Purity and separating mixtures

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Question 2
Medium

A chemist measures the melting points of three food preservatives.

PreservativeMelting point (°C)
X122
Y175
Z99
a.

All of the preservatives are pure substances.

Explain what is meant by a pure substance.

[1]
b.

How can you tell that the three preservatives are pure from their melting points?

[1]
c.

The chemist uses gas chromatography to investigate a mixture of preservative X and preservative Z.

How many peaks will the chemist see in the gas chromatogram?

[1]
d.

The chemist then analyses preservative W using thin layer chromatography.

The chromatogram is shown in the diagram.

A thin layer chromatogram diagram showing a vertical rectangular plate next to a centimeter ruler. The ruler on the left runs from 0 cm at the bottom to 10 cm at the top, graduated with millimeter marks. At the 0 cm mark, a horizontal line labeled 'Pencil line' is drawn across the plate. At the 8.0 cm mark, a horizontal line labeled 'Solvent front' is drawn. A single dark circular spot for Preservative W is shown on the plate, with its center perfectly aligned at the 4.6 cm mark. The label 'Preservative W' is written at the bottom of the plate.

Calculate the RfR_{\text{f}}Rf​ value of preservative W.

Use the formula: Rf=distance travelled by substancedistance travelled by solventR_{\text{f}} = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}Rf​=distance travelled by solventdistance travelled by substance​

Give your answer to 2 decimal places.

[3]
e.

Which components are needed for thin layer chromatography?

Choose the two correct components from the list below:

  • balance
  • Bunsen burner
  • mobile phase
  • paper
  • stationary phase
  • thermometer
[2]
f.

The chemist thinks that an impure preservative will only have two spots on the thin layer chromatogram.

Give two reasons why the chemist is incorrect.

[2]

Purity and separating mixtures Questions

  1. GCSE
  2. /Chemistry
  3. /Purity and separating mixtures