Year 12 Chemistry Module 8 ⏱ ~35 min 7 MC · 3 Short Answer Lesson 3 of 16 Core

Precipitation Reactions & Qualitative Analysis

Build an ion identity from reaction evidence: choose a selective test, record the observation, write the net ionic equation, then confirm before concluding.

Today's hook: An unknown clear solution produces a white precipitate with one reagent, a coloured hydroxide with another, and a distinctive flame. Which observations identify an ion, and which only narrow the possibilities?

0/5TASKS

Connect: read the evidence

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Prediction Before Identification

A dissolved sample of the white powder is tested in the lab. Adding silver nitrate produces a white precipitate. Adding sodium hydroxide to a second portion gives a pale blue precipitate. The sample also produces a blue-green flame.

What ions might already be present in the unknown sample?
Why do these tests tell you what is present, but not necessarily how much is present?

By the end of this lesson you will:

Know

How common anions and cations can be identified using precipitation reactions
The difference between complete ionic equations and net ionic equations
The characteristic flame colours for key metal ions

Understand

Why qualitative analysis is about presence or absence rather than amount
How solubility rules explain why certain reagents are chosen for tests
How a combination of tests provides stronger evidence than one test alone

Can Do

Write ionic and net ionic equations for precipitation reactions
Classify test results to identify likely ions in an unknown solution
Interpret flame-test colours and distinguish qualitative from quantitative evidence

Vocabulary flip cards+10 XP

Predict before you flip. Say what the term means, reveal the definition, then mark it “Got it” or “Again”.

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Term 1Precipitation reactionClick to reveal ↻

DefinitionA reaction between aqueous ions that forms an insoluble solid called a precipitate.

Term 2Qualitative analysisClick to reveal ↻

DefinitionIdentifying which ions or substances are present, rather than measuring their amount.

Term 3Flame testClick to reveal ↻

DefinitionA screening test that uses a characteristic flame colour as evidence for a metal ion.

Term 4Confirmatory testClick to reveal ↻

DefinitionA second test using different chemistry to strengthen an identification suggested by a screening test.

Term 5Spectator ionClick to reveal ↻

DefinitionAn ion present in solution that remains unchanged and is removed from the net ionic equation.

Term 6Net ionic equationClick to reveal ↻

DefinitionAn equation showing only the species that undergo chemical change after spectators are cancelled.

Cross-lesson links: Precipitation reactions here link to L02 (gravimetric analysis uses selective precipitation to quantify ions). Net ionic equations connect to Module 4 equilibrium chemistry. The AAS technique in L04 provides a complementary instrumental method for confirming metal identity when visual tests are ambiguous.

Safety: Wear safety glasses and gloves. Silver nitrate stains skin and is corrosive, barium salts are toxic if ingested, and aqueous ammonia gives irritating fumes, so work in a well-ventilated area. Use small test-tube quantities and place all silver, barium and other heavy-metal residues in labelled waste containers, never down the sink.

Core Content

Explain: identity, not amount

What Qualitative Analysis Does Core

Presence or absence, not amount

Qualitative analysis answers the question "what is in this sample?" It does not, by itself, answer "how much is there?"

In qualitative analysis, chemists use specific observations such as precipitate formation, colour changes, gas evolution or flame colours to determine whether particular ions are present. A positive test gives evidence for identity, not concentration.

This differs from quantitative analysis, where the aim is to determine the amount of a substance, usually through measured volumes, masses or instrument signals. In Module 8, students need to be able to move clearly between these two styles of analysis.

Qualitative analysis identifies what ions or substances are present (not how much). A positive test gives evidence for identity: the observation must match a known pattern for that specific ion. A single test rarely proves identity conclusively, multiple consistent tests build stronger evidence.

Pause, copy the highlighted distinction into your book.

Must know: A white precipitate with AgNO₃(aq) tells you chloride may be present. It does not tell you whether the sample contains 0.001 mol L^-1 chloride or 1.0 mol L^-1 chloride without further measurement.

