Purity & Separating Mixtures (OCR GCSE Chemistry A (Gateway))

A student is separating a mixture of three solid substances, A, B and C.

Look at the table. It gives information about these substances.

Suggest how the student can separate the mixture to get pure, dry samples of substances A, B and C.

Explain why your methods work.

A student is separating some mixtures. She wants to make pure water from a solution of salt water. She filters the mixture.

Her method does not work.

Explain why her method does not work and describe the method she should use.

The student wants to separate a mixture of two liquids.

The liquids are:

Which separation technique should she use?

Explain how the method works.

The student separates two solid substances A and B. She wants to check that they are pure.

i) What is meant by a pure solid?

[1]

ii) The student measures the melting points of four samples of solid A.

Look at her results.

She knows that a pure sample of solid A has a melting point of 120 °C.

She concludes that sample 4 is the purest sample of solid A.

Do the results support her conclusion? Explain your answer using evidence from the table.

[3]

A painting has arrived at a local museum for a new exhibit.

To make sure the painting is genuine, a sample of the paints from the painting is sent to a laboratory for analysis.

The laboratory uses thin-layer chromatography to compare the sample of the paints from the painting with some paints used in 1849 and 1973.

The chromatograms are shown below.

i) Look at the chromatogram for the sample of the paints from the painting.

Which paint colour(s) is/are pure?

[1]

ii) The red, blue and yellow paint from the painting are dissolved in different solvents, X, Y and Z.

The results are shown below.

Which solvent, X, Y or Z, should the laboratory use for the thin-layer chromatography?

Explain why this would be better than the other solvents.

[2]

iii) Scientists use R_f values to compare the different spots on the chromatogram.

Calculate the R_f value for the red paint in the sample of paints from the painting.

Use the equation: R_f = 

Give your answer to 2 significant figures.

R_f = .......................................................

[3]

The laboratory think that the painting was completed in 1849.

Is the laboratory correct?

Explain your answer.

A scientist analyses a sample of tomato sauce using chromatography.

The tomato sauce is compared to four known food additives, A, B, C and D, as shown in the chromatogram.

The start line is not drawn in ink.

Explain why

Food additive D is insoluble in the solvent used.

Draw on the diagram the spot for food additive D at the end of the experiment.

Which additive is in the tomato sauce?

Tick () one box.

A   

B   

C   

Calculate the R_f value for additive C.

Use the equation: R_f = 

Give your answer to 2 significant figures.

R_f value = ........................................................... [3]

A student analyses a sample of tomato sauce using thin-layer chromatography.

The tomato sauce was compared to four known food additives, A, B, C and D, as shown in the chromatogram.

i) Calculate the R_f value for C.

Give your answer to 2 significant figures.

R_f value = ........................................................... [3]

ii) Which additives are shown to be in the tomato sauce?

Tick () all the correct boxes.

   A   

   B   

   C   

   D   

[1]

iii) Suggest why D has not travelled as far up the plate as C.

[1]

Gas chromatography is another type of chromatography.

The diagram shows the equipment used for gas chromatography.

i) State one similarity and one difference between gas chromatography and thin-layer chromatography.

Similarity .........................................................

Difference .....................................................

[2]

ii) Explain why thin-layer chromatography is used instead of gas chromatography to analyse the tomato sauce.

[1]

A student has a mixture of three substances.

Look at some information about these substances.

Describe how the student can separate the mixture to get pure samples of all three substances.

Explain why each method of separation works.

The student separates two solid substances A and B.

She wants to check that they are pure.

She measures the melting points of four samples of solid B.

Look at her results.

The student knows that a pure sample of solid B has a melting point of 110 °C.

She concludes that sample 4 is the purest sample of solid B.

Do the results support her conclusion?

Explain your answer using evidence from the table.

A student is making perfume using violet flowers.

The student does an experiment to extract the perfume from the flowers using the apparatus shown in Fig. 21.1.

The teacher says the apparatus is not set-up correctly.

i) What is the name of the piece of apparatus labelled X?

[1]

ii) Name the process shown in Fig. 21.1.

[1]

iii) Suggest two changes that the student should make to set-up the apparatus correctly.

[2]

The student finds out that phosphate fertilisers are used to help to grow flowers.

Compound A is a fertiliser of a metal M with the formula M₃PO₄.

The relative formula mass of compound A is 164.0.

Do a calculation to identify metal M.

Use the relative atomic masses in the Periodic Table.

Metal M = ..........................................................