0620 · 12.3Chromatography

Paper chromatography method

0:00A single spot of coloured ink can contain several different substances. They overlap so completely that your eyes cannot distinguish the separate components. Paper chromatography spreads those substances apart, revealing what was hidden inside the original mixture. By the end of this video, you will understand the method, interpret its results, and calculate how far a substance travels relative to the solvent. A mixture contains substances that are together without being chemically combined.

0:27Separating its components does not require making new substances. For paper chromatography, the components must be soluble in a suitable solvent. Soluble means able to dissolve, and a solvent is the liquid in which they dissolve. The solvent does more than wet the paper. As it moves through the fibres, it carries dissolved substances with it. Different components travel at different rates, so they gradually separate. The resulting pattern of spots is called a chromatogram, and each separated spot represents a substance.

0:57Begin by drawing a pencil baseline near the bottom of the chromatography paper. Place a small spot of the sample on that line and allow it to dry. A small starting spot helps produce clear, separate spots rather than broad, overlapping patches. If several samples are being compared, place them at separate positions along the same baseline. Pencil matters because graphite does not dissolve in the solvent used for this method.

1:20An ink line could dissolve and separate, adding unwanted spots to the chromatogram. Those extra components would come from your marking pen rather than the sample. The baseline must therefore stay in place without becoming part of the experiment. Put a shallow layer of suitable solvent in a beaker. Suspend the paper with its lower edge dipping into the liquid, but keep the baseline above the solvent surface. If the sample starts below that surface, it can dissolve directly into the liquid in the beaker.

1:47Instead of travelling up the paper, it may wash away. The solvent rises through the paper by capillary action, which is the movement of liquid through narrow spaces. You only need that basic idea here. When it reaches the sample, soluble components dissolve and begin moving upwards. Covering the container helps reduce solvent evaporation while the separation takes place. Separation happens because the substances interact differently with the solvent and the paper.

2:13A component carried more readily by the liquid, and held less strongly by the paper, generally travels farther. Another component may spend more time held back, so it moves a shorter distance. These differences spread the original sample into separate spots. Imagine walkers moving along a path with different amounts of stopping time. Someone who pauses frequently falls behind someone who keeps moving. In chromatography, the differences come from chemical interactions rather than choices.

2:39Spot height does not simply tell you how heavy the particles are, or how much substance was present.

Interpreting chromatograms

2:44Allow the solvent to rise, but remove the paper before it reaches the top. Immediately mark the solvent front in pencil. The solvent front is the furthest position reached by the liquid. Marking it promptly matters because the solvent evaporates, making that boundary difficult to see later. After the paper dries, examine the separated coloured spots. A pure substance gives one spot, while an impure substance or mixture gives more than one.

3:10Count spots within each sample’s lane, rather than across the entire sheet. Several pure reference substances placed side by side will naturally produce several spots overall. For a simple chromatogram, one spot is interpreted as a pure substance. In practice, different components can sometimes overlap, so one spot is not an absolute guarantee of purity. More than one separated spot clearly shows that the sample contains different substances. This distinction keeps the simple interpretation useful without claiming more than the evidence supports.

3:41Here is a worked interpretation using three known reference substances and an unknown sample. All were placed on the same baseline and developed together. The unknown produces two spots, so it is a mixture. Its lower spot matches reference one, while its upper spot matches reference three. The matching heights show that those components travelled the same distance under the same conditions. We therefore identify the unknown as containing reference one and reference three in this comparison.

4:09There is no spot matching reference two. Matching colour alone is not enough, because different substances can have similar colours. Comparisons must use the same solvent and the same type of paper under the same conditions. Changing the solvent can change how far a substance travels. Running known and unknown samples together makes the comparison straightforward. Do not compare spot heights from unrelated experiments without considering those differences.

Locating agents and Rf values

4:35This next part is for the Extended paper. Colourless substances can also be separated by paper chromatography if they dissolve in a suitable solvent. The process is the same, but their spots are not visible afterwards. An apparently blank sheet therefore does not necessarily mean that nothing has separated. After developing the chromatogram, remove the paper, mark the solvent front, and allow it to dry. Spray it with a locating agent to make the positions of the colourless substances visible.

5:02The locating agent reveals the spots; it does not perform the separation. You do not need to know the names of specific locating agents for this syllabus. This next part is for the Extended paper. The Rf value equals the distance travelled by the substance divided by the distance travelled by the solvent. Both distances start at the baseline. Measure to the centre of the substance’s spot, and take the solvent distance up to the marked front.

5:27This ratio describes how far the substance travelled compared with the solvent. A raw distance depends partly on how long you allowed the experiment to run. Dividing by the solvent distance gives a relative measure instead. Under the same conditions, a matching Rf value supports identification by comparison with a known substance. Suppose the substance travels three point zero centimetres from the baseline. The solvent front travels seven point five centimetres from that same starting line.

5:55Substituting gives three point zero divided by seven point five, both in centimetres. The Rf value is zero point four zero. This means the substance travelled forty per cent of the distance reached by the solvent front. The units cancel because both measurements are lengths expressed in centimetres. The Rf value therefore has no unit. It is always less than one, because no substance travels farther than the solvent front.

6:19Common calculation mistakes include measuring from the bottom edge of the paper and using the top edge of a spot. Neither gives the required distance from the baseline to the spot centre. Reversing the division is another warning sign because it gives a value greater than one. Use the same units for both measurements before dividing.

Exam answers and recap

6:37Chromatography can appear in theory questions and in the practical or alternative to practical paper. You may be asked to describe the method, interpret a chromatogram, or state and use the equation for the Rf value. Read the command word before answering. A method needs experimental steps, while an interpretation needs conclusions supported by the spots. Suppose an exam asks you to describe how to separate a mixture of soluble coloured substances using paper chromatography.

7:05Draw a pencil baseline and place a spot of the sample on it. Put the paper in a suitable solvent with the baseline above the solvent level. Allow the solvent to rise through the paper and separate the soluble coloured substances. For an interpretation question, connect each conclusion to a visible observation. State that an unknown matches a known substance because their spots are at the same height under the same conditions.

7:28Identify a sample producing one spot as pure in the simple chromatogram. More than one spot shows that the sample is a mixture. Paper chromatography works because soluble substances travel at different rates as the solvent rises through the paper. A pencil baseline above the liquid keeps the starting point fixed and prevents the sample washing away. Matching spots help identify components, while the number within a lane indicates purity.

7:53Locating agents reveal colourless substances, and the Rf value compares their travel distance with the solvent’s.