A white precipitate forms when AgNO₃(aq) is added to an unknown solution. What information does this provide?

Represent: reveal the reacting ions

Ionic and Net Ionic Equations Core

Show the reacting species, then strip away spectators

We just saw that qualitative analysis identifies ions by specific observations. That raises a question: how do we write the chemistry behind those observations at the ionic level? This card answers it → by writing complete ionic equations and then removing spectator ions to get the net ionic equation that shows what actually happens.

A precipitation test makes more sense when written at the ionic level. The net ionic equation shows the actual chemistry, not just the labels on the bottles.

Consider the chloride test:

Ag⁺ + NO₃^- + Na⁺ + Cl^- → AgCl(s) + Na⁺ + NO₃^-

Reacting species only: Ag⁺(aq) + Cl^-(aq) → AgCl(s)

Spectator ions such as Na⁺(aq) and NO₃^-(aq) are not involved in the actual precipitation step, so they are removed from the net ionic equation.

Chloride · precipitateAgCl(s) Ag⁺ + Cl^- → AgCl(s)

Sulfate · precipitateBaSO₄(s) Ba²⁺ + SO₄^2- → BaSO₄(s)

Carbonate · gasCO₂(g) CO₃^2- + 2H⁺ → CO₂ + H₂O

Hydroxide · gasNH₃(g) NH₄⁺ + OH^- → NH₃ + H₂O

Copper(II) · precipitateCu(OH)₂(s) Cu²⁺ + 2OH^- → Cu(OH)₂(s)

Copper(II) · complexationdeep-blue complex Cu²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄]²⁺

Net ionic equation: remove all spectator ions (those that appear identically on both sides of the complete ionic equation) so only species that change chemically remain. Chloride test: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Sulfate test: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Pause, copy the highlighted net ionic equations into your book.

Common error: "Every ion in the flask belongs in the final ionic equation." Not in the net ionic equation. Only species that change chemically should remain.

Which net ionic equation represents the test for sulfate ion using barium chloride?

Respond: test common anions

Testing for Common Anions Core

Use selective reagents and interpret the observation

We just saw how to write net ionic equations for precipitation tests. That raises a question: which specific reagents and observations tell you which anion is present? This card answers it → a table of the three key anion tests with their reagents, characteristic observations, and net ionic equations.

Anion tests work because certain ions form characteristic precipitates or gases when combined with appropriate reagents.

Anion	Reagent	Positive observation	Net ionic equation	Interference and confirmation
Cl^-	Acidify with dilute HNO₃, then AgNO₃(aq)	White precipitate of AgCl(s), darkens on standing in light	Ag⁺(aq) + Cl^-(aq) → AgCl(s)	Carbonate and phosphate also precipitate with silver; acidifying first destroys them. Confirm: AgCl dissolves in dilute ammonia.
Br^-	Acidify with dilute HNO₃, then AgNO₃(aq)	Cream precipitate of AgBr(s)	Ag⁺(aq) + Br^-(aq) → AgBr(s)	Separated from the other halides by ammonia solubility: AgBr dissolves only in concentrated ammonia.
I^-	Acidify with dilute HNO₃, then AgNO₃(aq)	Yellow precipitate of AgI(s)	Ag⁺(aq) + I^-(aq) → AgI(s)	AgI stays insoluble even in concentrated ammonia, which is what distinguishes it from AgCl and AgBr.
OH^-	Ammonium salt (e.g. NH₄Cl(aq)), gentle heat	Pungent ammonia gas, turns damp red litmus paper blue	NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)	Check first that the solution itself is basic to litmus. Waft the gas towards you; never inhale over the tube.
CH₃COO^-	Dilute H₂SO₄(aq), warm gently	Vinegar-like smell as ethanoic acid is released	CH₃COO^-(aq) + H⁺(aq) → CH₃COOH(aq)	Confirm with neutral FeCl₃(aq), which gives a deep red-brown colour. Waft to smell.
CO₃^2-	Dilute acid	Effervescence; the gas turns limewater milky	CO₃^2-(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)	Sulfite also effervesces, but its gas is sharp-smelling and decolourises acidified permanganate. Confirm: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s).
SO₄^2-	Acidify with dilute HCl, then BaCl₂(aq)	White precipitate of BaSO₄(s) that does not dissolve in acid	Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)	Carbonate and sulfite also give white barium precipitates, but those dissolve in acid. Acidifying first is what makes this test specific.
PO₄^3-	AgNO₃(aq) in neutral solution	Yellow precipitate of Ag₃PO₄(s)	3Ag⁺(aq) + PO₄^3-(aq) → Ag₃PO₄(s)	The solution must be neutral, or the phosphate protonates. Confirm: Ag₃PO₄ dissolves in dilute nitric acid, unlike the silver halides.

