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Cambridge IGCSE0620

Experimental design

Chemistry 0620 Chapter Notes

What this chapter covers

Experimental designAcid–base titrationsChromatographySeparation and purificationIdentification of ions and gases
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Experimental design notes

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In this chapter

  1. 1Measuring time and temperature
  2. 2Measuring mass and volume of liquids
  3. 3Exact volumes: burette and volumetric pipette
  4. 4Measuring the volume of a gas
  5. 5Advantages and disadvantages of methods and apparatus
  6. 6Solvent, solute, solution and saturated solution
  7. 7Residue and filtrate
  8. 8Burette and volumetric pipette
  9. 9Rinsing the burette
  10. 10A suitable indicator
  11. 11Carrying out a titration
  12. 12Identifying the end-point
  13. 13Separating coloured mixtures
  14. 14Identifying unknown substances
  15. 15Pure and impure substances
  16. 16Colourless substances and locating agents
  17. 17Calculating Rf values
  18. 18Solvent and filtration
  19. 19Crystallisation
  20. 20Simple distillation
  21. 21Fractional distillation
  22. 22Choosing a technique
  23. 23Purity from melting and boiling points
  24. 24Testing for gases
  25. 25Halide ions: chloride, bromide and iodide
  26. 26Carbonate, sulfate and sulfite ions
  27. 27Nitrate and ammonium ions
  28. 28Aluminium, calcium and zinc ions
  29. 29Chromium(III), copper(II) and iron ions
  30. 30Flame tests

1. Measuring time and temperature

Time is measured with a stop-watch. Temperature is measured with a thermometer.

Record each reading as the apparatus allows. In the practical papers, times are recorded in whole numbers of seconds and temperatures to the nearest 0.5 °C.

A digital thermometer has a greater resolution than a liquid-in-glass thermometer. It is not more accurate.

Examiner insight

“Some candidates proposed using a digital thermometer, suggesting a confusion between the resolution of the reading and the accuracy.” “It should be noted that digital thermometers (or thermocouples) are not necessarily more accurate than liquid-in-glass thermometers; while they do have a greater resolution, they are no more accurate (and often less accurate) than liquid-in-glass thermometers.”

Worked example 12 marks

A student reacts a solid with dilute acid. Name the apparatus used to measure(i) the time taken for the reaction and(ii) the temperature of the mixture.

  1. 1

    (i) stop-watch [1]

  2. 2

    (ii) thermometer [1]

2. Measuring mass and volume of liquids

A balance measures mass. A measuring cylinder measures the volume of a liquid.

Pick the smaller cylinder for a small volume. A 10 cm³ measuring cylinder is more accurate than a 50 cm³ one for a small volume.

A measuring cylinder is a quick way to measure a volume. It is less accurate than a burette or a volumetric pipette.

Examiner insight

“A significant number thought a (graduated) beaker or a measuring cylinder, which they had just used, would be more accurate.” “Many candidates decided that a measuring cylinder should be used instead of a burette, which was an inappropriate idea.”

Worked example 11 mark

The volume of the dilute ethanoic acid is measured using a 10 cm³ measuring cylinder. State why a 10 cm³ measuring cylinder is used rather than a 50 cm³ measuring cylinder to measure the volume of the dilute ethanoic acid.

  1. 1

    a 10 cm³ measuring cylinder is more accurate [1]

3. Exact volumes: burette and volumetric pipette

A burette delivers any volume you choose. Read it from its scale. On a burette the numbers increase downwards, so the final reading is always greater than the initial reading. Record every reading to 1 decimal place, such as 24.0 cm³.

A volumetric pipette measures one fixed volume only, such as exactly 25.00 cm³. A safety pipette filler must always be used with it.

volume added = final reading - initial reading

Examiner insight

“Candidates should recall that a beaker is not used to give precise measurements and that a 25 cm3 pipette will measure only 25 cm3.” “A significant number of candidates did not realise that burettes are accurate to one decimal place and gave a reading of 24 cm3 instead of 24.0 cm3.”

Worked example 11 mark

Which piece of apparatus is used to measure exactly 25.00 cm³ of dilute hydrochloric acid?

A a balance · B a beaker · C a measuring cylinder · D a volumetric pipette

  1. 1

    D: a volumetric pipette. [1]

4. Measuring the volume of a gas

A gas syringe collects a gas and measures its volume. A measuring cylinder can also be used, but only over water: it is filled with water and inverted in a trough of water.

Time the collection with a stop-watch to find the mean rate of reaction.

mean rate of reaction = (volume of gas collected)/(time taken to collect the gas)

Examiner insight

“Some candidates thought that a measuring cylinder could be used but did not know that it must be over water.” “Using a gas syringe was a common correct method but those candidates who used collection over water, often used a test-tube instead of a measuring cylinder but still claimed they would be able to measure the volume.”

Worked example 11 mark

Magnesium carbonate reacts with dilute sulfuric acid.

MgCO₃(s) + H₂SO₄(aq) → MgSO₄(aq) + CO₂(g) + H₂O(l)

Name the piece of apparatus used to collect and measure the volume of carbon dioxide produced in this reaction.

  1. 1

    (gas) syringe [1]

5. Advantages and disadvantages of methods and apparatus

Judge apparatus by what the task needs. A burette is more accurate than a measuring cylinder and can measure variable volumes. A volumetric pipette is also more accurate, but it measures one volume only.

A measuring cylinder is quick and easy to use, but it is less accurate. A powder is used rather than large lumps to increase the rate of dissolving.

If gas escapes before the stopper is replaced, the volume of gas collected is smaller. When a gas is collected over water in an inverted measuring cylinder, the cylinder also holds air from the flask. This has no effect on accuracy: the volume of air collected is the same as the volume of gas left in the flask.

Examiner insight

“Credit was given for mentioning the speed or ease of measurement. Reference to convenience was ignored.” “Many candidates incorrectly thought that the air in the apparatus would increase the volume of gas collected and so make the results inaccurate.”

