Qualitative Analysis For each test you should record all your observations in the spaces provided. Examples of observations include: colour changes seen the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write 'no change'.…

(a) Ammonia (NH₃) and hydrogen chloride (HCl) identified from litmus tests; observations recorded in order (condensation, sublimation, white smoke, litmus colour changes, white/grey residue) · (b)(i) Observations recorded for all four tests (see working) · (b)(ii) Chloride, Cl⁻ and sulfate, SO₄²⁻ · (b)(iii) Zn²⁺ confirmed by white ppt soluble in excess NH₃; NH₄⁺ confirmed by NH₃ gas with warm NaOH

Chemistry 9701/36 — October/November 2024

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Qualitative Analysis

Q1Medium-EasyPresentation of Data and ObservationsAnalysis, Conclusions and Evaluation

Sodium carbonate reacts with hydrochloric acid to release carbon dioxide as shown.

\text{Na}_2\text{CO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

You will find the percentage purity in a sample of impure sodium carbonate by reacting it with excess hydrochloric acid and measuring the volume of carbon dioxide formed. You may assume that the impurity does not react with acid to produce a gas.

FB 1 is impure sodium carbonate, $\text{Na}_2\text{CO}_3$ .
FB 2 is hydrochloric acid, $\text{HCl}$ .

(a)

Method

Weigh the container with FB 1. Record the mass.
Fill the tub with water to a depth of approximately $5\text{ cm}$ .
Fill the $250\text{ cm}^3$ measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub.
Use the $50\text{ cm}^3$ measuring cylinder to transfer $50.0\text{ cm}^3$ of FB 2 into the flask labelled X. Check the bung fits tightly into the neck of flask X, clamp flask X and place the delivery tube into the inverted $250\text{ cm}^3$ measuring cylinder.
Remove the bung from the neck of the flask. Tip all the FB 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents.
Replace the flask in the clamp and leave until the fizzing has stopped.
Remove the flask from the clamp occasionally, swirl it and replace the flask in the clamp.
Weigh the empty container that held FB 1. Record the mass.
Calculate and record the mass of FB 1 added.
When no more gas is collected, record the final volume of gas produced.

You may wish to start Question 2 while the gas is being produced.

Results

4M Similar

DifficultyMedium-Easy

Worked solution

Answer

Results table (representative values):

Measurement	Value
Mass of container + FB 1 / g	22.45
Mass of empty container / g	21.75
Mass of FB 1 / g	0.70
Volume of gas / cm³	150

All balance readings recorded to 2 decimal places.
Mass of FB 1 = 22.45 − 21.75 = 0.70 g
Volume of gas recorded to the nearest 1 cm³ and within the range 50–250 cm³.

Final answer

See working: results table with headings and units, balance readings to 2 dp, volume to 1 cm³

Detailed explanation

Background Concept

This is a gas-collection experiment to determine the percentage purity of an impure sample of sodium carbonate. The key idea is that the volume of carbon dioxide produced is directly proportional to the amount of pure sodium carbonate present, because the impurity does not produce gas. Recording data correctly is essential in practical work: every measurement must be associated with a clear heading and unit, and readings must be taken to an appropriate precision.

Understanding the Question

Part (a) is about recording the results of the experiment. The marks are awarded for the structure and quality of the results table, not for the actual values (which depend on the candidate's own readings). You need to record: the mass of the container with FB 1, the mass of the empty container, the calculated mass of FB 1, and the final volume of gas produced.

Approach

Set up a table with clear headings, each with its unit. Record balance readings to 2 or 3 decimal places. Calculate the mass of FB 1 by subtraction. Record the gas volume to the nearest 1 cm³. Check the volume is between 50 and 250 cm³.

