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GCSE Chemistry · GCSEChapter 17 · Chemical analysis: purity, chromatography and identifying substances
GCSE · Chapter 17

Chemical analysis: purity, chromatography and identifying substances

By the end of this chapter you will be able to

  • Distinguish a pure substance from a mixture, using melting point
  • Calculate an Rf value from a chromatogram
  • Identify a gas from its test result
  • Identify a metal ion from a flame colour or a precipitate colour
  • Choose the correct test for a carbonate, halide or sulfate ion

You should already know

  • Elements, compounds and mixtures (Chapter 1)
  • Ions and their charges (Chapter 4)
  • Reactions of acids and carbonates (Chapter 10)

Where this leads

  • Quantitative analysis and titration (Chapter 10)
  • Tests in the atmosphere and water (Chapter 18)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Explain pure substances, mixtures and formulations, and use melting point as a test for purity, Carry out paper chromatography to separate coloured substances, and calculate Rf values, Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine gases (teacher demonstration), Identify metal ions by flame colour and by precipitates with sodium hydroxide, and anions by their tests, [H] Explain instrumental analysis as a method of identifying substances.

Exercise P  Do you remember?

1
FWhat is the chemical formula of limewater’s active ingredient, calcium hydroxide? Write the formula.
2
FWhich group do the halogens belong to? Write the number.
3
FWhat is 2.0 / 8.0? Write the number as a decimal.
4
FWhich gas relights a glowing splint? Write the name.
5
FWhat is a precipitate? Write the words.
17.1

Pure substances, mixtures and melting point

GCSE
Explain pure substances, mixtures and formulations, and use melting point as a test for purity
Video lesson: Pure substances, mixtures and melting pointComing soon
Context. A pharmacist checks that a tablet contains only one substance. “How can I tell if a sample is pure?” asks a student. In chemistry, a pure substance is one element or compound with nothing else mixed in, and melting point is a simple test.

A pure substance contains only one element or compound. In everyday language, “pure” can mean something useful; in chemistry it means only one substance. A mixture contains two or more substances that are not chemically joined. A formulation is a useful mixture designed for a purpose, such as paint or medicine.

A pure substance has a sharp melting point: it melts at one fixed temperature. An impure substance melts over a range and at a lower temperature than the pure substance.

SampleMelting behaviourPure or impure?
a substance that melts at exactly 114 degrees Csharp, at one temperaturepure (iodine, for example)
a substance that melts over 2 to 5 degrees C below its usual valuemelts over a range, lower than expectedimpure
Using melting point to test for purity

Key ideas

  • Pure substance: one element or compound only.
  • Mixture: two or more substances not chemically joined.
  • Formulation: a useful mixture designed for a purpose.
  • Pure substances melt sharply at one temperature; impure substances melt over a range, lower than expected.
Worked example 17.1.1
A sample of iodine melts at exactly 114 degrees C. Is it pure? Write yes or no.

Method. Compare the melting point with the pure value.

  1. Look. Pure iodine melts at 114 degrees C, and this sample melts sharply at the same temperature.
    sharp, 114
  2. Decide. So the sample is pure.
    yes
  3. Write the answer.
    yes

Check. A sharp melting point at the pure value shows purity. ✓

Worked example 17.1.2
A sample melts over a range, starting at 105 degrees C. Is it pure? Write yes or no.

Method. A range of melting points shows impurity.

  1. Look. The sample melts over a range and starts lower than the pure value.
    range
  2. Decide. So the sample is impure.
    no
  3. Write the answer.
    no

Check. Impurities lower and widen the melting point. ✓

Worked example 17.1.3
Name a formulation, a useful mixture designed for a purpose. Write the name. Accept any of: paint, medicine, fuel.

Method. Recall an example.

  1. Recall. Paint is a formulation: a mixture designed to coat surfaces.
    paint
  2. Check. It is a mixture, not a compound, and it is designed for its use.
    mixture
  3. Write the answer.
    Accept any of: paint, medicine, fuel.

Check. Formulations are mixtures with a purpose. ✓

Watch out

Watch out: a mixture is not a compound, even if it looks uniform. Use melting point or boiling point to test whether a substance is pure, not its appearance.

Exam tip

Exam tip. Say “sharp melting point” for a pure substance and “melts over a range” for an impure one.