Worked example 12 marks

The gas collected in the inverted measuring cylinder contains air from the conical flask. State what effect, if any, this air will have on the accuracy of the results of the investigation. Explain your answer.

  1. 1

    none: there is no effect on accuracy [1]

  2. 2

    the volume of air collected is the same as the volume of carbon dioxide left in the flask [1]

6. Solvent, solute, solution and saturated solution

A solvent is a substance that dissolves a solute. A solute is a substance that is dissolved in a solvent.

A solution is a mixture of one or more solutes dissolved in a solvent.

A saturated solution is a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature.

Examiner insight

“The temperature dependency part was occasionally omitted and weaker responses confused the terms solute and solvent.”

Worked example 14 marks

A student stirs solid X into 100 cm³ of water at 25 °C. The student keeps adding X until some solid stays undissolved.

(a) Name the solute and the solvent.(b) State what is meant by a saturated solution.

  1. 1

    (a) solute: X, because it is the substance that is dissolved [1]

  2. 2

    solvent: water, because it is the substance that dissolves the solute [1]

  3. 3

    (b) a solution containing the maximum concentration of a solute dissolved in the solvent [1]

  4. 4

    at a specified temperature, here 25 °C [1]

7. Residue and filtrate

A residue is a substance that remains after evaporation, distillation, filtration or any similar process. A filtrate is a liquid or solution that has passed through a filter.

In filtration, the insoluble solid stays on the filter paper in the funnel: this is the residue. The liquid that drips through into the beaker is the filtrate.

To make crystals from an insoluble base and an acid, filter off the excess solid first. Then heat the filtrate.

Examiner insight

“Filtrate or solution was credited for the general name given to the liquid in the dish. Common wrong answers were solute, solvent, water and residue.” “Candidates who performed less well overall tended to choose option A, where they confused the terms soluble and insoluble or option C where they incorrectly assumed that the filtrate is the solid extracted from the mixture.”

Worked example 16 marks

Hydrated chromium(III) nitrate is a salt which is soluble in water. Hydrated chromium(III) nitrate is made by reacting the insoluble base chromium(III) oxide with dilute nitric acid. Chromium(III) oxide reacts very slowly with dilute nitric acid at room temperature.

Describe how to make pure, dry crystals of hydrated chromium(III) nitrate.

You are provided with chromium(III) oxide, dilute nitric acid and common laboratory apparatus.

  1. 1

    add chromium(III) oxide to dilute nitric acid in a suitable named container, such as a beaker [1]

  2. 2

    the chromium(III) oxide is in excess [1]

  3. 3

    the mixture is stirred [1]

  4. 4

    the mixture is heated or warmed to speed up the reaction [1]

  5. 5

    filter the reaction mixture: the excess oxide is the residue and the solution is the filtrate [1]

  6. 6

    heat or warm the filtrate to remove water [1]

  7. 7

    then cool and filter out the crystals (an alternative point: any six score)

Recap

  • Time: stop-watch. Temperature: thermometer. Mass: balance.
  • Volume of a liquid: measuring cylinder. Exact variable volumes: burette. One exact volume: volumetric pipette.
  • Volume of a gas: gas syringe, or a measuring cylinder over water.
  • Burette and pipette are more accurate than a measuring cylinder. A measuring cylinder is quicker.
  • Air collected with a gas has no effect on accuracy. Gas escaping before the stopper is replaced makes the volume collected smaller.
  • Solvent dissolves the solute. Solute is dissolved in the solvent. Together they make the solution.
  • Saturated solution: the maximum concentration of solute dissolved in the solvent at a specified temperature.
  • Residue stays on the filter paper. Filtrate passes through.

8. Burette and volumetric pipette

An acid–base titration uses two pieces of glassware to measure volume accurately.

A burette can measure variable volumes. A volumetric pipette can only measure a single volume, such as 25 cm³.

A volumetric pipette is more accurate than a measuring cylinder.

Examiner insight

“Candidates that performed well were able to correctly state that a volumetric pipette can only measure a single volume while a burette can measure variable volumes.” “Many of the answers suggested that candidates were thinking of dropping or Pasteur pipettes.”

Worked example 11 mark

Which piece of apparatus is used to measure accurately the volume of a liquid in an acid–base titration?

A a balance · B a beaker · C a burette · D a conical flask

  1. 1

    C: a burette. [1]

9. Rinsing the burette

A burette is rinsed with distilled water, not tap water. Distilled water has fewer dissolved ions than tap water.

The water left from rinsing is removed before the burette is filled. Water left in the burette would dilute the solution put in it.

A greater volume would then be needed from the burette to reach the end-point.

Examiner insight

“Water left in the burette would decrease the concentration of the acid, and so a greater volume would need to be added from the burette to reach the endpoint.” “Many candidates correctly stated that the distilled water that remained in the burette needed to be removed.”

Worked example 11 mark

In a titration, why is distilled water and not tap water used to rinse the burette?

A Distilled water contains more bacteria than tap water. · B Distilled water contains fewer dissolved ions than tap water. · C Distilled water dissolves substances better than tap water. · D Distilled water has been filtered and tap water has not.

  1. 1

    B: distilled water contains fewer dissolved ions than tap water. [1]

10. A suitable indicator

The indicator is used to identify the end-point. A suitable indicator has a sharp single colour change at the end-point. Methyl orange is one.

Universal Indicator is not suitable. It shows a series of gradual colour changes, so there is no sharp end-point.

Examiner insight

“Weaker responses gave indicators unsuitable for titration such ‘Universal Indicator’ or ‘pH indicator’.” “It should be noted that universal indicator is not suitable for use in a titration as it does not have a sharp end point.”

Worked example 12 marks

A student suggests using Universal Indicator in a titration. Explain why this is not a suitable indicator.