Step-by-Step Reasoning

Headings: "Mass of container + FB 1 / g", "Mass of empty container / g", "Mass of FB 1 / g", "Volume of gas / cm³" (or "Volume of CO2 / cm³"). Each heading must have its unit.
Balance readings: record to 2 dp or 3 dp (e.g., 22.45 g, not 22.4 g). The mass of FB 1 is the difference: 22.45 − 21.75 = 0.70 g.
Volume: read the measuring cylinder to the nearest 1 cm³ (e.g., 150 cm³, not 150.2 cm³).
Range check: the volume must be between 50 and 250 cm³. If it is outside this range, the experiment may need repeating with a different mass of FB 1.

Key Takeaways

Results tables need clear headings with units.
Balance readings to 2 or 3 dp.
Gas volumes to nearest 1 cm³.
The mass of FB 1 is found by difference.

Common Mistakes

Missing units in headings.
Recording balance readings to 1 dp.
Recording gas volume to more precision than the measuring cylinder allows.
Writing "Mass of FB 1" without showing the subtraction.

Things to Be Careful About

The volume of gas must be within 50–250 cm³; if not, the experiment is invalid.
Keep the same decimal places for both balance readings.
The mass of FB 1 is a calculated value, so it may have fewer decimal places than the raw readings (e.g., 0.70 g from 22.45 − 21.75).

Techniques used

record balance readings to appropriate precisioncalculate mass by differencerecord gas volume to nearest 1 cm³

(b)

Calculations

(i)

Give your answers to each part of (b)(ii), (b)(iii) and (b)(iv) to an appropriate number of significant figures.

1M Similar

(ii)

Calculate the amount, in mol, of carbon dioxide collected in the $250\text{ cm}^3$ measuring cylinder.

1M Similar

(iii)

Use your answer to (b)(ii) to deduce the amount, in mol, of the sodium carbonate present in the FB 1 you used in your experiment.

Use your answer to calculate the mass, in g, of sodium carbonate in your sample of FB 1.

1M Similar

(iv)

Calculate the percentage purity of FB 1.

1M Similar

(c)

Even though the bung was replaced quickly, some carbon dioxide was lost. Suggest a change you could make to minimise gas loss at this stage.

1M Similar

(d)

Some carbon dioxide is not collected because it is slightly soluble in water. State a change you could make to reduce the solubility of the gas.

Do not suggest using a liquid other than water in your tub or changing the volume of water used.

1M Similar

(e)

State the uncertainty in a single reading of your balance.

Calculate the maximum percentage error in the mass of FB 1 used in (a).

1M Similar

Q2Medium-EasyManipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative Analysis

Many metal carbonates, such as magnesium carbonate, decompose to form the metal oxide when heated.

\text{MgCO}_3(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{CO}_2(\text{g})

Other metal carbonates, such as sodium carbonate, $\text{Na}_2\text{CO}_3$ , do not decompose at the temperature produced by a Bunsen burner.

FB 3 is a mixture that contains only sodium carbonate, $\text{Na}_2\text{CO}_3$ , and magnesium carbonate.

You will carry out an experiment involving thermal decomposition to find the percentage of each of these metal carbonates in this mixture.

(a)

Method

Weigh the empty crucible with its lid. Record the mass.
Transfer all the FB 3 from the container into the crucible.
Weigh the crucible, lid and FB 3. Record the mass.
Calculate and record the mass of FB 3 used.
Place the crucible and contents on the pipe-clay triangle.
Heat the crucible gently, with the lid on, for approximately 1 minute.
Heat strongly, with the lid off, for a further 5 minutes.
Leave the crucible with its contents until it is cool.

While the crucible is cooling, you may wish to begin work on Question 3.

When the crucible is cool, weigh the crucible with its lid and contents. Record the mass.
Heat the crucible strongly, with the lid off, for approximately 4 minutes.
Allow the crucible and contents to cool.
When the crucible is cool, weigh the crucible with its lid and contents. Record the mass.
Calculate and record the mass of residue.
Calculate and record the mass of carbon dioxide produced.

Leave the crucible and contents to become completely cool for use in Question 2(c).

