在典型的色谱实验中，样品混合物被放置在固定相上。然后流动相在固定相上或穿过固定相移动，携带着各组分。由于不同物质对两相的吸引力大小不同，它们以不同的速度移动并分离。
2. The Stationary and Mobile Phases | 固定相与流动相
In paper chromatography, the stationary phase is the water molecules trapped in the cellulose fibres of the chromatography paper. The paper itself is not the stationary phase — it is the very thin layer of water adsorbed onto the paper surface.
在纸色谱中，固定相是吸附在色谱纸纤维素纤维中的水分子。纸本身不是固定相——真正的固定相是吸附在纸表面的一层极薄的水膜。
The mobile phase is a solvent or mixture of solvents that moves up the paper. Common solvents include water, ethanol, or a mix of organic liquids. The solvent must be able to dissolve the components of the mixture being tested.
流动相是沿纸向上移动的溶剂或溶剂混合物。常用的溶剂包括水、乙醇或有机液体的混合物。溶剂必须能够溶解被测试混合物的各组分。
In thin-layer chromatography (TLC), the stationary phase is a thin layer of silica gel or aluminium oxide on a glass or plastic plate. The mobile phase is again a liquid solvent that rises up the plate by capillary action.
在薄层色谱（TLC）中，固定相是涂布在玻璃或塑料板上的薄层硅胶或氧化铝。流动相同样是一种液体溶剂，通过毛细作用沿板上升。
3. Paper Chromatography Setup | 纸色谱实验装置
A typical paper chromatography experiment uses a strip of chromatography paper, a small spot of the mixture to be analysed, and a suitable solvent in a beaker with a lid. The baseline is drawn in pencil — never in ink, because ink would dissolve and run.
典型的纸色谱实验使用一条色谱纸、一小点待分析混合物，以及盛有合适溶剂的带盖烧杯。基线用铅笔绘制——绝不能用墨水笔，因为墨水会溶解并扩散。
The spot of mixture is placed on the pencil line. The paper is then placed in the beaker so that the solvent level is below the baseline. This prevents the spot from simply dissolving into the bulk solvent. The beaker is covered to create a saturated atmosphere, which stops the solvent from evaporating too quickly.
混合物的斑点放在铅笔线上。然后将纸放入烧杯中，使溶剂液面低于基线。这可以防止斑点直接溶解到主体溶剂中。烧杯需盖上盖子以形成饱和气氛，避免溶剂过快蒸发。
4. How Paper Chromatography Works | 纸色谱的工作原理
As the solvent travels up the paper by capillary action, it carries the components of the mixture with it. A component that is more soluble in the mobile phase will spend more time dissolved in the solvent and travel further.
溶剂通过毛细作用沿纸上移时，携带着混合物中的各组分。在流动相中溶解度更高的组分会在溶剂中停留更长时间，移动距离更远。
A component that is more strongly attracted to the stationary phase (water on the paper) will spend more time adsorbed on the paper and travel less far. Each substance therefore moves at its own rate, producing separate spots at different heights on the paper.
对固定相（纸上的水）吸引力更强的组分会有更多时间吸附在纸上，移动距离较短。因此每种物质以自身的速率移动，在纸上不同高度形成分离的斑点。
Important: the spots may be colourless and need to be visualised using a locating agent, such as ninhydrin for amino acids, or by viewing under UV light.
重要提示：斑点可能是无色的，需要使用定位剂显色，例如氨基酸用茚三酮，或在紫外灯下观察。
5. Calculating Rf Values | Rf 值计算
The Rf (retention factor) value is used to identify a substance by comparing it to known values under identical conditions. Rf is defined as:
Rf（比移值）用于通过与相同条件下已知值的比较来鉴定物质。Rf 的定义为：
Rf = distance travelled by the substance spot / distance travelled by the solvent front
Rf = 物质斑点移动的距离 / 溶剂前沿移动的距离
Both distances are measured from the baseline (pencil line). The solvent front is the furthest point reached by the solvent. The Rf value is always between 0 and 1, and it has no units. For example, if a spot moves 2.5 cm and the solvent front moves 5.0 cm, then Rf = 2.5 / 5.0 = 0.50.
