TOPIC

Purificotion qnd Seporotion

E Multiple Choice Ques{ions

1. The following information on water and potassium chloride is given.

Which of the following shows the likely melting and boiling points of a sample of water which contains

a small quantity of potassium chloride?

A.

Melting point /'C-5

Boiling point / "C

95

B. 4 104

C. 4 104

D. 750 1450

A food technologist is given a sample of ester. What property of the sample can be used to check for the

purity of the ester before it is used as a flavouring in food products?

A. smellB. solubility in waterC. boiling pointD. whether it changes the colour of blue litmus paper

3. In the inorganic chemistry laboratory, some crystals of a compound were washed, filtered and then dried.A melting point determination gave a value of around 117 'C. The same crystals were then washed

and dried again. The melting point was then found to be 140 oC. What is the best explanation for the

difference in the melting points?

A. The thermometer was faulty and gave inconsistent results.

B. The crystals were impure when the first melting point determination was carried out.

C. A smaller quantity of crystals was used for the first melting point determination.

D. The crystals were impure when the second melting point determination was carried out.

4. You are given a mixture of organic liquid X and an aqueous solution of potassium iodide.

Properties of organic liquid X are as follows:. It does not dissolve in water.. Its density is similar to oil.. It is yellowish in colour.. It boils at l2O "C.

,

I

melting point /'C 0 770

boiling point / "C 100 1420

4Topic 2

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Properties Water Potassium chloride

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Which sequence can be used to obtain samples of organic liquid X and solid potassium iodide?A. filtration -) evaporation

B. evaporation -+ sublimationC. use of separating funnel -+ evaporationD. filtration -+ crystallisation

A manufacturer accidentally dropped some powdered pencil lead into silver nitrate powder. What steps

could be used on the mixture to recover the pencil lead?

A. shake with water -+ filter + evaporate filtrate to dryness

B. shake with water -+ filter -+ collect residue and wash with distilled water

C. shake with dilute hydrochloric acid -+ filter -+ collect residue and wash with distilled waterD. shake with ethanol -+ filter -+ collect residue and wash with distilled water

6; Tony was given a sample of an unknown colourless liquid. He heated the liquid to dryness in a crucible.

No residue was left behind. The unknown liquid could be

I a mixture of two or more colourless liquids.II a single pure substance.

III a solution of a gas dissolved in a liquid.

A. I onlyB. I and II onlyC. I and III onlyD. I, II and III only

When aqueous barium chloride and aqueous sodium sulfate are added together in a beaker, the chemical

reaction is represented by the equation below.

BaC/r(aq) + NarSOo(aq) + 2NaC/(aq) + BaSOo(s)

Which of the following methods can be used to prepare a pure, dry sample of each of the products?

A. distillation onlyB. filtration followed by evaporation to dryness

C. crystallisationonlyD. sublimation followed by filtration

8. The atmosphere of Venus contains mainly oxygen, argon and nitrogen. The melting and boiling points ofthese gases are shown in the table below.

If only liquid oxygen is to be obtained from a mixture of these gases, what should the temperature of themixture be decreased to?

A. -190,cB. -185'Cc. -187.cD. -1gg.c

7,

oxygen -219 -1 83

argon -189 -186nltrogen -210 -196

Purification and Separation5

Gas Melting point / oC Boiling point l'C

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Two liquids, dichloromethane and water, are separated using a separating funnel. Dichloromethane is

drained off first. Which statement is correct?

A. Dichloromethane is miscible with water and is denser than water.

B. Dichloromethane is immiscible with water and is less dense than water.

C. Dichloromethane is miscible with water and is less dense than water.

D. Dichloromethane is immiscible with water and is denser than water.

10. The table gives some information of three chloride salts.

9.

SaltSolubility incold water

Solubility inhot water

Effect of heat

silver (I) chloride insoluble insoluble no effect

ammonium chloride soluble soluble sublimes

lead (II) chloride insoluble soluble no effect

If you are tasked to separate silver (I) chloride from a mixture of these three salts, what should be the

sequence of steps to be carried out on the mixture?

A. shake with cold water -+ filter -+ heat

B. shake with hot water -> heat -+ filterC. heat -+ shake with cold water -+ fllterD. heat + shake with hot water -+ filter

11. A chromatography experiment is performed in the food laboratory to determine the number of amino

acids present in a sample of protein mixture Q.Chromatogram A shows the separation of amino acids in the mixture using solvent X.The chromatogram is then removed, rotated clockwise 90o and placed inside a second solvent, Y.

The results are shown in chromatogram B.

rotateclockwise

900..........-_...................*

firstIirst

linelinestarting starting original

location of Q

secondstarting

line

chromatogram A

How many different types of amino acids are present in protein mixture Q?

A.38.4c.5D.6

chromatogram B

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a

( oa

oO

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12. A mixture of two unknown liquids, X (boiling point 70 "C) and Y (boiling point 97 'C) in equal proportions

is fractionally distilled as shown below.

thermometer

water out

IC

glassbeads I

water ln

mrxture

When the thermometer first shows a steady reading, at which point will there be a greater proportion ofliquid Y?

13. Chromatography is a process by which

A. coloured dyes are removed using a bleaching solvent.

B. dyes are extracted from a plant using a solvent.

C. chemically similar substances in solution are separated.

D. dyes are separated into ink.

L4. What property must two substances have in order for them to be separated by paper chromatography?