Results

5M Similar

DifficultyMedium-Easy

Worked solution

Answer

Use a results table with these headings (include units):

mass of crucible + lid / g
mass of crucible + lid + FB 3 / g
mass of FB 3 added / g
mass of crucible + lid + contents after 1st heating / g
mass of crucible + lid + contents after 2nd heating / g
mass of residue / g
mass of $\text{CO}_2$ / g

Record all balance readings to the same number of decimal places (2 dp or 3 dp). The two post-heating readings should agree closely to show decomposition is complete.

Example using representative readings:

Reading	Mass / g
crucible + lid	15.00
crucible + lid + FB 3	17.50
FB 3 added	2.50
after 1st heating	16.88
after 2nd heating	16.86
residue	1.86
$\text{CO}_2$ produced	0.64

Calculations:

mass of FB 3 = 17.50 - 15.00 = 2.50 g
mass of residue = 16.86 - 15.00 = 1.86 g
mass of $\text{CO}_2$ = 2.50 - 1.86 = 0.64 g

Final answer

See working: completed results table with headings and example values (FB 3 2.50 g, residue 1.86 g, CO2 0.64 g); candidate-dependent.

Detailed explanation

Background Concept

Thermal decomposition of a metal carbonate produces the metal oxide and carbon dioxide. Magnesium carbonate decomposes at Bunsen-burner temperature:

\text{MgCO}_3(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{CO}_2(\text{g})

Sodium carbonate is much more stable and does not decompose under these conditions. Therefore, when a mixture of the two is heated, only the MgCO $_3$ loses mass. The mass lost is exactly the mass of CO $_2$ released, because CO $_2$ is the only gaseous product. Heating to constant mass (reweighing after further heating and getting the same mass) ensures decomposition is complete.

Understanding the Question

This part asks you to carry out the heating procedure and record your results in a clear table. You need to record the masses needed to find the mass of FB 3, the mass of residue, and the mass of CO $_2$ produced. The mark scheme rewards correct headings with units, consistent decimal places, and correct calculations.

Approach

Weigh the empty crucible with its lid, then weigh it again after adding FB 3. Heat, cool, and reweigh. Repeat heating until the mass is constant. Use mass differences to calculate the required quantities.

Step-by-Step Reasoning

Record the mass of the crucible and lid.
Record the mass of the crucible, lid and FB 3.
Mass of FB 3 = (mass of crucible + lid + FB 3) - (mass of crucible + lid).
After heating and cooling, record the mass of the crucible, lid and contents.
Heat again and reweigh. If the two post-heating masses agree closely, decomposition is complete.
Mass of residue = (mass of crucible + lid + contents after final heating) - (mass of crucible + lid).
Mass of CO $_2$ = mass of FB 3 - mass of residue (or mass loss on heating).

In the example, FB 3 = 2.50 g, residue = 1.86 g, and CO $_2$ = 0.64 g.

Key Takeaways

Mass loss on heating a carbonate equals mass of CO $_2$ released.
Heating to constant mass confirms complete decomposition.
A results table must have clear headings with units and consistent decimal places.

Common Mistakes

Forgetting units in headings.
Using different numbers of decimal places for different readings.
Calculating mass of CO $_2$ incorrectly (it is mass lost, not mass of residue).
Weighing before the crucible is cool, giving inaccurate readings.

Things to Be Careful About

Always cool the crucible before weighing.
Use the same balance and same precision throughout.
The two post-heating masses should differ by no more than about 0.02 g (allowing a small range) to show constant mass.

Techniques used

record balance readings to consistent precisioncalculate masses by differenceheat to constant massconstruct a results table with units

(b)

\text{MgCO}_3(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{CO}_2(\text{g})

(i)

Calculate the amount, in mol, of carbon dioxide produced in the decomposition.

1M Similar

(ii)

Calculate the mass of magnesium carbonate in FB 3.

