Chromatography and purityIB MYP Chemistry: Flashcards

Exam questions on Chromatography and purity

1. A student uses paper chromatography to analyse a black ink. The solvent front moves 8.0 cm from the start line. The ink separates into three spots, which move 2.0 cm, 4.8 cm and 6.4 cm from the start line. She runs three known dyes on the same paper: dye A moves 2.0 cm, dye B moves 3.2 cm and dye C moves 6.4 cm.
   Explain how the chromatogram shows that the ink is not a pure substance.2 marks
2. A student sets up paper chromatography for a sample of green food colouring. She draws a start line near the bottom of the paper in pencil, puts a small spot of the sample on the line and stands the paper in a beaker containing a shallow layer of solvent. The solvent level is below the start line, and she puts a lid on the beaker.
   Explain how the different colours in the food colouring become separated.2 marks
3. A student tests the purity of two samples of a white solid sold as pure benzoic acid, which has a melting point of 122 °C. She heats a small amount of each sample slowly in a melting point tube and records the range of temperatures over which it melts. Sample 1 melts between 121 °C and 122 °C. Sample 2 melts between 112 °C and 119 °C.
   Decide which sample is purer, using the data to justify your answer.3 marks