Exercise 17A  Pure substances, mixtures and melting point

1
FWhat is a pure substance? Write the words.
2
FWhat is a formulation? Write the words.
3
FAt what kind of point does a pure substance melt? Write the words.
4
FDoes an impure substance melt at a higher or a lower temperature than the pure substance? Write the word.
5
FDoes an impure substance melt over a range? Write yes or no.
6
FIs sea water a mixture or a pure substance? Write the word.
7
PSA sample of water melts at 0 degrees C. Is this consistent with pure water? Write yes or no.
8
PSA sample melts between 105 and 110 degrees C, but pure iodine melts at 114 degrees C. What does this suggest? Write the word: pure or impure.
9
RExplain why melting point can be used to test for purity, in one sentence. Accept any sentence that says impurities change the melting point and make it melt over a range.
10
RExplain why a formulation is a mixture rather than a compound, in one sentence. Accept any sentence that says its substances are not chemically joined in a fixed ratio.
17.2

Chromatography and Rf values: required practical

GCSE
Carry out paper chromatography to separate coloured substances, and calculate Rf values
Video lesson: Chromatography and Rf values: required practicalComing soon
Context. A forensic student wants to know which inks were used in a set of felt-tip pens. “How can I separate the colours?” she asks. Chromatography separates a mixture by how far each substance travels up paper.

In chromatography, a mixture is spotted on a pencil baseline on paper. The mobile phase (the solvent, usually water) moves up the paper and carries the substances with it. The paper is the stationary phase. Substances that dissolve more in the solvent and stick less to the paper travel further. Each substance has a characteristic Rf value:

Rf = distance moved by the spot / distance moved by the solvent front

Paper chromatogram (pencil baseline)pencil linesolvent frontA: Rf 0.25B: Rf 0.75Rf = distance moved by spot / distance moved by solvent
A paper chromatogram: Rf is the spot distance divided by the solvent distance
SpotDistance moved by spot (cm)Solvent front (cm)Rf
A2.08.00.25
B6.08.00.75
C3.28.00.40
Rf values calculated from measured distances

Try it (with your teacher). Required practical: paper chromatography of inks or food colourings. Draw a pencil line 1.5 cm from the bottom of the paper (never use ink for the baseline). Put a small spot of each sample on the line, and dip the bottom in a shallow layer of water. Keep the solvent below the baseline, and let it rise until near the top. Mark the solvent front, dry the paper, and measure each spot distance. Wear goggles and wash your hands at the end.

Key ideas

  • Mobile phase: the solvent; stationary phase: the paper.
  • Rf = spot distance / solvent distance; no units; always between 0 and 1.
  • Different substances have different Rf values in the same solvent.
  • Keep the baseline above the solvent level, so the spots do not dissolve into the solvent.
Worked example 17.2.1
A spot travels 2.0 cm and the solvent travels 8.0 cm. What is the Rf value? Write the number.

Method. Divide the spot distance by the solvent distance.

  1. Divide. Rf = 2.0 / 8.0.
    2.0 / 8.0
  2. Answer. Rf = 0.25.
    0.25
  3. Write the answer.
    0.25

Check. Rf values are always between 0 and 1. ✓

Worked example 17.2.2
A spot moves 6.0 cm and the solvent front is 8.0 cm from the baseline. What is the Rf value? Write the number.

Method. Divide the two distances.

  1. Divide. Rf = 6.0 / 8.0 = 0.75.
    0.75
  2. Check. The spot is nearer the solvent front, so the Rf is high.
    high Rf
  3. Write the answer.
    0.75

Check. 0.75 x 8.0 = 6.0 cm. ✓

Worked example 17.2.3
Which spot is the most soluble in the solvent: one with Rf 0.25 or one with Rf 0.75? Write the Rf value.

Method. The more soluble substance travels further.

  1. Compare. Rf 0.75 travels further than Rf 0.25.
    0.75
  2. Decide. So the Rf 0.75 substance is more soluble in the solvent.
    0.75
  3. Write the answer.
    0.75

Check. A higher Rf means more solubility in the solvent. ✓

Watch out

Watch out: the baseline must be drawn in pencil, not ink, because ink is a mixture and would run up the paper with the sample. The solvent must not cover the baseline.

