Zadanie 3a, Paper 1B, listopad 2025, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, November 2025, TZ1, pytanie 3(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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A student used paper chromatography to find out which coloured dyes had been mixed to make the brown ink of a marker pen, and to compare the polarities of those dyes. A spot of the ink was placed on the start line of each of two identical paper strips. The start line was drawn in pencil 1.5 cm from the bottom edge of the strip, and each strip was placed in a covered beaker containing a different solvent system, both mixtures of ethanol and water. System 1 was 25 % ethanol and 75 % water by volume; system 2 was 75 % ethanol and 25 % water by volume. Each run gave four coloured spots. The positions of the start line, the solvent front and the spots were measured from the bottom edge of each strip.
| Position measured from the bottom edge / cm | System 1 (25 % ethanol) | System 2 (75 % ethanol) |
|---|---|---|
| Start line | 1.5 | 1.5 |
| Solvent front | 17.5 | 15.5 |
| Violet spot | 16.2 | 5.6 |
| Blue spot | 14.1 | 11.3 |
| Orange spot | 11.1 | 7.8 |
| Yellow spot | 3.3 | 3.0 |
PowiększPolecenie
Each beaker used as a chromatography chamber was covered with a watch glass while the solvent rose up the paper.
Explain why the chamber has to be covered for the experiment to be properly designed.
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Odpowiedź z klucza
so that the air in the chamber becomes saturated with solvent vapour ✔
which stops the solvent evaporating from the paper «before the run is complete»
OR
which keeps the composition/ethanol to water ratio of the solvent constant ✔
Schemat punktowania
M1: saturation of the chamber with solvent vapour (accept 'otherwise the distance travelled by the solvent front changes'). M2: prevents evaporation of the solvent from the paper, or maintains the solvent composition. Award [1] only for an answer limited to safety precautions with a volatile solvent. Do not accept 'moisture absorbed from the air changes the concentration of ethanol' for M2.