  1. 1

    Universal Indicator shows a series of gradual colour changes [1]

  2. 2

    so it does not have a sharp single colour change at the end-point [1]

11. Carrying out a titration

A volumetric pipette puts a fixed volume of one solution into a conical flask. A few drops of indicator are added.

The other solution goes in the burette. The area below the tap will be full of air. This is removed before the titration begins.

The solution is added from the burette dropwise. The flask is swirled so that its contents are mixed. The volume added is read from the burette.

Examiner insight

“Only better performing candidates realised that the area below the tap will be full of air and this needs to be removed before the titration begins.” “With questions of this type, candidates need to think whether the solution in question is in the conical flask or the burette.”

Worked example 13 marks

A student has dilute hydrochloric acid and aqueous sodium hydroxide. Describe how the student sets up and starts a titration of the acid against the alkali.

  1. 1

    use a volumetric pipette to add the dilute hydrochloric acid to a conical flask [1]

  2. 2

    add a few drops of a suitable indicator, such as methyl orange [1]

  3. 3

    add the aqueous sodium hydroxide from a burette, dropwise [1]

12. Identifying the end-point

The end-point is reached when the colour of the indicator just changes. Adding the solution from the burette quickly overshoots the end-point.

A white tile under the flask makes the colour change easier to see.

The reactions used in titrations are almost instantaneous, so the rate does not matter. Warming the solution in the flask gives no change to the volume added.

Examiner insight

“While the increase in temperature would increase the rate of the reaction between the hydrogen ions from the acid and the carbonate ions from the sodium carbonate, the reaction is already almost instantaneous (as are all reactions used in titrations), hence any increase in the rate is irrelevant.” “Rapid addition will lead to the end point being overshot.”

Worked example 11 mark

Dilute hydrochloric acid is titrated against aqueous sodium hydroxide.

At the start of the titration, dilute hydrochloric acid is added to a conical flask using a …⁠1⁠…

A few drops of methyl orange indicator are added to the dilute hydrochloric acid in the conical flask.

Aqueous sodium hydroxide is added …⁠2⁠… until the colour of the indicator just …⁠3⁠…

Which row completes the gaps 1, 2 and 3?

123
Aburettedropwisedisappears
Bburette1 cm³ at a timechanges
Cvolumetric pipettedropwisechanges
Dvolumetric pipette1 cm³ at a timedisappears
  1. 1

    C: volumetric pipette, dropwise, changes. [1]

Recap

  • A burette measures variable volumes. A volumetric pipette measures a single volume.
  • The burette is rinsed with distilled water, which has fewer dissolved ions than tap water.
  • Water left in the burette dilutes the solution, so a greater volume is needed to reach the end-point.
  • A suitable indicator has a sharp single colour change. Universal Indicator is not suitable.
  • Add from the burette dropwise. The end-point is where the indicator colour just changes.
  • The rate of the reaction is irrelevant in a titration.

13. Separating coloured mixtures

Paper chromatography separates a mixture of soluble coloured substances. A spot of the mixture goes on a baseline drawn in pencil. The paper stands in a suitable solvent, with the baseline above the solvent level.

The solvent rises up the paper. The substances travel different distances, so they separate. A more soluble substance travels further than a less soluble one.

A lid on the container prevents loss of the solvent. Water is no use for dyes that are not soluble in water.

Examiner insight

“Stronger answers referred to keeping the base line above the solvent level or stated that the paper should not fall into the solvent as the dyes would mix.” “Common errors included stating that it prevented the loss of water (the solvent was not water); that it provided insulation or that it held the paper/clips up.”

Worked example 14 marks

Describe how paper chromatography is used to separate a mixture of soluble coloured substances. Use a suitable solvent.

  1. 1

    draw a baseline on the paper in pencil [1]

  2. 2

    put a spot of the mixture on the baseline [1]

  3. 3

    stand the paper in the solvent, with the baseline above the solvent level [1]

  4. 4

    leave it while the solvent rises up the paper: the substances travel different distances and separate [1]

14. Identifying unknown substances

Identify an unknown by comparison with known substances. A spot that travels the same distance as a known substance's spot is that substance.

Compare heights, not colours. Two spots of the same colour at different heights are different substances.

An unknown contains a known dye only if every spot of that dye matches a spot of the unknown.

Examiner insight

“A third of the other candidates chose option D, by counting the number of ‘red’ spots without considering whether they represented the same or different dyes.”

Worked example 11 mark

The diagram shows a chromatogram for an ink sample and four other dyes, 1, 2, 3 and 4. Which dyes could be part of the ink sample?

A 1 and 2 · B 1 and 3 · C 2 and 3 · D 2 and 4

  1. 1

    the ink has three spots, at three different heights

  2. 2

    dyes 1 and 3 each have a spot at a height where the ink has none, so they are not in the ink

  3. 3

    every spot of dyes 2 and 4 matches a spot of the ink

  4. 4

    D: 2 and 4 [1]

15. Pure and impure substances

A pure substance gives one spot. An impure substance, a mixture, gives more than one spot.

A substance that is insoluble in the solvent stays on the baseline.

Count the spots in a column before judging a sample. A spot that travels far can still belong to a mixture.

Examiner insight

“A large number of incorrect responses referred to dye C being colourless, pure, only one substance or too dense to move. Stronger responses realised that dye C was insoluble in the solvent.” “Many candidates selected option B, which contained a substance with the largest Rf value but did not recognise that this sample was impure.”

Worked example 11 mark

Four coloured substances, P, Q, R and S, are analysed by chromatography. The chromatogram is shown.

Which statements are correct?

1 P is less soluble in the solvent than R. · 2 R is a pure substance. · 3 Q is insoluble in the solvent used. · 4 S is a mixture of P and R.

A 1, 2 and 3 · B 1, 2 and 4 · C 2, 3 and 4 · D 3 and 4 only

  1. 1

    statement 1 is false: P travels further than R, so P is more soluble

  2. 2

    statement 2 is true: R gives one spot

  3. 3

    statement 3 is true: Q stays on the baseline

  4. 4

    statement 4 is true: S gives two spots, at the heights of P and R

  5. 5

    C: 2, 3 and 4 [1]

16. Colourless substances and locating agents

Colourless substances leave no visible spots. A locating agent makes the spots visible, so the separation can be read.

