Required practical 6: separate and identify actual food dyes
| English | 中文 | Pinyin |
|---|---|---|
| origin line | 起始线 | qǐ shǐ xiàn |
| solvent front/ˈsɒlvənt frʌnt/ | 溶剂前沿 | róng jì qián yán |
What would explain this observation?
- An unknown food colouring can contain two known dyes and another dye absent from the reference set. A good result must allow an unmatched spot to remain unmatched.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- RP6 investigates separation and comparison of coloured substances and requires Rf calculation. The acquired handbook uses four known food colourings A–D, an unknown U, chromatography paper, water and separate capillary tubes. Its teacher notes suggest U contain two known dyes and a third not in A–D. Known single dyes are preferable references; a commercial colouring may itself be a mixture. Written predictions do not certify that the practical has been performed.
- origin line 起始线: The marked starting line from which chromatographic distances are measured; solvent front 溶剂前沿: The farthest position reached by the moving solvent in the stated chromatogram.
Why must the spotted origin stay above the initial water level?
Draw a horizontal pencil origin 2 cm above the lower paper edge. Mark positions away from the edges, place small concentrated spots using separate clean tubes and label in pencil. The handbook aims for 2–3 mm spots and no more than 1 cm water depth, keeping the origin above the water while the paper bottom dips into it. Support the paper without touching the beaker walls and avoid moving the vessel during the run.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Draw a horizontal pencil origin 2 cm above the lower paper edge. Mark positions away from the edges, place small concentrated spots using separate clean tubes and label in pencil. The handbook aims for 2–3 mm spots and no more than 1 cm water depth, keeping the origin above the water while the paper bottom dips into it. Support the paper without touching the beaker walls and avoid moving the vessel during the run.
- Stop before the solvent reaches the paper top, remove the paper and mark the solvent front immediately in pencil. Let the paper dry before measuring from the origin to each spot centre and front. Preserve the actual chromatogram, measurements, solvent identity and calculated Rf. Compare U with references in the same run; record an additional spot as unidentified rather than inventing a name. The school method and risk assessment govern capillary handling and any drying equipment.
Which two habits make the investigation or model in this case more defensible?
Stop before the solvent reaches the paper top, remove the paper and mark the solvent front immediately in pencil. Let the paper dry before measuring from the origin to each spot centre and front. Preserve the actual chromatogram, measurements, solvent identity and calculated Rf. Compare U with references in the same run; record an additional spot as unidentified rather than inventing a name. The school method and risk assessment govern capillary handling and any drying equipment.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: in an original model run the front travels 50 mm. A, B and an unmatched U spot travel 10, 30 and 40 mm respectively. Their Rf values are 0.20, 0.60 and 0.80. If U also has spots at 10 and 30 mm, it supports A and B plus another detectable component. These model readings are practice data; the practical record must contain the student’s real observations.
In an actual run a spot centre travels 33 mm and the front 60 mm from the origin. Calculate Rf. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In an actual run a spot centre travels 33 mm and the front 60 mm from the origin. Calculate Rf.
The result is 0.55 . Known: in an original model run the front travels 50 mm. A, B and an unmatched U spot travel 10, 30 and 40 mm respectively. Their Rf values are 0.20, 0.60 and 0.80. If U also has spots at 10 and 30 mm, it supports A and B plus another detectable component. These model readings are practice data; the practical record must contain the student’s real observations.
Check the conclusion and its limits
- Ink can move and interfere, so the baseline and labels use pencil. An origin submerged in solvent lets samples dissolve into the reservoir. Large spots can overlap; touching walls can distort the front. No spot matching A–D does not mean the unknown contains no dye. RP6 applies on both tiers and assesses actual separation skills alongside written interpretation.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A written chromatography calculation alone proves RP6 was completed. This claim is false: Ink can move and interfere, so the baseline and labels use pencil. An origin submerged in solvent lets samples dissolve into the reservoir. Large spots can overlap; touching walls can distort the front. No spot matching A–D does not mean the unknown contains no dye. RP6 applies on both tiers and assesses actual separation skills alongside written interpretation.
Required practical 6: separate and identify actual food dyes: Draw a horizontal pencil origin 2 cm above the lower paper edge. Mark positions away from the edges, place small concentrated spots using separate clean tubes and label in pencil. The handbook aims for 2–3 mm spots and no more than 1 cm water depth, keeping the origin above the water while the paper bottom dips into it. Support the paper without touching the beaker walls and avoid moving the vessel during the run.
A written chromatography calculation alone proves RP6 was completed.
Ink can move and interfere, so the baseline and labels use pencil. An origin submerged in solvent lets samples dissolve into the reservoir. Large spots can overlap; touching walls can distort the front. No spot matching A–D does not mean the unknown contains no dye. RP6 applies on both tiers and assesses actual separation skills alongside written interpretation.
The marked starting line from which chromatographic distances are measured: write the technical term.
origin line means The marked starting line from which chromatographic distances are measured.