A student is given a solid mixture that contains insoluble sand and a soluble coloured salt. The salt itself is a mixture of coloured dyes. The student sepa...
Question 1 Report
A student is given a solid mixture that contains insoluble sand and a soluble coloured salt. The salt itself is a mixture of coloured dyes. The student separates and purifies the mixture following the scheme in Fig. 11.1.
(a) Name the process taking place at Step 1, Step 2 and Step 3. [3]
(b) Name the apparatus and material used at Step 1, and state what forms the residue. [3]
(c) Describe how pure, dry crystals of the salt are obtained from the filtrate at Step 2. [3]
A sample of the coloured salt is analysed by paper chromatography. The solvent front moved 9.0 cm from the baseline. Dye D1 moved 3.6 cm and dye D2 moved 6.3 cm. The Rf values of four reference dyes in the same solvent are shown in the table.
| reference dye | Rf value |
|---|---|
| red | 0.40 |
| yellow | 0.55 |
| blue | 0.70 |
| green | 0.85 |
(d) Calculate the Rf value of dye D1 and of dye D2. Show your working. [4]
(e) Use the table to identify the two dyes present in the salt. [2]
(f) At Step 3, some of the filtrate is used to obtain pure water. Name the method used and state what is collected and what remains in the flask. [3]
(g) The melting point of the dry salt crystals is measured as 799 to 801 °C. A data book gives the melting point of the pure salt as 801 °C. Deduce whether the crystals are pure and justify your answer. [2]
(h) Suggest one improvement to the method that would increase the purity of the salt crystals. [2]
Answer Details
This question follows a solid mixture through filtration, crystallisation and distillation, then analyses the coloured salt by paper chromatography. Rf values let you compare an unknown dye with reference dyes run in the same solvent.
(a) Naming the processes: Step 1 is filtration [1]; Step 2 is crystallisation (evaporation) [1]; Step 3 is simple distillation. [1]
(b) Step 1 uses a filter funnel and filter paper (over a beaker or flask). [1] The residue is the insoluble sand [1], and the filtrate that passes through is the salt solution. [1]
(c) To get pure, dry salt crystals from the filtrate: [3]
- Heat the filtrate in an evaporating basin to evaporate some water and concentrate the solution. [1]
- Heat until crystals just start to form (test with a cold glass rod), then allow it to cool slowly so crystals grow. [1]
- Filter off the crystals and dry them between sheets of filter paper. [1]
(d) Rf is the distance moved by the dye divided by the distance moved by the solvent front (here 9.0 cm): [4]
\[ R_f(\text{D1}) = \frac{3.6}{9.0} = 0.40 \]working [1], answer [1];
\[ R_f(\text{D2}) = \frac{6.3}{9.0} = 0.70 \]working [1], answer [1]. Rf has no units because it is a ratio of two lengths.
(e) Matching to the reference table: D1 with Rf 0.40 is the red dye [1], and D2 with Rf 0.70 is the blue dye. [1] A spot is identified as a reference dye only when it has the same Rf in the same solvent.
(f) Pure water is obtained by simple distillation. [1] The pure water is collected as the distillate after evaporating and condensing [1], while the salt remains behind in the flask. [1]
(g) The crystals are essentially pure. [1] They melt over a very narrow range (799 to 801 °C) that includes the data-book value of 801 °C, and a pure solid melts sharply at its correct melting point; an impurity would lower and broaden the melting range. [1]
(h) To increase purity, wash the crystals with a little cold distilled water to remove soluble impurities from their surface [1], then dry them again or recrystallise them to remove the last traces of impurity. [1] Cold water is used so that very little of the salt itself dissolves and is lost.
Examination reminder: identify a chromatography spot by matching Rf values, never by how far the spot travelled, because distance depends on how long the paper was run.
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