Separation & Purification

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Paper Chromatography

Chromatography separates substances based on different solubilities in a solvent.
Draw a pencil line on chromatography paper (ink would run) and place sample spots on it.
Lower paper into solvent so the pencil line is above the solvent to avoid washing samples off.
Solvent travels up by capillary action, carrying substances at different rates.
More soluble substances travel further up the paper.
A pure substance produces one spot; an impure mixture produces multiple spots.
Identical substances produce identical chromatograms (spots at same height).

Setting up and running paper chromatography

Locating Agents & Rf Values (Extended)

Locating agents (e.g., ninhydrin) react with colourless substances like amino acids to form visible coloured spots.
Apply locating agent after the chromatogram has run.
Retention factor (Rf) = distance moved by substance ÷ distance moved by solvent.
Rf is a ratio with no units, always less than 1.
Rf value is constant for a given compound under the same solvent conditions.
Compare Rf values to known substances to identify unknowns.

Measuring distances on a chromatogram

Filtration

Separates an insoluble solid from a liquid (e.g., sand from water).
Use filter paper in a funnel; liquid passes through as filtrate, solid remains as residue.
Works because solid particles are too large to pass through filter paper pores.
Also called centrifugation when using a centrifuge.

Filtration apparatus

Crystallisation

Separates a dissolved solid from a solution (e.g., copper(II) sulfate from water).
Heat solution to evaporate solvent until saturated (test with cold glass rod – crystals form).
Allow saturated solution to cool slowly; crystals form as solubility decreases.
Collect crystals by filtration, wash with distilled water, and dry (e.g., between filter paper or in oven).

Crystallisation by evaporation and cooling

Simple Distillation

Separates a liquid from a dissolved solid (e.g., water from salt solution) or a pure liquid from a mixture.
Heat solution; liquid with lower boiling point evaporates first.
Vapour passes through a condenser where it cools and condenses back to liquid.
Collect distillate (pure liquid); solid remains in the flask.

Simple distillation apparatus

Fractional Distillation

Separates two or more miscible liquids with different boiling points (e.g., ethanol and water).
Heat to the lowest boiling point; that substance evaporates first.
Vapour passes through a fractionating column (or condenser) and is collected.
Ethanol (b.p. 78°C) distils first; water (b.p. 100°C) remains.
Used industrially for crude oil separation.

Fractional distillation apparatus

Assessing Purity

Pure substances melt and boil at sharp, specific temperatures (e.g., water: 0°C, 100°C).
Mixtures melt/boil over a range of temperatures.
Impurities lower the melting point and raise the boiling point.
Compare experimental melting/boiling points to literature values to check purity.

Melting point apparatus

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Practice questions

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1.In paper chromatography, why is a pencil line used to mark the starting position instead of a pen?
Easy
- APencil is insoluble in the solvent and will not run up the paper.
- BPencil marks are more visible than pen marks.
- CPen ink would react with the sample.
- DPencil is cheaper than pen.
2.What is the correct order of steps to separate a mixture of sand and salt to obtain pure salt crystals?
Medium
- AFiltration, then crystallisation
- BCrystallisation, then filtration
- CDistillation, then filtration
- DFiltration, then distillation
3.Which separation technique would be most suitable to obtain pure water from a solution of salt water?
Easy
- ASimple distillation
- BFractional distillation
- CCrystallisation
- DFiltration
4.In filtration, what is the name of the solid that remains on the filter paper?
Easy
- AResidue
- BFiltrate
- CDistillate
- DPrecipitate
5.A student performs paper chromatography on a sample of black ink. The chromatogram shows three spots. What does this indicate about the black ink?
Easy
- AIt is a mixture of at least three substances.
- BIt is a pure substance.
- CIt is a single compound.
- DIt contains only one pigment.
6.Which of the following is a physical property that can be used to separate mixtures?
Easy
- ABoiling point
- BReactivity with acid
- CColour
- DTaste
7.The Rf value of a substance in paper chromatography is 0.6. The solvent front moved 10 cm. How far did the substance travel?
Medium
- A6.0 cm
- B0.6 cm
- C16.7 cm
- D6.0 mm
8.A mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) is separated by fractional distillation. Which liquid distils over first?
Easy
- AEthanol
- BWater
- CBoth together
- DNeither