Notice that not every qualitative test forms a precipitate. Carbonate is identified through gas evolution when acid is added, hydroxide is identified through gas evolution from a heated ammonium salt, and acetate is identified through a characteristic smell rather than a solid or gas colour. The same logic still applies: the observation gives evidence for the presence of a particular ion.

Key anion tests: Cl⁻ → AgNO₃(aq) → white AgCl(s); SO₄²⁻ → BaCl₂(aq) → white BaSO₄(s); CO₃²⁻ → dilute acid → effervescence (CO₂ gas); OH⁻ → warm with an NH₄⁺ salt → pungent NH₃(g) that turns damp red litmus blue; CH₃COO⁻ → warm with dilute acid → vinegar-smelling CH₃COOH(aq). No precipitation or reaction with a test reagent means that anion is likely absent.

Pause, copy the highlighted anion test table into your book.

Forensic anchor: In a forensic context, one white precipitate alone is rarely enough to identify a powder with confidence. A stronger conclusion comes from combining anion tests with cation tests and flame-test evidence to build a consistent ion profile.

Alternative acetate test: Warming an acetate salt with ethanol and a few drops of concentrated H₂SO₄ produces the sweet, fruity smell of ethyl ethanoate (an ester). This esterification reaction is a second confirmatory test for CH₃COO^-, alongside the vinegar-smell acidification test above.

Effervescence is observed when dilute acid is added to a solution. Which anion is most likely present?

A solution is warmed with an NH₄⁺ salt and releases a pungent gas that turns damp red litmus paper blue. Which anion is most likely present?

Apply: distinguish common cations

Testing for Common Cations Core

Hydroxide and carbonate tests reveal characteristic solids

We just saw the three key anion tests and their observations. That raises a question: how do you identify the positive ions (cations) in the same sample? This card answers it → NaOH(aq) is added to test for transition metal cations, each producing a characteristic coloured precipitate.

Cation tests often depend on the colour or behaviour of the precipitate formed when hydroxide or carbonate ions are added.