The method is otherwise the same: a suitable solvent, and different distances travelled. Knowledge of specific locating agents is not required.

An insoluble colourless substance is still on the baseline, and the locating agent shows it there.

Examiner insight

“For any of the colourless amino acids to be seen a locating agent would be used and if an amino acid was insoluble, it would still be present on the baseline.”

Worked example 11 mark

Locating agents are often used in paper chromatography.

Which mixture requires a locating agent?

A a mixture of two colourless substances · B a mixture of two insoluble substances · C a mixture of two different-coloured substances with the same Rf value · D a mixture of two substances that are the same colour

  1. 1

    colourless spots cannot be seen, so a locating agent is needed

  2. 2

    B is about solubility, not colour: insoluble substances stay on the baseline

  3. 3

    C and D describe coloured substances, which can already be seen

  4. 4

    A: a mixture of two colourless substances [1]

17. Calculating Rf values

The Rf value is the distance travelled by substance divided by the distance travelled by solvent. Measure both from the baseline, in the same unit.

It is a ratio of two distances, so it has no units. In the figure, Rf = a ÷ b: substance on top, solvent underneath.

Two substances with the same Rf value do not separate at all.

Rf = (distance travelled by substance)/(distance travelled by solvent)

Examiner insight

“Despite the instructions given, many candidates were unable to measure the distances required to score full credit.” “Candidates did not realise that two substances with the same Rf value would not separate at all.”

Worked example 11 mark

A chemical mixture is separated using paper chromatography.

The chromatogram is shown.

Which statements are correct?

1 The spot nearest the solvent front has an Rf value greater than 0.50. · 2 Insoluble compounds rise with the solvent front. · 3 The lower spot is a less soluble substance than the higher spot.

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

  1. 1

    statement 1 is true: the upper spot is over half way up, so Rf > 0.50

  2. 2

    statement 2 is false: insoluble compounds stay on the baseline

  3. 3

    statement 3 is true: the lower spot travels less far

  4. 4

    C: 1 and 3 only [1]

Recap

  • Paper chromatography: pencil baseline, spot of mixture, baseline above the solvent level.
  • Unknowns are identified by comparison with known substances: same distance, same substance.
  • Pure = one spot. Impure = more than one spot. Insoluble = stays on the baseline.
  • Extended: colourless substances need a locating agent to show the spots.
  • Extended: Rf = distance travelled by substance ÷ distance travelled by solvent.

18. Solvent and filtration

Adding a suitable solvent dissolves the soluble solid in a mixture. Filtration then separates an insoluble solid from a liquid.

The solid left on the filter paper is the residue. The liquid that passes through the filter paper is the filtrate.

For sand and solid sodium chloride: add a solvent, then filter. The sand is the residue.

Examiner insight

“Many candidates confused filtrate and residue, putting the residue in the collecting beaker and the filtrate in the funnel.” “Soluble impurities are not removed by filtration.”

Worked example 11 mark

Which processes are required to separate sand from a mixture of solid sodium chloride and sand?

1 add a suitable solvent · 2 filter · 3 distil

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

  1. 1

    B: add a suitable solvent, then filter. Distilling is not needed. [1]

19. Crystallisation

Crystallisation obtains a dissolved solid from its solution.

Heat the solution until it is saturated, at the point of crystallisation. Then leave it to cool so that crystals form. Filter off the crystals, rinse them and dry them.

Evaporating to dryness is wrong. Hydrated copper(II) sulfate heated until all the water has gone gives a white solid, anhydrous copper(II) sulfate.

CuSO₄.5H₂O →(heat to dryness) CuSO₄ + 5H₂O

Examiner insight

“Answers such as ‘crystallise the solution’ did not gain credit since the question asked for a description of how this is done.” “Once the solution of calcium chloride had been obtained, it should have then been heated until it was saturated/had reached crystallisation point.”

Worked example 12 marks

A student heats aqueous copper(II) sulfate until all the water has evaporated. The student obtains a white solid instead of blue hydrated copper(II) sulfate.

Describe what the student should do to get crystals of hydrated copper(II) sulfate from the aqueous copper(II) sulfate.

  1. 1

    heat until saturated / to the point of crystallisation, not to dryness [1]

  2. 2

    leave the heated solution to cool and crystallise [1]

20. Simple distillation

Simple distillation separates a liquid from a mixture by its boiling point. Heat the mixture in a flask.

The liquid with the lower boiling point boils off first. Its vapour condenses in the condenser and is collected. The substance with the higher boiling point remains in the flask.

Examiner insight

“Many answers were too vague to gain credit, e.g. ‘separates them at the corresponding temperatures’ or ‘water is lower than crotyl alcohol’ (without mentioning boiling point).”

Worked example 13 marks

Pure water is obtained from aqueous sodium chloride by simple distillation. Describe how the water is separated from the sodium chloride.

  1. 1

    heat the solution until the water boils [1]

  2. 2

    the water vapour condenses in the condenser and is collected [1]

  3. 3

    the sodium chloride remains in the flask [1]

21. Fractional distillation

Fractional distillation separates a mixture of liquids that have different boiling points.

The liquid with the lowest boiling point is collected first. A liquid whose boiling point is above the temperature of the heating stays in the flask.

Examiner insight

“Many candidates correctly identified the process as fractional distillation, although a few named the method as ‘distillation’ or ‘simple distillation’.”

Worked example 13 marks

A student separates a mixture of the three alkanes in the table. The flask containing the mixture stands in a beaker of water, and the water is heated.

(d) State which alkane is collected first. Give a reason for your answer.

