State — 5 marks
A student is testing gases produced during different chemical reactions in the laboratory. They need to identify the gases using chemical tests and separate a mixture of coloured dyes using paper chromatography.
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(a) State the name of the gas produced when a metal reacts with dilute hydrochloric acid.
[1 mark]
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(b) State the test and the expected observation to identify oxygen gas.
[2 marks]
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(c) A student uses paper chromatography to separate a mixture of food colourings. State the name of the liquid used to carry the dyes up the paper and state why this liquid must not be allowed to reach the top of the paper.
[2 marks]
Show mark scheme
- (a) Hydrogen (gas)
- (b) Test: A glowing splint is placed in the gas / insert a burning splint into the gas
- (b) Observation: The splint relights / the splint burns brightly / the splint reignites
- (c) The liquid used is the solvent / water (or named solvent such as ethanol)
- (c) If the liquid reaches the top of the paper, the dyes cannot travel any further / the dyes will not separate properly / there is no distance for separation to occur
Show — 3 marks
A student is investigating the composition of air samples collected from different locations in their school. They use gas tests to identify the gases present and paper chromatography to separate coloured dyes from a food sample.
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(a) Show that oxygen is present in an air sample by describing the test you would carry out and the expected result.
[1 mark]
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(b) A student carries out paper chromatography on a dye mixture using water as the solvent. The dye mixture travels 6 cm up the paper and one of the separated dyes travels 4.8 cm. Show that the Rf value for this dye is 0.8.
[1 mark]
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(c) The student tests a gas sample and observes that a lighted splint burns more brightly than in air. Show what this indicates about the composition of the gas sample and explain your reasoning.
[1 mark]
Show mark scheme
- (a) Insert a lighted splint into the gas sample / air sample; the splint relights (or burns more brightly) indicating oxygen is present
- (b) Rf = distance travelled by dye / distance travelled by solvent = 4.8 / 6 = 0.8
- (c) The gas sample contains a higher concentration of oxygen than air / the sample is enriched in oxygen; a lighted splint burns more brightly in pure oxygen or oxygen-rich mixtures because oxygen supports combustion more vigorously
Calculate — 2 marks
A food safety laboratory uses paper chromatography to check if a bright orange soft drink contains an unapproved artificial food dye. The chromatogram shows a spot that travelled 4.8 cm from the base line, and the solvent front travelled 12.0 cm.
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(a) State the formula used to calculate the Rf value for the food dye spot.
[1 mark]
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(b) Calculate the Rf value for the food dye spot. Give your answer to two decimal places.
[1 mark]
Show mark scheme
- (a) Rf = distance moved by substance ÷ distance moved by solvent (front)
- (b) 0.40
Explain — 3 marks
A food testing laboratory uses chromatography to check that a brand of orange juice contains natural ingredients and no artificial colours. They also test for gases produced during chemical reactions.
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(a) A spot of orange juice is placed on chromatography paper and water is used as the solvent. Explain why the different colours in the orange juice separate.
[2 marks]
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(b) A chemist tests for hydrogen gas. Explain how the chemist can confirm that the gas is hydrogen.
[1 mark]
Show mark scheme
- (a) different colours/components have different solubilities (in the solvent)
- (a) different colours/components travel at different speeds / have different affinities for the paper
- (b) burning splint gives a squeaky pop sound
Describe — 2 marks
A student is carrying out experiments in the laboratory to identify the gases produced during chemical reactions. The student collects samples of gas in test tubes and needs to test them to determine their identities.
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(a) Describe the test for hydrogen gas.
[1 mark]
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(b) Describe the positive result of the hydrogen test.
[1 mark]
Show mark scheme
- (a) use a burning splint / lighted splint (at the mouth of the test tube)
- (b) (burns with a) squeaky pop
GCSE Perfect