1M Similar

(iii)

Calculate the percentages by mass of magnesium carbonate and sodium carbonate in FB 3.

1M Similar

(c)

(i)

Add a few drops of water to the cool residue in the crucible.
Use universal indicator to test the pH of the solution formed.
Tick ( $\checkmark$ ) one box to show the direction of the temperature change.

\text{pH} = \text{..............................}

temperature goes up	temperature goes down
[ ]	[ ]

1M Similar

(ii)

Use these observations and the information about the thermal decomposition of magnesium carbonate to write an equation for the reaction in (c)(i). Include state symbols and the sign of $\Delta H$ .

2M Similar

(iii)

Suggest how you would show that sodium carbonate had not decomposed during the reaction in (a). State the reagent(s) and observations.

Do not carry out your test.

2M Similar

Q3MediumQualitative AnalysisManipulation, Measurement and Observation

Qualitative Analysis

For each test you should record all your observations in the spaces provided.

Examples of observations include:

colour changes seen
the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added
the formation of any gas and its identification (where appropriate) by a suitable test.

You should record clearly at what stage in a test an observation is made.

Where no change is observed, you should write 'no change'.

Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.

If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used.

Rinse and reuse test-tubes and boiling tubes where possible.

No additional tests should be attempted.

FB 4 is a mixture of two salts that each contain one cation and one anion.
All the ions present are in the Qualitative analysis notes.

(a)

Place a small spatula measure of FB 4 into a hard-glass test-tube. Heat the tube gently at first and then more strongly.

Record all your observations and identify any gas given off.

4M Similar

DifficultyMedium

Worked solution

Answer

On gentle heating:

Condensation/steam forms on the cooler part of the tube (water of crystallisation driven off)
A gas is given off that turns damp red litmus paper blue — ammonia, $\text{NH}_3$
A white solid sublimes onto the cooler part of the tube
The solid dissolves/melts in its own water of crystallisation and the liquid bubbles

On stronger heating:

White smoke is produced
The gas now turns damp blue litmus paper red — hydrogen chloride, $\text{HCl}$
A white/grey residue remains on cooling

Gases identified: ammonia ( $\text{NH}_3$ ) and hydrogen chloride ( $\text{HCl}$ )

Final answer

Ammonia (NH₃) and hydrogen chloride (HCl) identified from litmus tests; observations recorded in order (condensation, sublimation, white smoke, litmus colour changes, white/grey residue)

Detailed explanation

Background Concept

When a solid mixture is heated in a test-tube, several types of change can occur: loss of water of crystallisation (dehydration of a hydrated salt), thermal decomposition, and sublimation. Hydrated salts such as $\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}$ lose their water of crystallisation on heating, producing steam which condenses on the cooler parts of the tube. Ammonium chloride ( $\text{NH}_4\text{Cl}$ ) is unusual: it sublimes, and at the same time undergoes reversible thermal decomposition into ammonia ( $\text{NH}_3$ ) and hydrogen chloride ( $\text{HCl}$ ):

\text{NH}_4\text{Cl}(\text{s}) \rightleftharpoons \text{NH}_3(\text{g}) + \text{HCl}(\text{g})

Ammonia is a basic gas — it turns damp red litmus paper blue. Hydrogen chloride is an acidic gas — it turns damp blue litmus paper red. Because ammonia is the lighter gas and is released first as the salt begins to decompose, the litmus test often shows blue first, then red as more $\text{HCl}$ accumulates.

Understanding the Question

FB 4 is a mixture of zinc sulfate heptahydrate ( $\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}$ ) and ammonium chloride ( $\text{NH}_4\text{Cl}$ ). The question asks you to heat a spatula measure in a hard-glass test-tube, gently at first and then more strongly, recording all observations and identifying any gas given off. The instruction to heat gently first then strongly is deliberate: gentle heating drives off water of crystallisation and begins the decomposition of $\text{NH}_4\text{Cl}$ , while stronger heating completes decomposition and may cause further changes. The marking scheme explicitly rewards observations "in the expected order" — gentle heating towards stronger heating.