Cation	Reagent	Positive observation	Net ionic equation	Interference and confirmation
Ba²⁺	Dilute H₂SO₄(aq)	White precipitate of BaSO₄(s), insoluble in acid	Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)	Ca²⁺ and Pb²⁺ also give white sulfates. Confirm by flame test: yellow-green.
Ca²⁺	Na₂CO₃(aq)	White precipitate of CaCO₃(s)	Ca²⁺(aq) + CO₃^2-(aq) → CaCO₃(s)	Nearly every cation here precipitates with carbonate, so this is weak evidence alone. Confirm by flame test: brick red.
Mg²⁺	NaOH(aq)	White precipitate that stays insoluble in excess NaOH(aq)	Mg²⁺(aq) + 2OH^-(aq) → Mg(OH)₂(s)	Al³⁺ also gives a white hydroxide, but it redissolves in excess NaOH. Magnesium gives no flame colour.
Pb²⁺	KI(aq)	Bright yellow precipitate of PbI₂(s)	Pb²⁺(aq) + 2I^-(aq) → PbI₂(s)	Ag⁺ gives a paler yellow AgI. Confirm: PbI₂ dissolves in hot water and recrystallises as golden plates on cooling.
Ag⁺	Dilute HCl(aq)	White precipitate of AgCl(s), darkens in light	Ag⁺(aq) + Cl^-(aq) → AgCl(s)	Pb²⁺ also gives a white chloride. Confirm: AgCl dissolves in dilute ammonia, whereas PbCl₂ does not but does dissolve in hot water.
Cu²⁺	NaOH(aq)	Pale blue precipitate of Cu(OH)₂(s)	Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)	Confirm with excess ammonia, which gives the deep blue [Cu(NH₃)₄]²⁺ complex. Flame test: blue-green.
Fe²⁺	NaOH(aq)	Green precipitate that darkens to red-brown on standing in air	Fe²⁺(aq) + 2OH^-(aq) → Fe(OH)₂(s)	The darkening is oxidation to iron(III) and can make a fresh Fe²⁺ result look like Fe³⁺. Read the colour immediately.
Fe³⁺	NaOH(aq)	Red-brown precipitate of Fe(OH)₃(s)	Fe³⁺(aq) + 3OH^-(aq) → Fe(OH)₃(s)	Confirm with KSCN(aq), which gives a blood-red solution.

Deep dive — Extension: ions beyond the required list

The eight cations and eight anions above are the ones the Module 8 syllabus names, and they are what you can be examined on. The tests below are worth knowing and appear in the practice bank, but a miss here is not a syllabus gap.

Ion	Reagent	Positive observation	Net ionic equation
NH₄⁺	NaOH(aq), warm gently	Pungent ammonia gas, turns damp red litmus paper blue	NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)
Na⁺	Flame test	Persistent yellow-orange flame	No precipitation reaction; the colour comes from electronic excitation
K⁺	Flame test	Lilac flame, best seen through cobalt-blue glass to filter out sodium	No precipitation reaction
Li⁺	Flame test	Crimson flame	No precipitation reaction
Al³⁺	NaOH(aq), then excess NaOH(aq)	White precipitate that redissolves in excess NaOH(aq)	Al³⁺(aq) + 3OH^-(aq) → Al(OH)₃(s), then Al(OH)₃(s) + OH^-(aq) → [Al(OH)₄]^-(aq)

Point to the **solid particles**, not the supernatant. Fresh green Fe(OH)₂(s) can oxidise and brown on standing. Colour and texture are supporting evidence only under controlled conditions; use confirmatory evidence before identifying a cation.

These tests are more powerful when used in combination. For example, a blue-green flame plus a pale blue precipitate with NaOH(aq) strongly supports the presence of Cu²⁺(aq).

Common error: "Every white metal hydroxide precipitate dissolves in excess NaOH(aq)." Not so. Only amphoteric hydroxides such as Al(OH)₃(s) redissolve in excess NaOH(aq). Mg(OH)₂(s) is also white but stays insoluble in excess, which is exactly how a white Mg²⁺ precipitate is distinguished from Al³⁺ in an unknown sample.

Key cation tests with NaOH(aq): Fe²⁺ → green Fe(OH)₂(s); Fe³⁺ → red-brown Fe(OH)₃(s); Cu²⁺ → pale blue Cu(OH)₂(s); NH₄⁺ → pungent ammonia gas on heating; Mg²⁺ → white Mg(OH)₂(s), insoluble in excess NaOH(aq) (unlike amphoteric Al³⁺). Multiple consistent observations from different methods greatly strengthen a qualitative conclusion.

Pause, copy the highlighted cation test table into your book.

A solution gives a pale blue precipitate with NaOH(aq). Which cation is most likely present?

A solution gives a white precipitate with NaOH(aq) that stays undissolved even when excess NaOH(aq) is added. Which cation is most likely present?

Feedback: confirm before concluding

Flame Tests as Supporting Evidence Core

Characteristic colours from excited metal ions

We just saw how precipitation reactions with NaOH identify cations through characteristic coloured precipitates. That raises a question: is there a quicker way to get a first clue about which metal is present? This card answers it → flame tests give rapid characteristic colour signals, but must always be treated as supporting evidence rather than definitive proof.