Exam tip

Exam tip. Measure to the nearest mm, from the baseline to the centre of the spot and to the solvent front. Show the division with its units cancelled.

Exercise 17B  Chromatography and Rf values

1
FWhat is the stationary phase in paper chromatography? Write the word.
2
FWhat is the mobile phase in paper chromatography? Write the word.
3
FWhy must the baseline be drawn in pencil? Write the words.
4
FWhat is the Rf value of a spot that moves 3.0 cm when the solvent moves 6.0 cm? Write the number.
5
FWhat is the Rf value of a spot that moves 1.0 cm when the solvent moves 4.0 cm? Write the number.
6
FA spot moves 4.0 cm and the solvent 10.0 cm. What is the Rf? Write the number.
7
FDoes an Rf value have units? Write yes or no.
8
FWhich spot in a chromatogram is the most soluble in the solvent: the one with the higher or the lower Rf? Write the word.
9
PSTwo spots of the same substance are run in the same solvent. What should their Rf values be? Write the words.
10
RExplain why the solvent level must be below the pencil baseline, in one sentence. Accept any sentence that says otherwise the sample would dissolve into the solvent instead of moving up the paper.
11
RExplain how two substances with different Rf values can be identified as different, in one sentence. Accept any sentence that says different substances travel different distances in the same solvent.
17.3

Tests for gases

GCSE
Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine gases (teacher demonstration)
Video lesson: Tests for gasesComing soon
Context. A teacher collects a gas from a reaction and asks the class to identify it. “How can we tell what it is?” asks a pupil. Each gas has a test that gives a characteristic result.

The tests for gases are done by a teacher demonstration only. Hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper.

GasTestPositive result
hydrogenlighted splint held at the mouth of the tubesqueaky pop
oxygenglowing splint placed in the gasrelights
carbon dioxidebubble through limewaterlimewater turns milky
chlorinedamp litmus paper held in the gasbleached (turns white)
Tests for gases (teacher demonstration only)

Key ideas

  • Hydrogen: lighted splint gives a squeaky pop.
  • Oxygen: a glowing splint relights.
  • Carbon dioxide: limewater turns milky.
  • Chlorine: damp litmus paper is bleached.
Worked example 17.3.1
Which gas turns limewater milky? Write the name.

Method. Recall the test.

  1. Recall. Carbon dioxide turns limewater milky.
    carbon dioxide
  2. Check. Limewater is calcium hydroxide solution, which goes cloudy with CO2.
    cloudy
  3. Write the answer.
    carbon dioxide

Check. Limewater is calcium hydroxide solution. ✓

Worked example 17.3.2
A gas makes a squeaky pop with a lighted splint. Which gas is it? Write the name.

Method. Recall the test result.

  1. Recall. A squeaky pop shows hydrogen.
    hydrogen
  2. Decide. So the gas is hydrogen.
    hydrogen
  3. Write the answer.
    hydrogen

Check. Hydrogen burns to form water. ✓

Worked example 17.3.3
A glowing splint is relit in a gas. Which gas is present? Write the name.

Method. Recall the test result.

  1. Recall. A glowing splint relights in oxygen.
    oxygen
  2. Decide. So the gas is oxygen.
    oxygen
  3. Write the answer.
    oxygen

Check. Oxygen supports combustion. ✓

Watch out

Watch out: the test for hydrogen is a squeaky pop, not a bang. The test for oxygen is relighting a glowing splint, not a flame.

Exam tip

Exam tip. Write the test and the result together, for example “limewater turns milky”.

Exercise 17C  Tests for gases

1
FWhat is the result of the test for hydrogen? Write the words.
2
FWhat is the result of the test for oxygen? Write the words.
3
FWhat is the result of the test for carbon dioxide? Write the words.
4
FWhat is the result of the test for chlorine? Write the words.
5
PSA gas relights a glowing splint. Which gas is it? Write the name.
6
PSA gas gives a squeaky pop with a lighted splint. Which gas is it? Write the name.
7
FWhich gas bleaches damp litmus paper? Write the name.
8
FWhich test identifies carbon dioxide? Write the name of the reagent. Write the reagent.
9
RWhy is the gas-test practical carried out as a teacher demonstration only? Write one reason.
10
RExplain why a limewater test does not identify oxygen, in one sentence. Accept any sentence that says oxygen does not make limewater milky.
17.4

Tests for metal ions

GCSE
Identify metal ions by flame colour and by precipitates with sodium hydroxide
Video lesson: Tests for metal ionsComing soon
Context. A tube contains a solution of a metal salt, and the label has been lost. “Which metal ion is it?” asks a pupil. Two tests help: the colour of a flame, and the colour of a precipitate formed with sodium hydroxide solution.

