- Specification route
- RP7
- Question bank
- 13 questions
- Course stage
- Year 13 / A-level only
Sample questions
When setting up paper chromatography to separate photosynthetic pigments, why must the line at the origin be drawn in pencil rather than ink pen?
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Think about what the solvent is meant to dissolve and carry.
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Answer: Ink contains soluble pigments that would dissolve in the solvent and travel up the paper, obscuring the results.
Required practical 7. Pencil graphite is insoluble in the chromatography solvent. Pen ink contains dyes that would dissolve and run up the chromatogram, interfering with the plant pigments.
When setting up paper chromatography, it is essential that the solvent level in the beaker sits below the pencil origin line. Why?
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The pigments must be drawn upwards by capillary action, not washed away.
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Answer: If the solvent covers the origin line, the concentrated plant pigments will dissolve directly into the solvent pool instead of travelling up the paper.
Required practical 7. The pigment spot must sit above the solvent. If submerged, the pigments will simply wash off the paper into the bulk liquid at the bottom of the beaker.
Why must the position of the solvent front be marked with a pencil immediately upon removing the chromatogram from the beaker?
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The distance the solvent travels is the denominator in the $R_f$ calculation.
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Answer: Because the highly volatile solvent will evaporate very quickly, making the solvent front invisible and preventing the calculation of the $R_f$ value.
Required practical 7. Chromatography solvents (like propanone or petroleum ether) are highly volatile. If the front isn't marked immediately, it evaporates, making it impossible to measure the distance the solvent moved.
When spotting the plant extract onto the chromatography paper, why must the student allow the drop to dry before adding another drop exactly on top of it?
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You want a tiny, dark dot. If you keep adding liquid without drying, the puddle just gets wider and wider.
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Answer: To build up a highly concentrated spot of pigment without allowing the spot to spread out too widely, which would blur the final separated bands.
Required practical 7. The pigment spot must be as small and concentrated as possible. Drying between applications prevents the drop from spreading out, which ensures the separated pigment bands will be sharp and distinct rather than blurred together.
Why is the chromatography beaker or tank tightly covered with a lid during the experiment?
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The solvents used (like propanone or petroleum ether) are highly volatile and turn into gas very easily at room temperature.
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Answer: To saturate the air inside the tank with solvent vapour, preventing the solvent from evaporating off the surface of the paper as it rises.
Required practical 7. Chromatography solvents are highly volatile. Covering the tank prevents the solvent from evaporating from the paper before it reaches the top, ensuring the pigments are carried the maximum possible distance.
What determines how far a specific photosynthetic pigment travels up the chromatography paper?
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Chromatography separates mixtures based on how much the molecules 'like' the liquid moving them versus the solid they are moving across.
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Answer: Its relative solubility in the mobile phase (the solvent) compared to its affinity for the stationary phase (the paper).
Required practical 7. Separation in chromatography is based on differing solubilities and affinities. A pigment highly soluble in the solvent and with low affinity for the paper will travel further up the chromatogram.
A student calculates the $R_f$ value of a pigment to be $1.2$. What is the most likely explanation for this result?
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Look at the formula: $R_f$ = distance moved by spot / distance moved by solvent. The numerator can never exceed the denominator.
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Answer: A calculation error has occurred; the $R_f$ value cannot be greater than $1.0$ because the pigment cannot travel further than the solvent front.
Required practical 7 / MS 0.3. The $R_f$ value is a ratio. Because the solvent front is the absolute maximum distance anything can travel, the distance moved by the pigment can never be larger than the distance moved by the solvent.
What is the primary safety hazard associated with the solvents (e.g., propanone, petroleum ether) used in plant pigment chromatography, and what is the key control measure?
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These liquids vaporize easily and catch fire very quickly.
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Answer: Hazard: Highly flammable. Control Measure: Ensure there are no naked flames (like Bunsen burners) in the laboratory.
CPAC 3: Assess risks. The organic solvents used to extract and separate plant pigments are highly flammable. The primary safety precaution is keeping them strictly away from naked flames.
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