Hold a nichrome wire loop in a bunsen flame, steady blue. Dip it into a mystery solution, hold it back in the flame, sudden yellow-orange. That flash of colour is not a definition; it is an observation that points to sodium ions. But a second yellow sample might contain a different sodium compound, or a contaminant. That is why flame tests are supporting evidence, not definitive proof on their own.

Flame colours arise when electrons in metal ions are excited and then release light of characteristic wavelengths as they return to lower energy levels. In practice, sodium contamination can dominate flame tests, so chemists do not rely on flame colour alone when identifying an unknown.

1 · Screening clue One colour or precipitate narrows the possibilities but can be ambiguous.

2 · Confirmatory test Use a different chemical basis, such as complexation after a hydroxide precipitate.

3 · Defensible conclusion Require consistent observations, controls and equations; report limitations.

Limitations: similar precipitate colours can cause false positives, sodium can mask flame colours, contaminated wire loops distort observations, and mixed-ion samples may react with more than one reagent. Use fresh portions, blanks and at least one confirmatory test before identifying an ion.

Flame test colours: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ brick red, Ba²⁺ pale green, Cu²⁺ blue-green. Flame tests are supporting evidence only, sodium's bright yellow flame can mask weaker colours from other elements.

Pause, copy the highlighted flame colour list into your book.

Flame test colours mapped to metal ions in a comparison table

Flame tests give fast qualitative clues for some metal ions, but they are not definitive on their own. Sodium contamination is especially important because its bright yellow flame can mask weaker colours.

Worked example · identify an unknown

Problem: An unknown gives a white precipitate with AgNO₃ and a pale-blue precipitate with NaOH. Identify likely ions and justify each with a net ionic equation.

Interpret the white precipitate
Ag⁺ + Cl^- → AgCl(s)

This supports chloride, but another halide could require confirmation.

Interpret the pale-blue precipitate
Cu²⁺ + 2OH^- → Cu(OH)₂(s)

The characteristic hydroxide colour supports Cu²⁺.

Confirm copper(II)
Add excess NH₃: a deep-blue complex forms.

A second reaction with different chemistry strengthens the identification.

Conclude cautiously
Cl^- and Cu²⁺ are supported.

Report evidence and limitations rather than claiming one observation is absolute proof.

Why is a flame test usually considered supporting evidence rather than final proof of identity?

Interactive Tool, Qualitative Analysis Flowchart Open fullscreen ↗

The Qualitative Analysis tool shows that adding AgNO₃ to a halide salt and seeing a WHITE precipitate indicates…

🔬Predict, Then Reveal+8 XP

A clear solution is tested with dilute HCl, no precipitate forms. It is then tested with AgNO₃(aq), a pale yellow precipitate forms. Predict: which halide ion is present, and what is the precipitate?

Your predictionExpert answerCompare

✓

Practice: build an identification case

Complete the Learn phase to unlock Practice.

Activities

Sort the Test to the Ion

Classify each observation by the most likely ion identified. If more than one interpretation is possible, say what extra test would help.

1. White precipitate with AgNO₃(aq).

2. Red-brown precipitate with NaOH(aq).

3. Effervescence when dilute acid is added.

4. Pale green flame in a flame test.

Classify Statements as Qualitative or Quantitative

For each statement, classify it as qualitative or quantitative and justify your choice in one sentence.

1. "The unknown solution contains chloride ion because a white precipitate formed with silver nitrate."

2. "The sample contains 0.025 mol L^-1 chloride ion."

3. "The flame test suggests sodium ion is present because the flame was yellow."

4. "The concentration of Cu²⁺ in the sample is 0.10 mol L^-1."

Check Your Understanding

Practice questions · Randomised bank

Answer five questions from the L03 bank. Use the observation and equation, not a memorised answer position.