Approach

Work through the heating in stages. First, gentle heating: expect water loss (steam/condensation) and the start of $\text{NH}_4\text{Cl}$ decomposition ( $\text{NH}_3$ released, litmus blue). Then stronger heating: more complete decomposition, $\text{HCl}$ released (litmus red), white smoke from $\text{NH}_3$ and $\text{HCl}$ recombining in the air, and a residue of anhydrous $\text{ZnSO}_4$ . Test any gas with damp litmus paper and record observations in the order they occur.

Step-by-Step Reasoning

Gentle heating: $\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}$ loses its seven waters of crystallisation. Steam is produced and condenses as droplets on the cooler upper part of the tube — this is the "condensation / steam produced" observation.
As heating continues, $\text{NH}_4\text{Cl}$ begins to decompose: $\text{NH}_4\text{Cl}(\text{s}) \rightarrow \text{NH}_3(\text{g}) + \text{HCl}(\text{g})$ . Ammonia is released and turns damp red litmus blue — the first gas identification.
The $\text{NH}_3$ and $\text{HCl}$ recombine on cooler surfaces, depositing white $\text{NH}_4\text{Cl}$ solid — this appears as sublimation (solid on the colder tube) and as white smoke where the gases meet the air.
The solid may appear to melt/dissolve in its own water of crystallisation, and the liquid bubbles as gases escape.
Stronger heating: more $\text{HCl}$ is released; the atmosphere in the tube becomes acidic and damp blue litmus turns red.
On cooling, the residue (anhydrous $\text{ZnSO}_4$ ) is white/grey.

Gas identification: ammonia ( $\text{NH}_3$ ) — turns damp red litmus blue; hydrogen chloride ( $\text{HCl}$ ) — turns damp blue litmus red.

Key Takeaways

Heating a solid mixture can produce multiple gases; test each with damp litmus.
$\text{NH}_4\text{Cl}$ sublimes and decomposes reversibly to $\text{NH}_3 + \text{HCl}$ .
Damp red litmus → blue = basic gas ( $\text{NH}_3$ ); damp blue litmus → red = acidic gas ( $\text{HCl}$ ).
Hydrated salts lose water on heating → steam condenses on cooler surfaces.
Record observations in the order they occur, distinguishing gentle vs strong heating.

Common Mistakes

Not recording observations in the order seen (the mark scheme requires this).
Missing the condensation/steam observation.
Confusing which gas turns litmus which colour ( $\text{NH}_3$ = blue, $\text{HCl}$ = red).
Not identifying the gases by name or formula.
Using a soft-glass test-tube instead of hard-glass for heating a solid.

Things to Be Careful About

Use a hard-glass test-tube when heating a solid.
Use damp litmus paper, not dry.
Write "no change" if nothing happens at a stage.
The white smoke is $\text{NH}_3 + \text{HCl}$ recombining — record it as a separate observation.

Techniques used

heat a solid mixture in a hard-glass test-tubetest gases with damp litmus paperidentify gases from their characteristic propertiesrecord observations in the order they occur

(b)

To a $5\text{ cm}$ depth of distilled water in a boiling tube, add a spatula measure of FB 4. Shake the tube to dissolve the FB 4.

12M

(i)

Carry out the following tests using a $1\text{ cm}$ depth of this FB 4 solution in a test-tube for each test. Record your observations in Table 3.1.

Table 3.1

test	observations
Test 1 Add aqueous sodium hydroxide, then transfer the mixture into a boiling tube, add a piece of aluminium foil and heat gently.
Test 2 Add an equal volume of dilute nitric acid, then add a few drops of aqueous silver nitrate.
Test 3 Add aqueous barium chloride or barium nitrate, then add dilute nitric acid.
Test 4 Add aqueous sodium carbonate dropwise with shaking until in excess.