More questions

9.In a chromatography experiment, the solvent front moves 8 cm and a substance moves 5 cm. What is the Rf value?
Medium
- A0.625
- B1.6
- C0.625 cm
- D1.6 cm
10.A pure substance melts at a sharp temperature, while an impure substance melts over a range. Which statement is correct?
Medium
- AImpurities lower the melting point and broaden the melting range.
- BImpurities raise the melting point and sharpen it.
- CImpurities do not affect melting point.
- DImpurities always cause the melting point to increase.
11.A student wants to separate a mixture of two miscible liquids with boiling points of 80 °C and 82 °C. Which technique is most appropriate?
Medium
- AFractional distillation
- BSimple distillation
- CFiltration
- DCrystallisation
12.A chromatogram shows two spots from an unknown sample. The Rf values are 0.45 and 0.70. A known substance has Rf 0.70 under the same conditions. What can be concluded?
Medium
- AThe unknown sample contains the known substance and another compound.
- BThe unknown sample is pure and identical to the known substance.
- CThe known substance is not present in the unknown sample.
- DThe unknown sample contains only one substance.
13.A student is given a mixture of calcium sulfate, magnesium chloride and water. The table shows some information about these substances. | Substance | Solubility in water | State at room temperature | |-----------|---------------------|---------------------------| | calcium sulfate | insoluble | solid | | magnesium chloride | soluble | solid | Which sequence of steps should the student use to obtain magnesium chloride crystals?
Hard
- AFiltration followed by crystallisation
- BCrystallisation followed by filtration
- CDistillation followed by crystallisation
- DFiltration followed by distillation
14.A student performs paper chromatography on a mixture of colourless amino acids. After the solvent front has moved 10 cm, the chromatogram is dried and sprayed with a locating agent. Two spots appear at distances 4.0 cm and 7.5 cm from the baseline. What is the Rf value of the slower-moving amino acid?
Medium
- A0.40
- B0.75
- C0.40 cm
- D0.75 cm
15.The diagram shows the apparatus for fractional distillation of a mixture of ethanol and water. Which labelled part allows the rising vapour to evaporate and condense repeatedly, so that only the vapour of the lower-boiling liquid reaches the condenser?
Medium
- AFractionating column
- BCondenser
- CThermometer
- DElectric heater
16.A pure sample of a solid melts at 150 °C. An impure sample of the same solid would be expected to melt:
Medium
- Aat a lower temperature and over a range
- Bat a higher temperature and over a range
- Cat exactly 150 °C
- Dat a lower temperature but at a sharp point
17.A student carries out paper chromatography on a sample of leaf extract. The chromatogram shows three spots. The Rf values of known pigments are: chlorophyll a = 0.60, chlorophyll b = 0.45, carotene = 0.95, xanthophyll = 0.70. The spots have Rf values 0.45, 0.60 and 0.95. Which pigments are present in the leaf extract?
Hard
- Achlorophyll b, chlorophyll a, carotene
- Bchlorophyll a, chlorophyll b, xanthophyll
- Ccarotene, xanthophyll, chlorophyll a
- Dchlorophyll a, carotene, xanthophyll
18.A student wants to separate a mixture of two miscible liquids with boiling points of 80 °C and 130 °C. Which method is most suitable?
Medium
- AFractional distillation
- BSimple distillation
- CFiltration
- DCrystallisation
19.In paper chromatography, why must the baseline be drawn in pencil?
Easy
- APencil marks are insoluble in the solvent and will not run.
- BPencil is cheaper than pen.
- CPencil marks are more visible.
- DPencil does not react with the substances.
20.A mixture of sand and salt is separated by adding water, stirring, and filtering. What is the residue?
Easy
- ASand
- BSalt
- CSalt solution
- DBoth sand and salt
21.A student measures the boiling point of a liquid sample and finds it boils at 101 °C at standard pressure. Pure water boils at 100 °C. What does this suggest?
Medium
- AThe sample contains dissolved impurities.
- BThe sample is pure water.
- CThe thermometer is faulty.
- DThe pressure is lower than standard.
22.Which separation technique relies on differences in boiling points?
Easy
- ADistillation
- BFiltration
- CCrystallisation
- DChromatography
23.A student performs paper chromatography on a dye and obtains a single spot with Rf = 0.8. The solvent front moved 10 cm. How far did the dye travel?
Medium