Flame tests are a teacher demonstration only. A metal ion gives a characteristic flame colour: lithium is red, sodium is yellow, potassium is lilac, and copper is green-blue. With sodium hydroxide solution, many metal ions form a coloured precipitate (an insoluble solid). Aluminium forms a white precipitate that dissolves in excess; copper(II) forms a light blue precipitate; iron(II) forms a green one; iron(III) forms a red-brown one.

Metal ionFlame colour (teacher demonstration)
lithiumred
sodiumyellow
potassiumlilac
coppergreen-blue
Flame colours of some metal ions (teacher demonstration only)
IonColour of precipitate with sodium hydroxide
Al3+white (dissolves in excess)
Ca2+white
Cu2+light blue
Fe2+green
Fe3+red-brown
Mg2+white
Colours of the precipitates formed with sodium hydroxide solution

Key ideas

  • Flame test: lithium red, sodium yellow, potassium lilac, copper green-blue.
  • Sodium hydroxide: a coloured precipitate identifies the metal ion.
  • Aluminium gives a white precipitate that dissolves in excess sodium hydroxide.
  • Iron(II) is green; iron(III) is red-brown; copper(II) is light blue.
Worked example 17.4.1
A solution forms a light blue precipitate with sodium hydroxide. Which metal ion is present? Write the ion.

Method. Match the colour to the table.

  1. Look. A light blue precipitate is formed with copper(II) ions.
    light blue
  2. Decide. So the ion is Cu2+.
    Cu2+
  3. Write the answer.
    Cu2+

Check. Copper(II) hydroxide is light blue. ✓

Worked example 17.4.2
A flame turns lilac. Which metal ion is present? Write the name.

Method. Recall the flame colour.

  1. Recall. A lilac flame shows potassium.
    potassium
  2. Decide. So the metal ion is potassium.
    potassium
  3. Write the answer.
    potassium

Check. Potassium is in group 1. ✓

Worked example 17.4.3
A red-brown precipitate forms with sodium hydroxide. Which ion is it? Write the ion.

Method. Match the colour.

  1. Recall. A red-brown precipitate is iron(III).
    red-brown
  2. Decide. So the ion is Fe3+.
    Fe3+
  3. Write the answer.
    Fe3+

Check. Iron(III) hydroxide is red-brown. ✓

Watch out

Watch out: iron(II) and iron(III) give different precipitate colours. Green is iron(II); red-brown is iron(III).

Exam tip

Exam tip. Give the ion, its charge, and the colour in your answer.

Exercise 17D  Tests for metal ions

1
FWhat flame colour does lithium give? Write the colour.
2
FWhat flame colour does sodium give? Write the colour.
3
FWhat flame colour does potassium give? Write the colour.
4
FWhat flame colour does copper give? Write the colour.
5
FWhat colour precipitate does copper(II) form with sodium hydroxide? Write the colour.
6
FWhat colour precipitate does iron(II) form with sodium hydroxide? Write the colour.
7
FWhat colour precipitate does iron(III) form with sodium hydroxide? Write the colour.
8
FWhich ion gives a white precipitate that dissolves in excess sodium hydroxide? Write the ion.
9
PSA flame turns yellow. Which metal ion is present? Write the name.
10
PSA solution forms a green precipitate with sodium hydroxide. Which ion is present? Write the ion.
11
RExplain why a flame test is useful for identifying a metal ion, in one sentence. Accept any sentence that says each metal ion gives a characteristic flame colour.
12
RExplain why sodium hydroxide solution is used to test for metal ions, in one sentence. Accept any sentence that says many metal ions form coloured precipitates with it.
17.5

Tests for anions and instrumental analysis [H]

GCSE
Identify anions by their tests, [H] Explain instrumental analysis as a method of identifying substances
Video lesson: Tests for anions and instrumental analysis [H]Coming soon
Context. A pupil tests a white solid and sees that it fizzes with acid, giving a gas that turns limewater milky. “What is the anion?” she asks. The fizz and the test for carbon dioxide point to a carbonate.