5M Similar

DifficultyMedium

Worked solution

Answer

Test 1 (NaOH, then Al foil + heat):

White precipitate forms
Precipitate dissolves in excess NaOH
On adding Al and warming: effervescence/fizzing
Gas evolved turns damp red litmus paper blue — ammonia ( $\text{NH}_4^+$ present)
Aluminium turns black/dark grey

Test 2 (dilute nitric acid, then silver nitrate):

No change on adding nitric acid
White precipitate on adding silver nitrate

Test 3 (barium chloride/nitrate, then dilute nitric acid):

White precipitate on adding barium chloride
No change on adding dilute nitric acid

Test 4 (sodium carbonate dropwise):

White precipitate forms

Final answer

Observations recorded for all four tests (see working)

Detailed explanation

Background Concept

Qualitative analysis uses characteristic reactions to identify ions in solution:

$\text{Zn}^{2+}$ : forms a white gelatinous precipitate of $\text{Zn(OH)}_2$ with NaOH; the hydroxide is amphoteric, so it dissolves in excess NaOH (forming zincate, $[\text{Zn(OH)}_4]^{2-}$ ). With $\text{Na}_2\text{CO}_3$ it forms white $\text{ZnCO}_3$ .
$\text{NH}_4^+$ : with warm NaOH releases ammonia gas, $\text{NH}_3$ , which turns damp red litmus blue. Adding aluminium foil and warming helps drive off the ammonia (and would also reduce any nitrate to ammonia, but here it simply assists $\text{NH}_3$ release).
$\text{Cl}^-$ : with $\text{AgNO}_3$ in dilute nitric acid gives a white precipitate of $\text{AgCl}$ , insoluble in the acid.
$\text{SO}_4^{2-}$ : with $\text{BaCl}_2$ or $\text{Ba(NO}_3)_2$ gives a white precipitate of $\text{BaSO}_4$ , insoluble in dilute nitric acid.

Understanding the Question

Four tests are carried out on separate 1 cm depths of FB 4 solution:

NaOH, then transfer to boiling tube, add Al foil, heat gently.
Dilute nitric acid, then a few drops of aqueous silver nitrate.
Aqueous barium chloride or barium nitrate, then dilute nitric acid.
Aqueous sodium carbonate dropwise until in excess.

Record all observations in Table 3.1. The marking scheme awards 2 observations per mark (round down).

Approach

For each test, predict the observation from the known composition ( $\text{Zn}^{2+}$ , $\text{NH}_4^+$ , $\text{Cl}^-$ , $\text{SO}_4^{2-}$ ). Record the colour of any precipitate, its solubility in excess reagent, and any gas evolved with its test result.

Step-by-Step Reasoning

Test 1: NaOH added to FB 4 solution → white ppt of $\text{Zn(OH)}_2$ (insoluble in water, but the hydroxide is amphoteric). On adding excess NaOH the ppt dissolves (zincate ion). Then Al + heat: the aluminium reacts with NaOH producing hydrogen (fizzing/bubbling/effervescence) and the $\text{NH}_4^+$ releases $\text{NH}_3$ which turns damp red litmus blue. The aluminium surface turns black/dark grey as it reacts.

Test 2: Dilute nitric acid added first — no change (no carbonate present to produce $\text{CO}_2$ ). Then $\text{AgNO}_3$ → white ppt of $\text{AgCl}$ (chloride confirmed).

Test 3: $\text{BaCl}_2$ / $\text{Ba(NO}_3)_2$ → white ppt of $\text{BaSO}_4$ . Adding dilute nitric acid → no change ( $\text{BaSO}_4$ is insoluble in acid).

Test 4: $\text{Na}_2\text{CO}_3$ → white ppt of $\text{ZnCO}_3$ (zinc carbonate is insoluble).