(e) State what, if anything, is left in the flask at the end of the experiment. Explain your answer.

alkaneboiling point / °C
hexane69
pentane36
undecane196
  1. 1

    (d) pentane, because it has the lowest boiling point [1]

  2. 2

    (e) undecane is left in the flask [1]

  3. 3

    its boiling point is over 100 °C. Water boils at 100 °C, so undecane does not reach its boiling point [1]

22. Choosing a technique

An insoluble solid in a liquid: filtration. A soluble solid wanted from its solution: crystallisation. A mixture of liquids: fractional distillation.

For a soluble and an insoluble solid: dissolve, filter, then crystallise the filtrate. Wash the residue with distilled water to remove a soluble impurity.

Examiner insight

“Candidates who performed less well overall tended to choose option A, where they confused the terms soluble and insoluble or option C where they incorrectly assumed that the filtrate is the solid extracted from the mixture.” “As the impurity is soluble in water, the expected answer was to wash the residue with distilled water.”

Worked example 11 mark

A mixture contains sand and an aqueous solution of sodium chloride.

Which processes are used to obtain a sample of solid sand and a sample of solid sodium chloride from the mixture?

A crystallisation followed by filtration · B evaporation followed by filtration · C filtration followed by crystallisation · D simple distillation followed by crystallisation

  1. 1

    C: filter off the sand, then crystallise the sodium chloride from the filtrate. [1]

23. Purity from melting and boiling points

A pure substance has a sharp melting point and boiling point, not a range of values.

Impurities lower the melting point and raise the boiling point. So a melting point or boiling point can show whether a sample is pure. Pure water boils at 100 °C.

Examiner insight

“This question required candidates to recognise that the boiling point would be higher than the melting point and that pure substances tend to have sharp melting and boiling points rather than a range of values.” “Most candidates recalled that the boiling point of a substance will increase when impurities are added. The decrease in melting point was not so well recalled.”

Worked example 11 mark

Two test-tubes each contain a sample of water. One test-tube contains pure water, but the other test-tube contains water and dissolved sodium chloride as an impurity.

Which tests may be used to identify the test-tube that contains pure water?

1 Measure the boiling point of each water sample. · 2 Test each sample with acidified aqueous silver nitrate. · 3 Test each sample with anhydrous copper(II) sulfate.

A 1 and 2 · B 1 only · C 2 and 3 · D 3 only

  1. 1

    A: the boiling point shows purity, and silver nitrate gives a test for chloride ions. Both samples contain water, so test 3 does not tell them apart. [1]

Recap

  • Solvent then filtration separates a soluble solid from an insoluble one.
  • Residue = solid on the filter paper. Filtrate = liquid that passes through.
  • Crystallisation: heat until saturated, cool, filter, rinse, dry. Never evaporate to dryness.
  • Simple distillation: lower boiling point boils off first, condenses in the condenser. The rest stays in the flask.
  • Fractional distillation separates a mixture of liquids. The lowest boiling point is collected first.
  • Soluble impurities are removed by washing the residue, not by filtration.
  • A pure substance has sharp melting and boiling points. Impurities lower the melting point and raise the boiling point.

24. Testing for gases

Each gas has one test and one positive result. Write both.

Hydrogen, H₂: a lighted splint gives a squeaky pop. Oxygen, O₂: a glowing splint relights.

Ammonia, NH₃: damp red litmus paper turns blue. Carbon dioxide, CO₂: limewater turns milky. Chlorine, Cl₂: damp litmus paper is bleached. Sulfur dioxide, SO₂: acidified aqueous potassium manganate(VII) is decolourised, from purple to colourless.

Examiner insight

“Candidates who performed less well often used vague terms such as ‘splint test’ or erroneously wrote ‘glowing splint’.” “As the question stated ‘test any gas produced’, candidates were expected to record the result of a positive test for the gas (damp red litmus paper turning blue).”

Worked example 11 mark

Which statement about chlorine gas is correct?

A It bleaches damp blue litmus paper. · B It turns damp red litmus paper blue. · C It decolourises acidified aqueous potassium manganate(VII). · D It turns limewater cloudy.

  1. 1

    A: chlorine bleaches damp litmus paper. B is ammonia, C is sulfur dioxide, D is carbon dioxide. [1]

25. Halide ions: chloride, bromide and iodide

To test for chloride, bromide and iodide ions, acidify the sample with dilute nitric acid. Then add aqueous silver nitrate.

The colour of the precipitate names the ion. Chloride, Cl⁻, gives a white precipitate. Bromide, Br⁻, gives a cream precipitate. Iodide, I⁻, gives a (pale) yellow precipitate.

Use nitric acid only. Hydrochloric acid contains chloride ions, so it would give a false precipitate.

Examiner insight

“Those who suggested silver nitrate sometimes disadvantaged themselves by suggesting the addition of hydrochloric acid, sulfuric acid or sodium hydroxide instead of nitric acid.” “A few candidates knew the test for iodide ions and again got confused with the correct colour of the silver iodide precipitate made in this reaction.”

Worked example 14 marks

Potassium iodide is an ionic compound. Describe tests to identify the two ions contained in potassium iodide.

  1. 1

    potassium ions, K⁺: test is a flame test [1]

  2. 2

    observation: a lilac flame [1]

  3. 3

    iodide ions, I⁻: acidify with dilute nitric acid, then add aqueous silver nitrate [1]

  4. 4

    observation: a (pale) yellow precipitate [1]

26. Carbonate, sulfate and sulfite ions

Carbonate, CO₃²⁻: add dilute acid, then test the gas. Carbon dioxide turns limewater milky.

Sulfate, SO₄²⁻: acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate shows sulfate. No change shows it is absent.

Sulfite, SO₃²⁻: react with acidified aqueous potassium manganate(VII). The manganate(VII) is decolourised, from purple to colourless.

Examiner insight

“A common error was to state that a white precipitate would form, this being the expected result for a positive sulfate test.” “Most candidates knew the acidified reagent but often did not give the correct oxidation state or placed it in the wrong position such as potassium(VII) manganate.”