- A8.0 cm
- B0.8 cm
- C12.5 cm
- D10.0 cm
24.Which apparatus is used to condense vapour during distillation?
Easy
- ACondenser
- BRound-bottomed flask
- CBeaker
- DFilter funnel
25.In paper chromatography, what is the purpose of using a pencil to draw the baseline?
Easy
- APencil is insoluble in the solvent and will not run
- BPencil is darker and easier to see
- CPencil marks are more precise
- DPencil does not react with the substances
26.A student places a spot of black ink on a pencil line on chromatography paper and places the paper in a solvent. The solvent level is above the pencil line. What is the likely result?
Medium
- AThe ink will dissolve into the solvent and not separate
- BThe ink will separate into its components
- CThe pencil line will dissolve and run up the paper
- DNo separation will occur because the solvent cannot rise
27.A chromatogram shows a solvent front at 8 cm and a spot at 4.8 cm. What is the Rf value?
Easy
- A0.6
- B1.67
- C0.48
- D0.4
28.A student obtains a chromatogram of a colourless amino acid mixture. After drying, the student sprays the paper with a locating agent and heats it. Which row describes the correct procedure?
Medium
- AAdd locating agent to the solvent before the experiment; compare Rf values to known amino acids
- BSpray locating agent on the paper before placing in solvent; compare Rf values to known solvents
- CSpray locating agent on the dried chromatogram; compare Rf values to known amino acids
- DDip the paper in locating agent after the solvent front has run; compare spot colours to a chart
29.Which separation technique would be used to obtain pure water from a mixture of water and ethanol?
Easy
- ASimple distillation
- BFractional distillation
- CFiltration
- DCrystallisation
30.A mixture of sand and salt is separated by adding water, stirring, and filtering. What is the filtrate?
Easy
- AA solution of salt in water
- BSand
- CSalt crystals
- DA mixture of sand and salt
31.A student wants to obtain crystals of copper(II) sulfate from a solution. After heating to evaporate some water, what should the student do next?
Medium
- AAllow the solution to cool slowly
- BFilter immediately
- CAdd more water
- DHeat to dryness
32.A mixture contains two miscible liquids with boiling points of 78°C and 82°C. Which method is most suitable to separate them?
Medium
- ASimple distillation
- BFractional distillation
- CFiltration
- DCrystallisation
33.A solid is heated and melts over a range of temperatures from 120°C to 125°C. What does this indicate?
Easy
- AThe solid is pure
- BThe solid is impure
- CThe solid is a liquid at room temperature
- DThe solid has a high boiling point
34.In the distillation apparatus, what is the purpose of the condenser?
Easy
- ATo heat the mixture
- BTo cool and condense the vapour
- CTo collect the solid residue
- DTo increase the boiling point
35.A student is given a mixture of calcium sulfate (insoluble), magnesium chloride (soluble), and water. Which order of techniques will produce dry magnesium chloride crystals?
Medium
- AFiltration → crystallisation
- BCrystallisation → filtration
- CDistillation → filtration
- DFiltration → distillation
36.A mixture of two liquids is separated by fractional distillation. The liquid with the lower boiling point distils first. Which statement about the process is correct?
Medium
- AThe vapour is richer in the lower boiling point liquid
- BThe vapour is richer in the higher boiling point liquid
- CBoth liquids distil at the same time
- DThe liquid with higher boiling point distils first
37.A student carries out paper chromatography on a mixture of colourless amino acids. After the solvent front has moved 10 cm, the chromatogram is dried and sprayed with a locating agent. Two spots appear at distances 4.0 cm and 7.0 cm from the baseline. What are the Rf values of the two amino acids?
Medium
- A0.40 and 0.70
- B0.40 and 0.30
- C0.60 and 0.30
- D4.0 and 7.0
38.A student is given a mixture of three solids: A (soluble in water, melting point 80 °C), B (insoluble in water, melting point 150 °C), and C (soluble in water, melting point 200 °C). The student adds water, stirs, and filters. The residue is then dried and weighed. Which solid(s) are in the residue?
Hard
- AB only
- BA and B
- CB and C
- DA, B and C
39.The table shows the Rf values of three pigments under the same solvent conditions. | Pigment | Rf value | |---------|----------| | P | 0.43 | | Q | 0.57 | | R | 0.75 | A student runs a chromatography experiment on an unknown sample. The solvent front moves 12 cm. A spot from the unknown appears at 6.84 cm from the baseline. Which pigment is present in the unknown?