Key Takeaways

$\text{Zn}^{2+}$ : white ppt with NaOH, soluble in excess; white ppt with $\text{Na}_2\text{CO}_3$ .
$\text{NH}_4^+$ : $\text{NH}_3$ with NaOH + heat (turns damp red litmus blue).
$\text{Cl}^-$ : white ppt with $\text{AgNO}_3$ in dilute nitric acid.
$\text{SO}_4^{2-}$ : white ppt with $\text{BaCl}_2$ / $\text{Ba(NO}_3)_2$ , insoluble in dilute nitric acid.

Common Mistakes

Saying "white precipitate" without noting solubility in excess reagent (this is a separate observation).
Missing the ammonia test result in Test 1.
Not recording "no change" for the nitric acid additions.
Confusing $\text{AgCl}$ (white) with $\text{AgBr}$ (cream) or $\text{AgI}$ (yellow).

Things to Be Careful About

Use dilute nitric acid (not sulfuric) before $\text{AgNO}_3$ — sulfuric would precipitate $\text{BaSO}_4$ and interfere.
Record at what stage each observation occurs (e.g. "on adding NaOH" vs "on adding excess").
Write "no change" where appropriate — it is a valid observation.
Use a boiling tube when warming solutions.

Techniques used

add aqueous sodium hydroxide to test for cationstest for ammonium ion with aluminium foil and heatingtest for halide ions with silver nitratetest for sulfate ions with barium chloridetest for metal cations with sodium carbonate

(ii)

Use your observations in Table 3.1 to identify two anions which must be present in FB 4.

2M Similar

DifficultyMedium-Easy

Worked solution

Answer

The two anions present in FB 4 are:

Chloride, $\text{Cl}^-$ — white precipitate with silver nitrate in dilute nitric acid ( $\text{AgCl}$ )
Sulfate, $\text{SO}_4^{2-}$ — white precipitate with barium chloride/nitrate, insoluble in dilute nitric acid ( $\text{BaSO}_4$ )

Final answer

Chloride, Cl⁻ and sulfate, SO₄²⁻

Detailed explanation

Background Concept

From the observations in Table 3.1:

Test 2: white ppt with $\text{AgNO}_3$ after acidification → $\text{Cl}^-$ ( $\text{AgCl}$ is white and insoluble in dilute nitric acid).
Test 3: white ppt with $\text{BaCl}_2$ / $\text{Ba(NO}_3)_2$ , insoluble in dilute nitric acid → $\text{SO}_4^{2-}$ ( $\text{BaSO}_4$ is white and insoluble in acid).

Understanding the Question

Use the observations from part (i) to identify the two anions present in FB 4. Each observation must be matched to the characteristic test. The question gives 2 marks — one per anion.

Approach

Match each observation to the known anion test:

$\text{AgNO}_3$ in acid → halide test; white ppt = chloride.
$\text{BaCl}_2$ / $\text{Ba(NO}_3)_2$ → sulfate test; white ppt insoluble in acid = sulfate.

Step-by-Step Reasoning

The white precipitate formed with silver nitrate in dilute nitric acid is $\text{AgCl}$ — chloride, $\text{Cl}^-$ , present.
The white precipitate formed with barium chloride/nitrate, which does not dissolve on adding dilute nitric acid, is $\text{BaSO}_4$ — sulfate, $\text{SO}_4^{2-}$ , present.

Key Takeaways

$\text{AgNO}_3$ test distinguishes halides: $\text{Cl}^-$ (white), $\text{Br}^-$ (cream), $\text{I}^-$ (yellow).
$\text{BaCl}_2$ / $\text{Ba(NO}_3)_2$ test confirms sulfate: white $\text{BaSO}_4$ insoluble in acid.

Common Mistakes

Confusing chloride with bromide or iodide (colour of ppt differs).
Saying "sulfate" without the acid-insolubility evidence.

Things to Be Careful About

Both anions must be stated with correct formulae ( $\text{Cl}^-$ , $\text{SO}_4^{2-}$ ).