两个距离均从基线（铅笔线）测量。溶剂前沿是溶剂移动到达的最远点。Rf 值始终在 0 到 1 之间，无单位。例如，斑点移动 2.5 cm，溶剂前沿移动 5.0 cm，则 Rf = 2.5 / 5.0 = 0.50。
Exam tip: When a question asks you to calculate Rf, always measure to the centre of the spot. If the spot is oval or smeared, take the middle point.
考试提示：当题目要求计算 Rf 时，始终测量到斑点的中心。如果斑点呈椭圆形或拖尾，取中点。
6. Factors Affecting Rf Values | 影响 Rf 值的因素
Rf values depend on the stationary phase, the mobile phase, the temperature, and the specific substance. Changing the solvent will change Rf values dramatically. That is why known reference compounds must be run on the same chromatogram as the unknown sample for reliable identification.
Rf 值取决于固定相、流动相、温度和具体物质。改变溶剂会显著改变 Rf 值。因此，必须将已知参照物与未知样品在同一色谱图上展开，才能进行可靠鉴定。
If the solvent front is not allowed to travel far enough, spots may not separate well. If the beaker is left uncovered, uneven evaporation can cause poor separation and unreliable Rf values. Consistency of conditions is key.
如果溶剂前沿移动距离不够远，斑点可能无法充分分离。如果烧杯未加盖，蒸发不均匀会导致分离效果差，Rf 值不可靠。条件的一致性至关重要。
7. Thin-Layer Chromatography (TLC) | 薄层色谱
TLC works on the same principle as paper chromatography but uses a coated plate as the stationary phase. The stationary phase is typically silica gel (SiO₂) or aluminium oxide (Al₂O₃) spread thinly on a glass or plastic support.
TLC 的原理与纸色谱相同，但使用涂覆板作为固定相。固定相通常是涂布在玻璃或塑料支撑体上的薄层硅胶（SiO₂）或氧化铝（Al₂O₃）。
TLC has several advantages over paper chromatography: plates are robust, separation is often faster and sharper, and a wider range of locating agents can be used, including corrosive sprays that would destroy paper.
TLC 相比纸色谱有几个优点：板体坚固，分离通常更快、更清晰，可使用更广泛的定位剂，包括会破坏纸的腐蚀性喷雾剂。
In WJEC GCSE Chemistry, you are expected to know that TLC is used in similar applications but may be able to handle smaller samples and produce better resolution.
在 WJEC GCSE 化学中，你要知道 TLC 用于类似应用，但可处理更小的样品，并产生更好的分辨率。
8. Interpreting Chromatograms | 色谱图解读
A chromatogram is the paper or TLC plate after the solvent has moved and the spots have been visualised. A pure substance will produce a single spot at its characteristic Rf. A mixture will produce multiple spots.
色谱图是溶剂移动并显色后的纸或 TLC 板。纯物质会在其特征 Rf 处产生单一斑点。混合物会产生多个斑点。
If a sample produces three spots, the mixture contains at least three components. If one spot matches the Rf and colour of a known reference substance run on the same chromatogram, that component is likely to be the same substance. However, Rf alone is not definitive proof of identity — other tests may be needed.
如果样品产生三个斑点，则混合物至少含有三种组分。如果一个斑点的 Rf 和颜色与同一色谱图上已知参照物的相匹配，则该组分很可能就是该物质。然而，仅凭 Rf 值不能作为确定的身份证明——可能还需要其他测试。
9. Choosing a Suitable Solvent | 选择合适的溶剂
The choice of solvent is critical. It must be able to dissolve all the components of the mixture. Often a mixture of solvents is used to achieve the best separation. For example, mixing water with ethanol or other organic solvents can tune the polarity of the mobile phase.
溶剂的选择至关重要。它必须能够溶解混合物的所有组分。通常使用混合溶剂以达到最佳分离效果。例如，将水与乙醇或其他有机溶剂混合，可调节流动相的极性。
If the solvent is too polar, all components may travel with the solvent front. If it is not polar enough, components may stay near the baseline. Students should be able to suggest why a certain solvent or solvent mixture is used in a given practical context.
