1 (a) Select, from the following list, one method by which each of the following may be separated from the stated mixture. You may use a method once, more than once, or not at all.

Synthesis chromatography filtration

Crystallization distillation electrolysis

Oxidation precipitation reduction sublimation

(i) lead iodide from a mixture of lead iodide and water

(ii) aqueous copper(II) sulphate from a mixture of copper(II) sulphate and iron(II) sulphate solution.

(iii) Iodine from a mixture of iodine and sodium chloride

(b) The composition of air is roughly:

(i) How could you remove water and carbon dioxide?

(ii) After water and carbon dioxide are removed, oxygen from the mixture is obtained by fractional distillation instead of simple distillation. Give a reason for this.

2. A student decides to investigate the substance responsible for the colour of

rose petals. A suitable solvent for extracting the dye is ethyl ethanoate. After the extraction is complete, the solution is freed from traces of water and filtered, thus removing chlorophyll pigments which are insoluble under these conditions. The solvent is then removed and the residue redissolved, with warming, in petroleum ether (a highly flammable liquid boiling over a temperature range of 60 – 80^oC)

(a) How can you show that the solution was free from traces of water?

(b) How can the first solvent, ethyl ethanoate, be removed from the mixture?

(c) Is petroleum ether a mixture or compound? Explain your answer

(d) The extracted colour pigments of the rose petals were separated by paper chromatography using chromatogram paper as stationary medium.

(i) What properties must a suitable stationary medium possess?

(ii) What do you think causes the separation of pigments to take place in the chromatogram?

3. An incomplete diagram of an apparatus which can be used to remove and collect

water from sea water is shown below.

 

 

(a) Complete the diagram by adding the apparatuses needed to show that the water being collected is pure.

(b) How would you adapt the apparatus so that a flammable liquid could be purified?

(c) A shopkeeper notices that a packet of baby food has been tampered with and sends it to a laboratory for investigation. The food is found to contain tiny pieces of glass. All of the glass, quite free from baby food, is needed for analysis. The baby food is soluble in water. Outline briefly the stages of the method you would use to separate all of the glass from the baby food.

4. Some information about three substances is given below.

Substance Heat Water

Sodium chloride no effect soluble

Sand no effect insoluble

Iodine sublimes insoluble

Use this information to describe the steps you would take to obtain pure dry samples of each substance from a mixture of all three. Draw diagrams only if these help in the description.

 

 

5. The figure below shows the apparatus used to obtain pure water from a sodium chloride solution.

 

 

 

 

 

 

 

(a) Name the process used in this experiment to obtain pure water from a sodium chloride solution.

(b) Besides tile chips, what remains in flask A at the end of the experiment?

(c) Briefly describe how you could determine whether the water collected in the beaker is indeed pure.

(d) Why is the thermometer placed with its bulb situated near the side-arm of the flask and not dipped into the solution?

6. To make crystals of pure salt from rock salt, the following procedure is used.

Crush the rock salt and stir into hot water. Filter the hot solution into an evaporating dish. Boil the solution in the dish until it is about one-third of its original volume and then allow it to cool. The crystals that form should be filtered off, rinsed with a little water and dried by pressing them between filter papers.

(a) What is the purpose of crushing the rock salt?

(b) What is the advantage of using hot water to dissolve the rock salt?

(c) Why must the rock salt solution be filtered when hot?

(d) Why is it not correct to boil or evaporate the solution in the evaporating dish to dryness, leaving behind the salt?

(e) Why must the filtered crystals be rinsed with a little water before drying?

 

 

 

7. The chromatogram below is for four dyes A, B, C and D. These dyes are supposed to have been made from only the colours blue, red and yellow.

 

 

 

 

 

 

 

(a) Which dye: (i) contains one colour only?

(ii) contains yellow and red only?

(b) Why is it necessary to check the result for B?

 

8. The labels from three bottles of Demure’s brand of food colouring are shown below. Sorbitol is used as a preservative in all these food colours.

 

(a)(i) What is the solvent in Demure’s blue food colour?