Techniques used

interpret silver nitrate test observationsinterpret barium chloride test observationsdeduce anions from characteristic precipitates

(iii)

Carry out further tests to confirm or identify which two cations are present in FB 4.

Record the reagents and conditions needed, your observations and your conclusions in a suitable table.

5M Similar

DifficultyMedium

Worked solution

Answer

Test	Observation	Conclusion
Add aqueous ammonia dropwise to FB 4 solution, then in excess	White precipitate forms; precipitate dissolves in excess ammonia giving a colourless solution	$\text{Zn}^{2+}$ present
Add aqueous sodium hydroxide and warm (no aluminium foil)	Gas evolved that turns damp red litmus paper blue (ammonia)	$\text{NH}_4^+$ present

Final answer

Zn²⁺ confirmed by white ppt soluble in excess NH₃; NH₄⁺ confirmed by NH₃ gas with warm NaOH

Detailed explanation

Background Concept

Cation confirmatory tests:

$\text{Zn}^{2+}$ : with aqueous ammonia forms a white gelatinous precipitate of $\text{Zn(OH)}_2$ ; this dissolves in excess ammonia forming the colourless complex $[\text{Zn(NH}_3)_4]^{2+}$ . This distinguishes $\text{Zn}^{2+}$ from $\text{Al}^{3+}$ (whose hydroxide does NOT dissolve in excess ammonia) and from other cations.
$\text{NH}_4^+$ : with warm aqueous NaOH (no aluminium) releases ammonia gas, $\text{NH}_3$ , which turns damp red litmus blue.

Understanding the Question

Design further tests to confirm the two cations in FB 4. Record the reagents and conditions, observations, and conclusions in a suitable table. The marking scheme requires a table with at least 2 tests, with Test/Observation headings and a Conclusion column.

Approach

Use aqueous ammonia to confirm $\text{Zn}^{2+}$ (white ppt soluble in excess). Use aqueous NaOH + warm (without aluminium) to confirm $\text{NH}_4^+$ (ammonia gas, litmus blue). Present both in a table with clear headings.

Step-by-Step Reasoning

$\text{Zn}^{2+}$ : add aqueous ammonia dropwise to FB 4 solution → white ppt; add excess ammonia → ppt dissolves giving a colourless solution. This behaviour is characteristic of $\text{Zn}^{2+}$ (amphoteric hydroxide forming a soluble ammine complex).
$\text{NH}_4^+$ : add aqueous NaOH and warm (no aluminium foil — aluminium is only needed if nitrate is suspected) → gas evolved that turns damp red litmus blue = ammonia. This confirms $\text{NH}_4^+$ .

Key Takeaways

$\text{Zn}^{2+}$ : white ppt with $\text{NH}_3$ , soluble in excess (forms $[\text{Zn(NH}_3)_4]^{2+}$ ).
$\text{NH}_4^+$ : ammonia gas with warm NaOH, litmus blue.
A results table must have clear headings: Test, Observation, Conclusion.

Common Mistakes

Using aluminium foil in the $\text{NH}_4^+$ confirmation (that's the nitrate test).
Not warming the NaOH for the $\text{NH}_4^+$ test (ammonia release is too slow cold).
Not presenting results in a table with proper headings.

Things to Be Careful About

Table must have "Test" and "Observation" headings, plus a "Conclusion" column.
Record reagents and conditions precisely (e.g. "aqueous ammonia, dropwise then excess").
Zinc hydroxide dissolves in excess $\text{NH}_3$ — this is the key distinguishing observation.

Techniques used

test for zinc ion with aqueous ammonia in excesstest for ammonium ion with sodium hydroxide and warmingpresent confirmatory tests in a suitable table

Chemistry 9701/36 — October/November 2024

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Working

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Working

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Working

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Working

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Answer

Background Concept

Understanding the Question

Approach

Step-by-Step Reasoning

Key Takeaways

Common Mistakes

Things to Be Careful About

Working

Answer

Background Concept