Pearson Edexcel International GCSE (9–1) · CHEMISTRY · Chapter
Elements, Compounds and Mixtures
69 practice questions
Topics covered
Practice questions
61–69 of 69
Question 61
5 marks · Easy
Answer and see the solution →A student is given a sample that may be pure aspirin, or may be aspirin mixed with impurities. The sample is white powder.
(a)[2 marks]Explain how you could use melting point to decide if the sample is pure or a mixture.
(b)[1 mark]The student finds the melting point is between 127°C to 134°C. What does this suggest about the sample? Give a reason.
(c)[2 marks]Suggest a separation technique the student could use to obtain pure aspirin from this sample, and briefly describe how it works.
Question 62
5 marks · Easy
Answer and see the solution →A chemist dissolves 10 g of sodium chloride in 1.0 litre of pure water.
(a)[2 marks]State whether the resulting solution is a pure substance, a compound, or a mixture. Give a reason.
(b)[1 mark]The boiling point of pure water is 100°C. Adding the salt changes the boiling point to 100.2°C. Calculate the increase in boiling point.
(c)[2 marks]Suggest a separation technique you could use to obtain pure water from this solution. State the process and what happens during it.
Question 63
5 marks · Easy
Answer and see the solution →Some students are studying how melting and boiling points differ between pure substances and mixtures, and how to separate mixtures.
(a)[1 mark]State what is meant by a pure substance in terms of melting and boiling points.
(b)[2 marks]Water is mixed with some sugar. Describe what happens to its melting point and boiling point, compared to pure water.
(c)[2 marks]Name one method that could be used to separate the sugar from the water and briefly describe how it works.
Question 64
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 65
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 66
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 67
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.
Question 68
7 marks · Hard
Answer and see the solution →A chemist receives a coloured powder described as a mixture of food dyes and metal salts. She wants to find out if the powder is a mixture or a compound, and which dyes are present.
(a)[2 marks]State two differences between a mixture and a compound, using the chemist’s powder as an example.
(b)The chemist dissolves a small amount of the powder in water and runs a paper chromatography experiment alongside three known food dyes (dye A, dye B and dye C). The chromatogram below was produced:
Chromatogram from the experiment Spot distances from pencil line (cm) Sample Distance travelled by dye spot (cm) Powder 2.0 Dye A 2.0 Dye B 4.3 Dye C 6.0 (i)[2 marks]Calculate the R_f value for the dye in the powder. Show your working.
(ii)[1 mark]Suggest which dye(s) are present in the powder, using the chromatogram results.
(iii)[2 marks]Explain why it is important to measure from the pencil line to the centre of each spot when calculating the R_f value.
Question 69
7 marks · Hard
Answer and see the solution →A chemist is given two samples labelled X and Y. She suspects that X is a compound and Y is a mixture. She performs paper chromatography on both samples along with three dyes labelled D1, D2, and D3 on the same sheet.
Chromatogram for samples and dyes Distances moved by spots (all from the pencil line at the bottom of the paper) Substance Distance moved by spot (cm) Solvent front 5.0 X 2.0 Y (spot 1) 1.0 Y (spot 2) 3.0 D1 1.0 D2 2.0 D3 3.0 (a)[3 marks]Classify samples X and Y as elements, compounds, or mixtures, using the results of the chromatogram. Explain your reasoning.
(b)[2 marks]For each spot found for Y, identify which dye(s), if any, are present in Y.
(c)[2 marks]Calculate the R_f value for the spot found for X.