Carbonates fizz with dilute acid, and the gas turns limewater milky. Halides (chloride, bromide, iodide) give a precipitate with silver nitrate, after dilute nitric acid is added: white (chloride), cream (bromide) or yellow (iodide). Sulfates give a white precipitate with barium chloride, after dilute hydrochloric acid is added.

[H] Higher tier only. Instrumental analysis uses machines such as mass spectrometers and infrared spectrometers to identify substances very quickly and accurately. It can detect very small amounts of a substance and identify the molecules by their mass or the bonds they contain. It is more accurate than simple chemical tests.

AnionTestResult
carbonate, CO32-add dilute acid; test gas with limewaterfizzes; gas turns limewater milky
chloride, Cl-add dilute nitric acid, then silver nitratewhite precipitate
bromide, Br-add dilute nitric acid, then silver nitratecream precipitate
iodide, I-add dilute nitric acid, then silver nitrateyellow precipitate
sulfate, SO42-add dilute hydrochloric acid, then barium chloridewhite precipitate
Tests for anions

Key ideas

  • Carbonate: fizzes with acid; the gas turns limewater milky.
  • Halide: silver nitrate after dilute nitric acid; white, cream or yellow precipitate.
  • Sulfate: barium chloride after dilute hydrochloric acid; white precipitate.
  • [H] Instrumental analysis is accurate and quick, and can detect small amounts.
Worked example 17.5.1
A white solid fizzes with dilute acid, and the gas turns limewater milky. What anion is present? Write the formula.

Method. The fizz with acid and the limewater test show carbon dioxide.

  1. Gas. The gas is carbon dioxide.
    carbon dioxide
  2. Anion. Carbon dioxide from acid shows a carbonate, CO32-.
    CO3 2-
  3. Write the answer.
    CO3 2-

Check. The carbonate ion is CO3 2-. ✓

Worked example 17.5.2
Silver nitrate, after dilute nitric acid, gives a cream precipitate. Which halide is present? Write the name.

Method. Recall the colours of the halide precipitates.

  1. Recall. Bromide gives a cream precipitate with silver nitrate.
    cream
  2. Decide. So the halide is bromide.
    bromide
  3. Write the answer.
    bromide

Check. Chloride is white and iodide is yellow. ✓

Worked example 17.5.3
[H] Name the technique that uses a machine to identify a substance quickly by its mass. Write the name.

Method. Recall the technique.

  1. Recall. A mass spectrometer identifies substances by their mass.
    mass spectrometry
  2. Decide. So the technique is mass spectrometry.
    mass spectrometry
  3. Write the answer.
    mass spectrometry

Check. It is an instrumental method. ✓

Watch out

Watch out: for halides and sulfates, the acid is added first, to remove other ions that would also form precipitates. Use dilute nitric acid for halides and dilute hydrochloric acid for sulfates.

Exam tip

Exam tip. Name the test, the reagent, and the result: for example “silver nitrate gives a cream precipitate”.

Exercise 17E  Tests for anions and instrumental analysis [H]

1
FWhat is the result of adding dilute acid to a carbonate? Write the words.
2
FWhat precipitate does silver nitrate give with chloride ions? Write the colour.
3
FWhat precipitate does silver nitrate give with bromide ions? Write the colour.
4
FWhat precipitate does silver nitrate give with iodide ions? Write the colour.
5
FWhich reagent is used to test for sulfate ions, with dilute hydrochloric acid? Write the name.
6
FWhat precipitate does barium chloride give with sulfate ions? Write the colour.
7
PSWhy is dilute nitric acid added before silver nitrate in a halide test? Write one reason.
8
PSA white solid gives a white precipitate with silver nitrate after dilute nitric acid. Name the anion that is likely. Write the name.
9
F[H] What is the main advantage of instrumental analysis over simple chemical tests? Write the words.
10
F[H] Name the instrument that identifies substances by their infrared absorption. Write the name.
11
R[H] Explain why instrumental analysis can detect a substance in a very small sample, in one sentence. Accept any sentence that says it is sensitive, so only a small amount of the substance is needed.