Worked example 14 marks

A student tests solid A. Table 3.1 shows the tests and the student’s observations for solid A.

(a) Identify the gas given off in test 2.

(b) State the conclusion about solid A that can be made from the observation in test 4.

(c) Identify solid A.

testsobservations
test 1: Place about half of solid A in a boiling tube. Heat solid A strongly in the boiling tube.solid A turns black
test 2: Place the remaining solid A in a boiling tube. Add 10 cm³ of dilute hydrochloric acid to the boiling tube. Test any gas given off. The solution that forms is solution C.effervescence is seen; some of solid A dissolves to form a blue solution; limewater turns milky
test 3: To the first portion of solution C, add aqueous sodium hydroxide dropwise and then in excess.blue precipitate forms which remains in excess
test 4: To the second portion of solution C, add 1 cm³ of dilute nitric acid followed by a few drops of aqueous barium nitrate.no visible change
  1. 1

    (a) carbon dioxide, CO₂ [1]

  2. 2

    (b) no visible change with barium nitrate, so solid A is not a sulfate [1]

  3. 3

    (c) test 3, a blue precipitate that remains in excess: copper(II), Cu²⁺ [1]

  4. 4

    test 2, effervescence and limewater turns milky: carbonate, CO₃²⁻ [1]

27. Nitrate and ammonium ions

Nitrate, NO₃⁻: add aluminium foil and aqueous sodium hydroxide, and warm. Test the gas with damp red litmus paper. It turns blue. The gas is ammonia.

Ammonium, NH₄⁺: add aqueous sodium hydroxide and warm. The same gas, ammonia, is given off and turns damp red litmus blue.

Examiner insight

“A common error was to confuse this test with the test for nitrate ions, where aluminium foil would also need to be added.”

Worked example 12 marks

To a portion of solution B, add aqueous sodium hydroxide dropwise until it is in excess. To the product, add a piece of aluminium foil. Warm the mixture gently. Test any gas produced.

Record your observations.

  1. 1

    fizzing / bubbles / effervescence [1]

  2. 2

    (damp) red litmus turns blue [1]

28. Aluminium, calcium and zinc ions

Add aqueous sodium hydroxide or aqueous ammonia dropwise, then in excess. Write what you see at each stage.

Aluminium, Al³⁺: sodium hydroxide gives a white precipitate that dissolves to a colourless solution. Ammonia gives a white precipitate that stays in excess.

Zinc, Zn²⁺: a white precipitate with either reagent. It dissolves in excess of either. Calcium, Ca²⁺: sodium hydroxide gives a white precipitate that stays in excess. Ammonia gives little or no precipitate.

Examiner insight

“A few candidates just noted that the mixture became ‘cloudy white’; this is not sufficient, and the term ‘precipitate’ should be used.” “A minority of candidates gave only one observation (such as ‘white precipitate’) or gave contradictory observations such as ‘soluble white precipitate’; a precipitate is not soluble, if it was then it would not be a precipitate.”

Worked example 12 marks

When a few drops of NH₃(aq) are added to Zn²⁺(aq) a white precipitate forms.

When a few drops of Zn²⁺(aq) are added to NH₃(aq) no precipitate forms.

Explain these observations.

  1. 1

    a few drops of ammonia: zinc hydroxide forms as a white precipitate [1]

  2. 2

    a few drops of zinc ions: NH₃(aq) is in excess, so the zinc ions are soluble and no precipitate forms [1]

29. Chromium(III), copper(II) and iron ions

Chromium(III), Cr³⁺: a green precipitate with either reagent. It dissolves in excess sodium hydroxide to a green solution.

Copper(II), Cu²⁺: a light blue precipitate. It stays in excess sodium hydroxide. In excess ammonia it dissolves to a deep blue solution.

Iron(II), Fe²⁺: a green precipitate that stays in excess sodium hydroxide. Iron(III), Fe³⁺: a red-brown precipitate with either reagent. It stays in excess sodium hydroxide.

Examiner insight

“A few candidates incorrectly stated that a colourless solution would form in excess aqueous sodium hydroxide, rather than green solution.” “Some candidates gave contradictory statements such as stating that a precipitate dissolves to form a deep blue precipitate.”

Worked example 12 marks

Solid C is shaken with water to form solution C. To a portion of solution C in a boiling tube, add aqueous sodium hydroxide dropwise until it is in excess.

Record your observations.

  1. 1

    green precipitate [1]

  2. 2

    the precipitate dissolves / disappears in excess [1]

30. Flame tests

Put the sample in a non-luminous (blue, roaring) Bunsen flame, on a wire or a splint. Report the colour first seen.

Lithium, Li⁺: red. Sodium, Na⁺: yellow. Potassium, K⁺: lilac. Calcium, Ca²⁺: orange-red.

Barium, Ba²⁺: light green. Copper(II), Cu²⁺: blue-green.

Examiner insight

“The placement of the test substance was commonly given as near the flame or over a flame rather than within the flame.” “Common wrong observations were a yellow precipitate instead of a yellow flame.”

Worked example 12 marks

Sea water contains sodium ions.

Name the test to identify sodium ions. Give the observations for a positive result.