Medium
- APigment P
- BPigment Q
- CPigment R
- DNone of these pigments
40.A student measures the boiling point of a liquid sample and finds it boils at 97 °C at standard pressure. Pure water boils at 100 °C. What does this suggest about the sample?
Medium
- AThe sample is pure water
- BThe sample contains a dissolved solid impurity
- CThe sample is a different pure liquid with a lower boiling point
- DThe sample contains an insoluble impurity
41.Which of the following is the correct reason for using a pencil to draw the baseline in paper chromatography?
Easy
- APencil lead is insoluble in the solvent and does not run
- BPencil marks are more visible than pen marks
- CPencil is cheaper than pen
- DPencil does not smudge on paper
42.In paper chromatography, the solvent level should be above the pencil line to allow the solvent to travel up the paper.
Easy
True or false?
43.Match each separation technique to its correct description.
Medium
- Filtration
- Crystallisation
- Simple distillation
- Separates an insoluble solid from a liquid
- Obtains a dissolved solid from a solution
- Separates a liquid from a solution of a solid in a liquid
44.Arrange the following steps in the correct order to obtain crystals of copper(II) sulfate from a solution.
Medium
- Heat the solution to evaporate some solvent
- Allow the saturated solution to cool slowly
- Filter the crystals and wash with distilled water
- Dip a cold glass rod into the solution to test for saturation
45.Which of the following are true about Rf values? (Select all that apply)
Medium
- ARf values have no units
- BRf values are always greater than 1
- CRf values depend on the solvent used
- DRf values are used to identify unknown substances
46.Which apparatus is used to cool and condense vapour during distillation?
Easy
- ACondenser
- BRound-bottomed flask
- CBeaker
- DFilter funnel
47.Fractional distillation is used to separate two or more miscible liquids with different boiling points.
Easy
True or false?
48.Arrange the following steps in the correct order to obtain pure dry crystals of copper(II) sulfate from a solution of copper(II) sulfate.
Medium
- Heat the solution to evaporate some water until saturated
- Allow the saturated solution to cool slowly
- Filter the mixture to collect the crystals
- Wash the crystals with distilled water
- Dry the crystals between sheets of filter paper
49.Which of the following statements about Rf values are correct? (Select all that apply)
Medium
- ARf values have units of cm
- BRf values are always less than 1
- CRf values depend on the solvent used
- DRf values can be used to identify unknown substances
- ERf values are calculated by dividing the distance moved by the solvent by the distance moved by the substance
50.A pure substance melts over a range of temperatures.
Easy
True or false?
51.Complete the sentence: In paper chromatography, substances with higher solubility travel ___ up the paper.
Easy
In paper chromatography, substances with higher solubility travel ____ up the paper.
52.In a chromatography experiment, the solvent front moved 12 cm and a substance moved 7.2 cm. Calculate the Rf value of the substance.
Easy
53.State what a pure substance would look like on a paper chromatogram.
Easy
54.Complete the sentence: If a sample contains impurities, its melting point will appear ___ than the pure substance.
Easy
If a sample contains impurities, its melting point will appear ____ than the pure substance.
55.In a paper chromatography experiment, the solvent front moved 8.0 cm and a substance moved 5.2 cm. Calculate the Rf value of the substance. Give your answer to two decimal places.
Medium
56.Complete the sentence about simple distillation.
Easy
In simple distillation, the vapour is cooled and condensed in a ____.
57.Explain why a student should remove the delivery tube from the water when heating is stopped in a distillation setup.
Medium
58.In a chromatography experiment, the solvent front moved 12 cm and a substance moved 7.2 cm. Calculate the Rf value of the substance.
Medium
- A1.6
- B-0.4
- C0.6
- D2.6
59.In a paper chromatography experiment, the solvent front moved 8.0 cm and a substance moved 5.2 cm. Calculate the Rf value of the substance. Give your answer to two decimal places.
Medium
- A-0.35
- B2.65
- C1.65
- D0.65

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