Chapter summary

SkillMethod
Pure substanceOne element or compound; sharp melting point at one temperature.
FormulationA useful mixture designed for a purpose.
ChromatographyRf = spot distance / solvent distance; no units; 0 to 1.
Gas testsHydrogen: squeaky pop. Oxygen: relights splint. CO2: milky limewater. Chlorine: bleaches litmus.
Metal ionsFlame: Li red, Na yellow, K lilac, Cu green-blue. Precipitates with NaOH: Cu2+ light blue, Fe2+ green, Fe3+ red-brown, Al3+ white (dissolves).
AnionsCarbonate: fizz and milky. Halides: silver nitrate (white, cream, yellow). Sulfate: barium chloride (white).
[H] Instrumental analysisMachines such as mass spectrometers and infrared spectrometers: quick, accurate, sensitive.

Can you…?

  • I can use melting point to test whether a substance is pure
  • I can carry out paper chromatography safely and calculate Rf values
  • I can identify hydrogen, oxygen, carbon dioxide and chlorine from their test results
  • I can identify metal ions from flame colours and precipitate colours
  • I can identify carbonate, halide and sulfate ions, and describe instrumental analysis [H]

Key terms

pure substance
a substance made of only one element or compound
formulation
a useful mixture designed for a purpose
mobile phase
the solvent that carries the substances up the paper in chromatography
stationary phase
the paper, which does not move, in chromatography
Rf value
the distance moved by a spot divided by the distance moved by the solvent
precipitate
an insoluble solid formed when two solutions react
instrumental analysis
[H] identifying substances using machines such as mass or infrared spectrometers

Exercise 17Z  Check-up

1
What is the Rf value of a spot that moves 2.0 cm when the solvent moves 8.0 cm? Write the number.
2
Does impurity lower or raise the melting point of a substance? Write the word.
3
Which gas turns limewater milky? Write the name.
4
What is the flame colour of lithium? Write the colour.
5
What colour precipitate does copper(II) give with sodium hydroxide? Write the colour.
6
Which gas gives a squeaky pop with a lighted splint? Write the name.
7
What precipitate does silver nitrate give with iodide ions? Write the colour.
8
Which reagent gives a white precipitate with sulfate ions? Write the name.
9
A spot moves 3.0 cm and the solvent 12.0 cm. What is the Rf value? Write the number.
10
Explain why a flame test is useful, in one sentence. Accept any sentence that says each metal ion gives a characteristic flame colour.

Total 24 marks. Allow about 24 minutes.

Exercise 17X  Exam-style questions

1
RA pure substance melts at 114 degrees C. A sample melts over a range starting at 105 degrees C. Which is pure, and why? Write one sentence. Accept the pure one melts sharply at 114 degrees C; the other is impure because it melts over a lower range.[3 marks]
2
PSA spot travels 5.0 cm and the solvent 10.0 cm in chromatography. Calculate the Rf value. Write the number.[2 marks]
3
FName the gas that turns limewater milky, and the gas that bleaches damp litmus paper. Write two names.[2 marks]
4
PSA flame turns lilac and a precipitate with sodium hydroxide is red-brown. Name the two metal ions. Write the two ion symbols.[2 marks]
5
FDescribe the test for a carbonate and the result for a positive test. Write two sentences. Accept any answer that says add dilute acid, which fizzes, and the gas turns limewater milky.[3 marks]
6
FDescribe the test for a chloride ion, including the reagents, in two sentences. Accept any answer that says add dilute nitric acid then silver nitrate, forming a white precipitate.[3 marks]
7
RExplain why flame tests are carried out by a teacher only, in one sentence. Accept any sentence that says the flames and the chemicals can be hazardous.[2 marks]
8
FName the test used to identify a sulfate ion and the precipitate formed. Write two names.[2 marks]
9
R[H] Explain what instrumental analysis is, in one or two sentences. Accept any answer that says machines such as mass spectrometers identify substances accurately and quickly.[3 marks]
10
RIn a chromatogram, the pencil baseline is above the solvent. Explain why, in one sentence. Accept any sentence that says the spots would otherwise dissolve into the solvent and be lost.[2 marks]
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