  1. 1

    test: flame test [1]

  2. 2

    observations: yellow (flame) [1]

Recap

  • Hydrogen: lighted splint, squeaky pop. Oxygen: glowing splint relights.
  • Ammonia: damp red litmus turns blue. Carbon dioxide: limewater turns milky. Chlorine: damp litmus bleached.
  • Sulfur dioxide and sulfite: acidified aqueous potassium manganate(VII) is decolourised.
  • Halides: dilute nitric acid, then aqueous silver nitrate. White chloride, cream bromide, yellow iodide.
  • Sulfate: nitric acid, then barium nitrate, white precipitate. Carbonate: acid, then limewater.
  • Nitrate: aluminium foil and sodium hydroxide, ammonia. Ammonium: sodium hydroxide and warm, ammonia.
  • Cations: add the reagent dropwise, then in excess, and write what you see at each stage.
  • Flame tests: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. A student dissolves a solid in water. Name the apparatus used to measure (a) the mass of the solid, (b) the temperature of the water and (c) the time taken for the solid to dissolve.3 marks
  2. Magnesium reacts with dilute hydrochloric acid. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) Name two ways to collect and measure the volume of the hydrogen produced.2 marks
  3. At the start of a titration the burette reads 1.8 cm³. At the end it reads 26.3 cm³. Calculate the volume of solution added from the burette.2 marks
  4. A student adds acid from a burette to a solution until the colour changes. The volume needed is not known beforehand. Explain why a volumetric pipette cannot be used to add the acid.2 marks
  5. A student measures 25 cm³ of a solution using a measuring cylinder instead of a volumetric pipette. State one advantage and one disadvantage of using the measuring cylinder.2 marks
  6. A student stirs 4 g of solid X with 100 cm³ of water until all of the solid has dissolved. Name the solute, the solvent and the solution.3 marks
  7. A student writes: ‘A saturated solution has the most solute in it.’ Write a definition of a saturated solution that gains full credit.2 marks
  8. Insoluble solid Z is mixed with aqueous solution Y. The mixture is poured through a filter paper in a funnel into a beaker. Name the residue and the filtrate, and state where each is found.2 marks
  9. A student compares two indigestion tablets, P and Q. Each tablet reacts with dilute hydrochloric acid and gives off carbon dioxide. The student puts the acid in a conical flask, adds one tablet, replaces the stopper and starts a stop-watch. The gas passes through a delivery tube into an inverted measuring cylinder full of water. After 80 s, tablet P has given 36 cm³ of gas and tablet Q has given 20 cm³ of gas. mean rate of reaction = (volume of gas collected)/(time taken to collect the gas) (a) Calculate the mean rate of reaction for tablet P. Give the units. (b) The first gas to reach the measuring cylinder is air from the flask. State the effect, if any, of this air on the accuracy of the results. Explain your answer. (c) Some gas escapes before the stopper is replaced. State the effect, if any, on the volume of gas collected in 80 s. Deduce which tablet loses more gas in this way. Explain your answer.7 marks
  10. A student investigates salt R, which is soluble in water. The student adds 25.0 g of solid R to 50 cm³ of water at 20 °C and stirs until no more R dissolves. Some solid is left. The mixture is filtered. All of the liquid collected is poured into a dish of mass 42.6 g and heated until all the water has gone. The dish and the solid left in it have a total mass of 60.6 g. (a) State whether each of these is a residue or a filtrate: (i) the solid on the filter paper, (ii) the liquid collected, (iii) the solid left in the dish. (b) Name the type of solution that is collected. State what this term means. (c) Calculate the mass of R that dissolved. Then calculate the mass of dry solid on the filter paper. Assume no R is lost.8 marks
  11. Name the apparatus that adds the second solution drop by drop in a titration.1 mark
  12. Name the apparatus that measures exactly 25.0 cm³ of aqueous sodium hydroxide into a conical flask.1 mark
  13. Give one reason why a volumetric pipette is used instead of a measuring cylinder to measure the alkali.1 mark
  14. State one way in which a burette differs from a volumetric pipette.2 marks
  15. A burette is rinsed with distilled water, not tap water. Give the reason.1 mark
  16. Before starting a titration, a student opens the burette tap and lets a little of the acid run into a waste beaker. Explain why.2 marks
  17. A student uses Universal Indicator in a titration. Explain why it is unsuitable.2 marks
  18. Aqueous sodium hydroxide is added from a burette to dilute hydrochloric acid containing methyl orange. The flask stands on a white tile. State how the student identifies the end-point, and how the white tile helps.2 marks
  19. A student titrates 25.0 cm³ of aqueous potassium hydroxide in a conical flask with dilute hydrochloric acid from a burette. She repeats the titration three times, making one change each time. For each change, state whether the volume of acid needed to reach the end-point is greater, smaller or unchanged. Give a reason. (a) The flask is rinsed with aqueous potassium hydroxide, and not dried, before the 25.0 cm³ is added. (b) The flask is rinsed with distilled water, and not dried, before the 25.0 cm³ is added. (c) The aqueous potassium hydroxide in the flask is warmed before the acid is added.6 marks
  20. A student plans a titration to find the volume of dilute hydrochloric acid that reacts with 25.0 cm³ of aqueous sodium hydroxide. This is the plan. Rinse the burette with distilled water, then fill it with the acid without removing the water left inside. Measure the aqueous sodium hydroxide into a conical flask with a measuring cylinder. Add a few drops of Universal Indicator. Run the acid in quickly until the colour has changed. Identify four mistakes in the plan. For each one, state what the student should do and give a reason.8 marks
  21. State why the baseline must be above the solvent level when the paper is put in the solvent.1 mark
  22. The dyes in a pen are not soluble in water. State the type of solvent that should be used.1 mark
  23. Dyes X and Y are separated using one solvent. X is more soluble in the solvent than Y. There is much more of Y than X. State which dye travels further up the paper and give a reason.2 marks
  24. A student compares a red ink, U, with three red dyes, A, B and C. U gives spots 2.0 cm and 4.5 cm above the baseline. A gives one spot at 2.0 cm, B one spot at 3.0 cm and C one spot at 4.5 cm. State which dyes could be in U.2 marks
  25. Three samples are separated by chromatography. Sample M gives one spot. Sample N gives three spots. Sample W stays on the baseline. State what each result shows.3 marks