Short Answer Practice

1. Explain how a chemist could test for the presence of chloride ion and sulfate ion in separate portions of an unknown solution. Include the relevant observations and net ionic equations. 4 marks

2. Distinguish between qualitative and quantitative analysis using one example of each from Module 8. 4 marks

3. Evaluate how useful precipitation reactions and flame tests are for identifying a white powder in a forensic investigation. In your answer, refer to strengths, limitations, and why multiple tests are preferable to relying on a single observation. 5 marks

Show All Answers

Activity 1

1. White precipitate with AgNO₃(aq): chloride ion is likely present. A confirmatory test could include checking consistency with other observations or using another chloride-specific method.

2. Red-brown precipitate with NaOH(aq): Fe³⁺ is likely present because Fe(OH)₃(s) is red-brown.

3. Effervescence with dilute acid: carbonate ion is likely present because CO₂(g) is released.

4. Pale green flame: Ba²⁺ is likely, but this should be treated as supporting evidence because flame colours can be contaminated or masked.

Activity 2

1. Qualitative, it identifies chloride presence from an observation, not the amount.

2. Quantitative, it gives a numerical concentration of chloride.

3. Qualitative, it uses flame colour to suggest identity, not concentration.

4. Quantitative, it states a measured numerical concentration for Cu²⁺.

Multiple Choice Concepts

Qualitative evidence: identifies whether specific substances or ions are present.

Sulfate equation: Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s).

Copper(II): a pale-blue precipitate with NaOH(aq) supports Cu²⁺.

Flame-test limitation: contamination and intense sodium emission can mask other colours.

Hydroxide test: NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l).

Short Answer Model Answers

Q1 (4 marks): To test for chloride ion, add AgNO₃(aq) to a portion of the unknown solution. A white precipitate of AgCl(s) indicates chloride is present. The net ionic equation is Ag⁺(aq) + Cl^-(aq) → AgCl(s). To test for sulfate ion, add BaCl₂(aq) to a separate portion of the solution. A white precipitate of BaSO₄(s) indicates sulfate is present. The net ionic equation is Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s).

Q2 (4 marks): Qualitative analysis determines whether a substance is present. An example is adding AgNO₃(aq) to test for chloride ion by observing formation of a white precipitate. Quantitative analysis determines how much of a substance is present. An example from Module 8 is using gravimetric analysis or titration to calculate concentration or percentage composition. The key difference is that qualitative analysis gives identity information, whereas quantitative analysis gives numerical amount information.

Q3 (5 marks): Precipitation reactions and flame tests are useful first-line forensic tools because they are quick, inexpensive and can narrow down the identity of ions in an unknown powder. A strength is that specific observations, such as AgCl(s) or a blue-green flame, provide strong evidence for certain ions. However, each individual test has limitations: some precipitates have similar appearances, and flame tests can be affected by contamination or weak colour intensity. Multiple tests are preferable because a combination of consistent results gives a much more defensible identification than one observation alone. Overall, these methods are highly useful for screening and preliminary identification, but strongest conclusions come from integrating several tests rather than relying on any single result.

Return to Think First

Return to the unknown clear solution from Think First. Use the evidence chain you have built to make a cautious, defensible identification.

Which observations provide screening evidence, and which different reaction would confirm the likely ion?
Why is a flame colour supporting evidence rather than definitive identification?
Write one sentence explaining why a net ionic equation is more useful than a molecular equation in a forensic qualitative-analysis report.

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Review: check what actually stuck

Quick Quiz

Review

What is the difference between qualitative and quantitative analysis?

State the reagent, observation, and net ionic equation for testing chloride ion.

Which cations give green and red-brown precipitates with NaOH(aq)?

List the flame colours for Na⁺, K⁺, Ca²⁺, and Cu²⁺.

What does spectator ion mean, and why are spectator ions removed from net ionic equations?

How would you distinguish Mg²⁺ from Al³⁺ using NaOH(aq)?

State the reagent, observation, and net ionic equation for testing hydroxide ion and acetate ion.

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