n line. 1. Identify the two distances: the substance moved 3.6 cm and the solvent moved 8.0 cm. 2. Substitute into the ratio: $$ R_f = \frac{3.6\ \text{cm}}{8.0\ \text{cm}} $$ 3. Calculate the value: $$ R_f = 0.45 $$ The centimetres cancel, so the `$R_f$` value is **0.45**, with no unit. Give `$R_f$` values to a sensible number of significant figures. If the distances are measured to two significant figures, an `$R_f$` value to two significant figures is usually appropriate unless the question says otherwise. Do not measure from the bottom of the paper. Measure from the **origin line** to the **centre of the spot**, and from the **origin line** to the **solvent front**. ## Using chromatography to identify substances Different compounds have different `$R_f$` values in different solvents. This means chromatography can help identify substances, but only if the comparison is fair. To identify an unknown substance, run it on the same chromatogram as known substances, using the same paper and the same solvent. If a spot in the unknown sample lines up with a spot from a known substance, they may be the same substance. The chromatogram below shows how an unknown mixture can be compared with known coloured substances. ![Chromatogram comparing known red, yellow, and blue substances with an unknown mixture](https://assets.yesgenie.com/notes/262ea7d9-1a04-469a-9e59-ffaa24a390f8.png) An unknown sample gives two spots. One spot is at the same height as known substance R. The other is at the same height as known substance B. There is no spot matching known substance Y. 1. Spots at the same height have travelled the same distance in the same solvent, so they have the same `$R_f$` value. 2. The unknown sample has two spots, so it contains more than one substance. 3. The unknown contains substances R and B, but there is no evidence that it contains Y. ## Pure and impure substances A pure compound produces a **single spot** in all solvents. A mixture may separate into two or more spots, because different substances in the mixture can travel different distances. However, one solvent may not separate every mixture clearly. Two different substances could accidentally have the same `$R_f$` value in that solvent and appear as one spot. A single spot in one solvent suggests a substance may be pure, but a stronger test is to repeat chromatography using different solvents. A pure compound should still produce a single spot in all solvents. This is why the solvent matters. The same compound can have a different `$R_f$` value in a different solvent, because its distribution between the stationary and mobile phases changes. An `$R_f$` value is only useful for comparison when the same solvent and conditions are used. Do not compare `$R_f$` values from different solvents as if they are automatically the same. ## Why the solvent changes the separation A solvent is not just “something wet”. It affects how well each substance dissolves and how strongly each substance stays on the paper. If the solvent is suitable, the substances in the mixture separate into clear spots. If the solvent is unsuitable, the spots may not move, may all move together, or may spread out too much. A good chromatogram has spots that are separated clearly enough to measure from the origin line to the centre of each spot. If every spot stayed on the origin line, the solvent probably did not carry the substances well. If every spot travelled with the solvent front, the substances may have been too soluble in the solvent. 1. When explaining separation, use the words **stationary phase**, **mobile phase**, and