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AQA GCSE · Question 04.5 · Chemical analysis
Why is the Rₑ value of a dye not affected by how far the solvent front is allowed to travel?
Why is the Rₑ value of a dye not affected by how far the solvent front is allowed to travel?
How to approach this question
Think about the formula for Rₑ value. It's a division of two distances. If you let the experiment run for longer, what happens to both the distance the solvent moves and the distance the spot moves? How does this affect the result of the division?
Full Answer
Because the Rₑ value is a ratio; the distance moved by the dye is proportional to the distance moved by the solvent front.
The Rₑ value is a ratio of two distances: (distance moved by spot) / (distance moved by solvent). If the experiment is allowed to run for a longer time, the solvent front will travel further up the paper. However, the spot will also travel proportionally further. For example, if the solvent travels twice the distance, the spot will also travel twice the distance it had previously. Because both the numerator and the denominator of the Rₑ fraction increase by the same factor, the value of the ratio itself remains constant. This makes the Rₑ value a characteristic property of a particular compound in a specific solvent and stationary phase, allowing for identification.
Common mistakes
✗ Vague answers like "it cancels out".
✗ Simply restating the formula.
✗ Incorrectly stating that the spot distance doesn't change.
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