  26. Two dyes in an ink travel the same distance up the paper in one solvent. Can this solvent separate them? Explain.2 marks
  27. Amino acids are colourless. Explain why a locating agent is used when they are separated by paper chromatography.2 marks
  28. A substance travels 3.6 cm from the baseline. The solvent travels 9.0 cm from the baseline. Calculate the Rf value of the substance.2 marks
  29. A laboratory tests three samples of a yellow food colouring, F, G and H, for a banned dye. In the solvent used, the banned dye has an Rf value of 0.65. The solvent travels 12.0 cm from the baseline. F gives one spot, 7.8 cm from the baseline. G gives two spots, 7.8 cm and 10.2 cm from the baseline. H gives one spot, 9.6 cm from the baseline. (a) Calculate the distance the banned dye travels from the baseline. [2] (b) State which samples contain the banned dye. [1] (c) Two of the samples are pure substances. Identify the one with the larger Rf value and calculate that value. [2]5 marks
  30. A student separates a mixture of colourless amino acids. The baseline is 20 mm above the bottom edge of the paper. After the run, the solvent front is 14.0 cm above the bottom edge. A locating agent is applied. Three spots are seen, at 2.0 cm, 6.8 cm and 11.0 cm above the bottom edge. The table gives the Rf values of three known amino acids in this solvent. (a) Calculate the Rf values of the two spots that moved. [3] (b) State what the spot at 2.0 cm shows about that substance. [1] (c) The student concludes that the mixture contains exactly three amino acids. Explain why this may be wrong. [2] | Amino acid | Rf | | --- | --- | | J | 0.40 | | K | 0.40 | | L | 0.75 |6 marks
  31. A mixture of sand and water is filtered. Name (a) the sand left on the filter paper and (b) the water that passes through.2 marks
  32. Sand and solid sodium chloride are stirred with water and the mixture is filtered. State which solid dissolves and which solid is left on the filter paper.2 marks
  33. Insoluble copper(II) carbonate is added in excess to dilute sulfuric acid. CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g) Describe how the excess copper(II) carbonate is removed and state what is collected in the filtrate.3 marks
  34. Describe how to obtain crystals of hydrated copper(II) sulfate, CuSO₄.5H₂O, from aqueous copper(II) sulfate.3 marks
  35. A student wants pure water from aqueous copper(II) sulfate, which is blue. The student filters the solution. The filtrate is still blue. (a) Explain why filtration does not give pure water. (b) Name the technique the student should use. State where the water is collected and what remains in the flask.4 marks
  36. Name the best technique to separate each mixture: (a) sand from water, (b) solid copper(II) sulfate from aqueous copper(II) sulfate, (c) pentane from hexane, two liquids with different boiling points.3 marks
  37. Ethanol boils at 78 °C and water boils at 100 °C. A mixture of the two liquids is separated by fractional distillation. State which liquid is collected first, which liquid is left in the flask when the temperature is 80 °C, and give the reason.3 marks
  38. A pure solid melts at exactly 80 °C. Sample P melts between 74 °C and 78 °C. Sample Q melts at 80 °C. State which sample is pure and give two reasons.3 marks
  39. The table gives data for three pure liquids, L, M and N. A sample is mostly one of these liquids, with a small amount of liquid N as an impurity. The sample melts between −99 °C and −96 °C and boils between 112 °C and 115 °C. (a) Identify the liquid that makes up most of the sample. Use the melting point data and the boiling point data to explain your answer. (b) Name the technique used to separate the two liquids in the sample. State which of the two is collected first and give a reason. | liquid | melting point / °C | boiling point / °C | | --- | --- | --- | | L | −114 | 78 | | M | −95 | 111 | | N | 13 | 138 |5 marks
  40. A powder is a mixture of three solids. R is insoluble in water and in ethanol. S is soluble in water but insoluble in ethanol. T is insoluble in water but soluble in ethanol. Describe how to obtain a pure, dry sample of R and crystals of S from the powder.6 marks
  41. Give the test and the positive result for hydrogen gas.1 mark
  42. A gas turns damp red litmus paper blue. A second gas bleaches damp litmus paper. Name the two gases.2 marks
  43. Describe the test for carbonate ions in a solid.2 marks
  44. Dilute nitric acid and then aqueous silver nitrate are added to solution Y. A cream precipitate forms. Name the ion in Y and state the colour of the precipitate that chloride ions would give.2 marks
  45. A flame test on compound P gives an orange-red flame. A flame test on compound Q gives a lilac flame. Identify the cation in each.2 marks
  46. Describe the test for sulfate ions in aqueous solution. State the result for a solution that has none.3 marks
  47. Aqueous sodium hydroxide is added to solid R and the mixture is warmed. The gas given off turns damp red litmus paper blue. Name the ion in R. State the extra reagent needed to test for nitrate ions.2 marks
  48. Solution S gives a white precipitate with aqueous sodium hydroxide. The precipitate dissolves in excess to give a colourless solution. Solution S also gives a white precipitate with aqueous ammonia. This precipitate does not dissolve in excess. Identify the cation in S.3 marks
  49. A technician tests waste water from a metal-plating works. A few drops of aqueous sodium hydroxide are added to one sample. A green precipitate forms. The technician writes ‘iron(II) ions’ and stops. Dilute hydrochloric acid and then aqueous silver nitrate are added to a second sample. A white precipitate forms. The technician writes ‘chloride ions’. (a) Explain why the first conclusion is not safe. State what the technician should do next and what would be seen for each possible ion. (b) Explain why the second conclusion is not safe. State the change that is needed.5 marks
  50. Solid Z is a mixture of two salts. It contains two different cations and two different anions. Test 1: a flame test on Z gives a yellow flame. Test 2: Z is warmed with aqueous sodium hydroxide. No precipitate forms. A gas is given off that turns damp red litmus paper blue. Test 3: aluminium foil is added to the mixture from test 2 and it is warmed again. The gas given off turns damp red litmus paper blue. Test 4: dilute nitric acid is added to Z. There is effervescence and the gas turns limewater milky. Aqueous silver nitrate is then added. A yellow precipitate forms. (a) Identify the four ions in Z. (b) A student says that test 3 shows nitrate ions. Explain why test 3 does not show this.6 marks

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