Pearson Edexcel International GCSE (9–1) · CHEMISTRY · Chapter
Elements, Compounds and Mixtures
69 practice questions
Topics covered
Practice questions
21–30 of 30 · Normal
Question 21
1 mark · Normal
Answer and check →Which of these would not be a correct safety precaution for a paper chromatography experiment using organic solvents?
- A.Wear gloves when handling solvents.
- B.Use a well-ventilated area.
- C.Taste the dyes after the experiment.
- D.Avoid contact of solvent with skin.
Question 22
2 marks · Normal
Answer and see the solution →Give two reasons why different dyes in a mixture travel different distances up the paper in chromatography.
Question 23
4 marks · Normal
Answer and see the solution →You want to check whether a green dye in a drink is a single substance or a mixture of dyes. You will use paper chromatography to investigate.
(a)[2 marks]Describe how you would set up and carry out this experiment, naming all key steps.
(b)[2 marks]Explain how you would use the chromatogram to decide if the green dye is pure or a mixture.
Question 24
2 marks · Normal
Answer and see the solution →State two reasons why the line for the origin is drawn with pencil, not pen, in a paper chromatography experiment.
Question 25
4 marks · Normal
Answer and see the solution →A student uses paper chromatography to separate a mixture of coloured dyes. The chromatogram below shows the results. The student also spotted two known dyes, X and Y, on the same piece of chromatography paper.
Chromatogram from a mixture and two known dyes Spots shown on chromatogram Sample Distance moved from origin (cm) Mixture (green) 4.8 Dye X (blue) 1.7 Dye Y (orange) 4.8 Solvent front 6.0 (a)[2 marks]State which dye(s), X or Y, is present in the mixture. Give a reason for your answer.
(b)[2 marks]Explain why the blue spot (Dye X) did not reach as far up the paper as the orange spot (Dye Y).
Question 26
3 marks · Normal
Answer and see the solution →In a chromatography experiment a spot of red dye moves 3.2 cm from the pencil line. The solvent front moves 6.4 cm from the pencil line.
Calculate the R_f value for the red dye. Show your working.
Question 27
6 marks · Normal
Answer and see the solution →A research chemist is given three samples labelled A, B, and C. She wants to classify them and analyse their composition using paper chromatography.
(a)[3 marks]Sample A consists only of nitrogen gas. Sample B contains sodium chloride crystals. Sample C is a solution made by dissolving food dye in water. Classify each sample as an element, compound, or mixture.
(b)[1 mark]The chemist uses paper chromatography to separate the dyes in Sample C. After running the experiment, she observes three coloured spots at different distances from the pencil line. What does this result tell you about the composition of Sample C?
(c)[2 marks]Explain how you could use the chromatogram to identify if Sample C contains the same dyes as a reference mixture of known dyes.
Question 28
6 marks · Normal
Answer and see the solution →A scientist wants to investigate the substances present in two unknown samples using paper chromatography. She also wants to classify the samples as elements, compounds, or mixtures.
(a)[2 marks]Describe how the scientist can use the appearance of the chromatogram to decide if each sample is a pure substance or a mixture.
(b)[2 marks]The scientist measures the distances on the chromatogram for one spot as follows: Distance from pencil line to spot centre: 2.4 cm Distance from pencil line to solvent front: 4.0 cm Calculate the Rf value for this spot. Show your working.
(c)[2 marks]Give the definitions for (i) element, (ii) compound, and (iii) mixture.
Question 29
6 marks · Normal
Answer and see the solution →A student finds a white powder in the laboratory labelled 'Sample X'. She wants to check if it is a pure substance or a mixture, and then investigate its components using paper chromatography.
(a)[2 marks]Describe how the student could use melting point measurements to determine whether Sample X is pure or a mixture.
(b)The student performs a paper chromatography experiment with a spot of Sample X and three reference dyes A, B and C on the same chromatography paper. The chromatogram is developed using a suitable solvent.
The diagram below shows the results.
Chromatogram of Sample X and reference dyes after development The solvent front has moved 6.0 cm from the pencil line. The spots are found at the following distances above the pencil line: Sample X: 2.0 cm, 4.5 cm Dye A: 2.0 cm Dye B: 4.5 cm Dye C: 3.7 cm
(i)[1 mark]How many different dyes are present in Sample X? Explain your answer.
(ii)[1 mark]State which reference dyes are found in Sample X. Give your reasoning.
(c)[2 marks]Calculate the R_f value for the spot at 3.7 cm. Show your working.
Question 30
6 marks · Normal
Answer and see the solution →A teacher finds a powdery sample on a lab bench. She wants to find out if it is a pure substance or a mixture. She records the melting point, then runs paper chromatography on the dissolved sample, using water as the solvent. The chromatogram below shows her results.
Melting behaviour of sample X Sample X observations Melting starts at 87°C Melting finishes at 97°C Chromatogram obtained from sample X and three known dyes (Dye A, Dye B, Dye C) The solvent front travelled 8.0 cm from the pencil line.
(a)[2 marks]Explain what the melting point results show about whether sample X is a pure substance or a mixture.
(b)[2 marks]Describe how paper chromatography was carried out in this investigation. Include the key steps.
(c)[1 mark]Use the chromatogram to identify which dyes are present in sample X. State your reasoning.
(d)[1 mark]Calculate the R_f value for the dye in sample X that matches Dye B. Give your answer to two decimal places.