A. Both substances must have low boiling points.

B. Both substances must be soluble in chromatography solvent.

C. Both substances must have the same R, values.

D. Both substances must be coloured.

15. The apparatus below can be used to extract propanone (boiling point 56 "C) from a solution of propanone

containing plant dyes.

thermometer

water out

Icondenser

flask

plantdyes in

Icold water in

distillate

B

D

A

theat

propanone

Purification and Separation7

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During the separation process, the plant dyes

A. are collected in the distillate.

B. react to form a colourless product.

C. remain in the flask.

D. are removed by the cooling water.

16. In an experiment, the melting point of an unknown solid sample X was found to be 70 "C which is the

siune as an 1 1-carbon hydrocarbon. The experiment was repeated by using one part of solid X mixed withtwo pafis of pure I l-carbon hydrocarbon. The mixture starts to melt at 55 oC.

What is the best deduction from the above observation?

A. Xisamixture.B. X is pure l1-carbon hydrocarbon.C. X isnotpure l1-carbon hydrocarbon.

D. X may contain 1l-carbon hydrocarbon.

17. Ink X was suspected to contain one or more of the dyes P, R and S. Spots of each dye were placed on the

starting lines of two pieces of chromatography paper. One paper was placed into water (diagram 1) and

the other with propanol (diagram 2).

solvent front

diagram I start line diagram 2

What is the identity of X?

A. P onlyB. R onlyC. Either P or RD. Either R or S

18. Which of the following is the best method of obtaining water from ink?

A. paperchromatography

B. fractionaldistillationC. distillationD. filtration

19. Which is the most likely temperature at which a concentrated solution of sodium chloride begins to boil?

A. 96 "CB. 99'Cc. 100'cD. 104'C

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1.

E Stn ctured Questions

A sample of seawater was collected from a beach located near an oil refinery which has reported cases ofoil spillage.

Several processes are listed below

' chromatography. evaporation. fractionaldistillation. sublimation. filtration. simple distillation. oxidation. separating funnel

Name the process used to separate each of the following substances from the seawater collected.

(a) (i) sand

(ii) salt

(iii) oil(iv) water

(b) How can the purity of the water separated from the contaminated seawater be tested?

(c) Many students have commented that anhydrous copper (II) sulfate can be used to determine the

purity of water. Do you agree?

2. In a school laboratory, solid sulfur and iron powder were added together into water, stirred and allowed

to stand. It was observed that the iron and sulfur separated into two distinct layers. When the experiment

was repeated with iron (II) sulfide alone, only one layer was observed. Explain the difference in

observations.

3. A solid mixture is known to be made up of copper metal, copper (II) carbonate, copper (II) oxide and

copper (II) sulfate. The effects of water and hydrochloric acid on these four substances are summarised

in the table below

Substance Action of water Action of dilutehydrochloric acid

copper insoluble in water no reaction with acid

copper (II) carbonate insoluble in waterdissolves in acid to produce

a colourless gas and agreen solution

copper (II) oxide insoluble in water dissolves in acid to producea green solution

copper (II) sulfate dissolves in water to producea blue solution

dissolves in acid to producea blue-green solution

Purifi cation and Separation

Describe how you would attempt to obtain a pure dry sample of copper metal from the mixture.

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4. A packet of jelly crystals used for research purposes in a laboratory became contaminated with some

sand.

(a) Outline a method to separate the sand from the mixture and to obtain a dry sample of pure jellycrystals.

(b) The label on the packet of jelly crystals states that only one approved dye is used for colouring.

Describe a test to find out if the claim is true.

5. The diagram below shows the apparatus used to separate hexane (boiling point 70 "C) and heptane

(boiling point 98 'C).thermometer

water outT /

/water in

distillate

(a) Name this separation technique.

(b) Usually, boiling chips will be added into the round-bottom flask. What is the purpose of doing so?

(c) Explain why water usually enters from the bottom of the condenser, rather than from the top.

(d) A mixture of the two liquids, hexane and heptane is heated at room temperature until both liquids

are collected in the conical flask.

(i) Draw the heating curve i.e. temperature against time.

(ii) Draw another graph showing total distillate against temperature.

6. The apparatus shown below can be used to determine the melting point of a solid which melts between

55 oC and 60 oC.

0 oC to 110 oC

thermometer

melting point tube

stirrerrubber band

beakercontainingwater

tgentle heat

0

0

+heat

l0Topic 2

solid sample

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(a) (i) What is the purpose of using a stirrer?

(ii) Why must the water not be heated too quickly?

(b) State and explain two changes that are necessary to be made to the apparatus in order to determine

the melting point of a solid that melts between 130 "C and 140 "C.

(c) How does the melting point of a solid show that the solid is impure?

Two methods are commonly used for paper chromatography. In the ascending method, the solvent travels

up the paper whereas in the descending method, the solvent flows down the paper.

lid lid

solvent

start line

paper

solvent

ascending method descending method

(a) Which method allows the solvent to travel faster? Give a reason to support your answer.

(b) For the ascending method, the start line must be above the solvent level. Why is this so?

(c) The start line is usually drawn using a pencil and not a pen. What is the reason for doing so?

(d) For both the ascending and descending methods, the container is covered with a lid. Why is this

necessary?

(e) Longer sheets of paper can be used in the descending method as compared to in the ascending

method. Why is the use of a longer piece of paper often preferred?

(f) Sometimes, a substance known as a locating agent is sprayed onto the chromatogram. Why might

this be necessary?

line

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