Show — 4 marks
A student is investigating different samples of water collected from various sources. Sample A is pure water from a distillation apparatus. Sample B is tap water from a local school. Sample C is salt water from a coastal location. The student measures the boiling point of each sample at atmospheric pressure.
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(a) Sample A boils at exactly 100°C. Explain why this observation shows that Sample A is a pure substance.
[1 mark]
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(b) Sample B boils at 100.2°C and Sample C boils at 101.8°C. Show that Sample C contains a greater concentration of dissolved solutes than Sample B.
[2 marks]
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(c) The student wants to separate the salt from Sample C. Show why distillation would be an appropriate method to obtain pure water from this mixture, and identify what would remain in the distillation flask.
[1 mark]
Show mark scheme
- (a) Award 1 mark for: Pure substances have a fixed/definite boiling point (at a given pressure) OR all molecules are identical so they require the same energy to change state.
- (b) Award 1 mark for: Sample C has a higher boiling point than Sample B (101.8°C > 100.2°C). Award 1 mark for: Dissolved solutes raise the boiling point / colligative property / boiling point elevation is proportional to solute concentration OR higher boiling point indicates higher concentration of solutes.
- (c) Award 1 mark for: Distillation separates components based on different boiling points AND salt has a much higher boiling point than water so water vaporises and condenses as pure liquid, leaving solid salt in the flask. (Accept: salt remains/stays behind in the distillation flask OR salt crystals remain)
Define — 4 marks
A student is investigating different materials found in a chemistry laboratory. They have four samples: distilled water, salt solution, pure copper, and bronze (a copper-tin alloy). The teacher asks them to classify these materials and explain the differences between them.
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(a) Define what is meant by a pure substance.
[1 mark]
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(b) Define what is meant by a mixture.
[1 mark]
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(c) Classify each of the four samples (distilled water, salt solution, pure copper, and bronze) as either a pure substance or a mixture. Explain your reasoning for one of your classifications.
[2 marks]
Show mark scheme
- (a) A pure substance contains only one type of particle/element/compound (1 mark)
- (b) A mixture contains two or more pure substances/elements/compounds that are not chemically bonded together (1 mark)
- (c) Correct classification: distilled water (pure substance), salt solution (mixture), pure copper (pure substance), bronze (mixture) (1 mark for all four correct)
- (c) Valid explanation for one classification, such as: 'Salt solution is a mixture because it contains salt and water that are not chemically combined' OR 'Bronze is a mixture because it contains copper and tin atoms that are physically mixed but not chemically bonded' OR 'Distilled water is a pure substance because it contains only H₂O molecules' (1 mark)
Define — 2 marks
A student is investigating different materials used in water treatment plants. The plant uses several processes to clean drinking water, including filtering sand and gravel, and adding chemicals that react with impurities. After treatment, the water is tested to ensure it meets safety standards.
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(a) Define what is meant by a pure substance.
[1 mark]
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(b) The treated drinking water leaving the plant contains dissolved minerals and chlorine. Explain why this treated water is not considered a pure substance.
[1 mark]
Show mark scheme
- (a) A substance that contains only one type of particle/element/compound (with no other substances mixed in)
- (b) Because it contains more than one substance / it is a mixture of water, dissolved minerals and chlorine / it contains impurities or additional substances
Show — 2 marks
A student is investigating different materials found in a school laboratory. They have three samples: pure water, salt water, and pure salt. The student wants to identify which samples are pure substances and which are mixtures.
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Show that salt water is a mixture and not a pure substance.
[2 marks]
Show mark scheme
- {'mark': 1, 'description': 'Identifies that salt water contains two or more substances (salt and water)'}
- {'mark': 1, 'description': 'States that a pure substance has a fixed composition / constant properties, whereas a mixture does not (or explains that the composition of salt water can vary)'}
Calculate — 2 marks
A technician analyses a 500 g sample of iron ore from a mining site. The sample is a mixture of iron oxide and unwanted rock. After processing, 340 g of pure iron oxide is separated.
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(a) Calculate the percentage by mass of iron oxide in the original ore sample.
[1 mark]
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(b) Calculate the mass of unwanted rock in the 500 g sample.
[1 mark]
Show mark scheme
GCSE Perfect