 

 

 

 

 

 

(ii) What is the solvent in Demure’s green food colour?

(iii) What is the solute in Demure’s yellow food colour?

(b) Which of these food colours should be kept away from naked flames? Explain your answer.

9.

 

 

 

 

 

The following chromatogram was obtained in an experiment to compare two brands of yellow coloured shampoo, X and Y.

(a) What is meant by the term ‘solvent front’?

(b) Name a suitable solvent for this experiment

(c) Suggest why the starting line should be drawn with a pencil, and not with a pen.

(d) Briefly describe and explain how the original spots of the two brands of shampoo, X and Y, could be treated for more effective results.

(e) Explain whether you think brands X and Y contain the same coloured chemicals.

(f) Why do the two spots from brand X appear at different distances from the starting line?

 

 

 

 

 

 

 

10 (a)  Some sweets and candies are coloured red by a mixture containing several dyes. The apparatus in the diagram below is used to analyse the mixture.

(i) How many different dyes are there in the mixture?

(ii) Give two reasons why the yellow dye in the mixture moves further than the

red dye.

(iii) Given that the Rf values for the yellow dye and the red dye are 0.750 and

0.500 respectively, if the yellow dye moved 7.20 cm on the chromatogram, calculate the distance traveled by the red dye.

 

(b) An alternative set of apparatus for this experiment is shown below.

 

Draw and label in the diagram below how you would expect the filter paper to appear after several minutes.

 

 

 

 

11. A scientist was employed by an ink manufacturer to investigate the nature of a rival’s ‘permanent’ purple ink. In the first step in his work, he decided to find out if the ink was made from a mixture of different commercial coloured dyes. To do this he set up the following apparatus.

 

 

 

 

 

 

 

 

He placed ink and dye spots at X and as the solvent moved up the filter paper, spots of ink and dye moved to the positions shown.

(a) What is the name given to this scientific process?

(b) Why do you think that the solvent chosen in this process would not be water?

(c) Explain why a cover would be necessary for the beaker.

(d) Which of the commercial dyes investigated was a mixture of two other dyes? Of which dyes was it made?

(e) Of which commercial dyes was the ink under investigation made?

(f) The scientist had about 200 cm³ of the ink. He wished to concentrate the ink by removing some of the solvent and collecting the solvent so that it too could be investigated. Name the process that he could use.

12. A group of students decides to see what happens when they heat an aqueous solution of a dye. They set up an apparatus like that shown in the diagram. As they heated the flask, the students noticed drops of colourless liquid gathered on the glass rod.

 

 

 

 

 

(a) Whilst the liquid was observed on the glass rod, the students noted the reading on the thermometer, and then added a second heater under the flask. What happened to the reading on the thermometer?

(b) What effect did the extra heat supplied by the second heater have on the contents of the flask?

(c) Sketch a graph, using the axes below, to show how you think the reading on the thermometer changed as the experiment went on when the first heater was used.

Temp/^oC

 

 

 

 

 

Time

13. An experiment was carried out with the dye extracted from elm tree leaves using propanone. The leaves were put into a mortar with a little sand and some propanone. The mixture was then crushed with a pestle. The mixture was then added to a round bottom flask and a further 50 cm³ of propanone was added. The round bottom flask and the contents were heated to about 50^oC.

(a) Why is sand added to the mortar?

(b) Propanone is very flammable. How could the round bottom flask and the contents be heated safely?

(c) The round bottom flask and its contents were heated as shown. What is the function of the condenser in this case?

(d)  The remains of the leaves and the sand were removed from the solution. What process can be used to bring about this separation?

 

 

 

 

 

 

 

 

(e) A drop of the dye solution from the leaves was placed in a center of a piece of filter paper(Figure 2) and allowed to dry. Then, a few drops of propanone were added to the dye spot. The solvent moved outwards and the dye was separated into two rings – a green ring due to chlorophyll and a yellow ring due to xanthrophyll. (Xanthrophyll is more soluble than chlorophyll in propanone) Complete the following diagram to show the appearance of the filter paper at the end of the experiment.

 

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