若溶剂极性过强，所有组分可能随溶剂前沿移动。若极性不足，组分可能停留在基线附近。学生应能解释在给定的实验情境中为何使用某种溶剂或混合溶剂。
10. Applications of Chromatography | 色谱的应用
Chromatography is widely used in forensic science to analyse inks, dyes, and drugs. It can identify compounds in food colourings, detect contaminants in water, and authenticate documents. In biochemistry, it is used to separate amino acids, sugars, and nucleic acid fragments.
色谱广泛用于法医科学，分析墨水、染料和药物。它可以鉴定食品色素中的化合物、检测水中的污染物、以及鉴定文件的真伪。在生物化学中，它用于分离氨基酸、糖类和核酸片段。
In the pharmaceutical industry, chromatography – including high-performance liquid chromatography (HPLC) – is used to purify products and check the purity of drugs. WJEC GCSE students may not need to know HPLC in detail, but they should understand that more advanced forms of chromatography exist for sensitive analytical work.
在制药工业中，色谱——包括高效液相色谱（HPLC）——用于纯化产品并检查药物纯度。WJEC GCSE 学生不需要详细了解 HPLC，但应理解存在更先进的色谱形式用于灵敏的分析工作。
11. Limitations and Common Mistakes | 局限性与常见错误
Common mistakes in chromatography experiments include: drawing the baseline with ink, placing the spot below the solvent level, and letting the solvent front run off the top of the paper. All of these will ruin the separation.
色谱实验中常见错误包括：用墨水画基线、将斑点放在溶剂液面以下、以及让溶剂前沿跑出纸张顶部。这些都会破坏分离效果。
Limitations include the inability to separate colourless components without a locating agent, and the difficulty of separating substances with very similar Rf values. Two different substances may coincidentally have the same Rf under certain conditions, so gas chromatography or mass spectrometry may be used for more reliable identification in higher-tier study.
局限性包括没有定位剂时无法分离无色组分，以及难以分离 Rf 值非常接近的物质。两种不同的物质在特定条件下可能偶然具有相同的 Rf，因此在更高层级的学习中，可能使用气相色谱或质谱进行更可靠的鉴定。
12. Exam Tips and Practice Questions | 考试技巧与练习
In WJEC exams, you may be asked to calculate Rf from a diagram, explain why a pencil is used for the baseline, or interpret a chromatogram to decide whether a sample contains a certain substance. Show all working clearly and always measure distances accurately from the baseline to the spot centre and to the solvent front.
在 WJEC 考试中，你可能需要根据图表计算 Rf、解释为何要用铅笔画基线，或解读色谱图以判断样品是否含有某种物质。清晰展示所有计算步骤，并始终准确测量从基线到斑点中心和溶剂前沿的距离。
Practice question: A chromatography experiment was carried out on a sample of food colouring. The solvent front moved 7.2 cm from the baseline. A blue spot moved 3.6 cm, and a yellow spot moved 4.8 cm. Calculate the Rf values for both spots. Which spot has a stronger attraction to the stationary phase?
练习题：对食品色素样品进行了色谱实验。溶剂前沿从基线移动了 7.2 cm。一个蓝色斑点移动了 3.6 cm，一个黄色斑点移动了 4.8 cm。计算两个斑点的 Rf 值。哪个斑点对固定相的吸引力更强？
Answer: Rf (blue) = 3.6 ÷ 7.2 = 0.50; Rf (yellow) = 4.8 ÷ 7.2 = 0.67. The blue spot has a lower Rf, meaning it spent more time adsorbed on the stationary phase – it has a stronger attraction to the stationary phase.
答案：Rf（蓝）= 3.6 ÷ 7.2 = 0.50；Rf（黄）= 4.8 ÷ 7.2 = 0.67。蓝色斑点的 Rf 较低，意味着它在固定相上吸附的时间更长——它对固定相的吸引力更强。
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