Identification of ions and gases

1. Overview

Qualitative analysis is the process used to identify the chemical components of an unknown substance. This topic focuses on the specific chemical tests used to detect anions (negatively charged ions) and cations (positively charged ions), and to identify common gases, by observing colour changes, the formation of precipitates, flame colours or the release of specific gases.

Key Definitions

• Anion: A negatively charged ion (e.g., $Cl^-$, $SO_4^{2-}$).
• Cation: A positively charged ion (e.g., $Na^+$, $Cu^{2+}$).
• Qualitative Analysis: Testing a substance to find out what it is made of, rather than how much of it there is.
• Precipitate: An insoluble solid that forms when two solutions are mixed together.
• Effervescence: Bubbling or fizzing seen in a liquid during a chemical reaction, indicating the production of a gas.
• Acidify: Adding a small amount of dilute acid to a sample to remove any interfering ions (like carbonates) before performing a specific test.

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Core Content: Tests for Anions, Cations and Gases

To identify an unknown anion in a solution, follow these specific procedures:

(a) Carbonate ($CO_3^{2-}$)

• Test: Add dilute hydrochloric acid to the solid or solution.
• Observation: Effervescence (fizzing).
• Confirmatory Test: Bubble the gas produced through limewater. The limewater turns cloudy/milky.
• Word Equation:
  Hydrochloric acid + Sodium carbonate → Sodium chloride + Water + Carbon dioxide
• Symbol Equation:
  $2HCl(aq) + Na_2CO_3(aq) \to 2NaCl(aq) + H_2O(l) + CO_2(g)$

(b) Halides (Chloride $Cl^-$, Bromide $Br^-$, Iodide $I^-$)

• Test: Acidify the solution with dilute nitric acid ($HNO_3$), then add aqueous silver nitrate ($AgNO_3$).
• Observations:
  • Chloride ($Cl^-$): White precipitate forms.
  • Bromide ($Br^-$): Cream precipitate forms.
  • Iodide ($I^-$): Yellow precipitate forms.
• General Symbol Equation (e.g., Chloride):
  $AgNO_3(aq) + NaCl(aq) \to AgCl(s) + NaNO_3(aq)$

(c) Nitrate ($NO_3^-$)

• Test: Add aqueous sodium hydroxide ($NaOH$), then add a small piece of aluminium foil. Warm the mixture gently.
• Observation: Ammonia gas is produced (has a pungent smell).
• Confirmatory Test: Ammonia gas turns damp red litmus paper blue.
• Explanation: The aluminium reduces the nitrate ion to ammonium/ammonia in alkaline conditions.

(d) Sulfate ($SO_4^{2-}$)

• Test: Acidify the solution with dilute nitric acid ($HNO_3$), then add aqueous barium nitrate ($Ba(NO_3)_2$).
• Observation: A thick white precipitate forms.
• Word Equation:
  Barium nitrate + Sodium sulfate → Barium sulfate + Sodium nitrate
• Symbol Equation:
  $Ba(NO_3)_2(aq) + Na_2SO_4(aq) \to BaSO_4(s) + 2NaNO_3(aq)$

(e) Sulfite ($SO_3^{2-}$)

• Test: Add a few drops of acidified aqueous potassium manganate(VII) to the solution.
• Observation: The acidified potassium manganate(VII) changes color from purple to colorless.
• Explanation: Sulfite ions are a reducing agent. They reduce the purple $MnO_4^-$ ions in the solution, so the purple colour disappears.

Tests for Aqueous Cations

Most of the metal ions in this topic form an insoluble hydroxide, so a precipitate appears when an alkali is added to their solution. The colour of the precipitate, and whether it dissolves when more alkali is added, identifies the cation. Two alkalis are used: aqueous sodium hydroxide and aqueous ammonia.

• Method:
  1. Pour a small volume of the unknown solution into a test tube.
  2. Add aqueous sodium hydroxide a few drops at a time and note the colour of any precipitate.
  3. Keep adding the alkali, shaking the tube, until it is in excess (far more than is needed to form the precipitate). Note whether the precipitate dissolves or stays.
  4. Repeat steps 1 to 3 on a fresh sample, using aqueous ammonia instead of sodium hydroxide.
• Why Two Reagents: some cations behave in the same way with sodium hydroxide, and only the result with aqueous ammonia tells them apart.

Cation	Aqueous sodium hydroxide	Aqueous ammonia
Aluminium ($Al^{3+}$)	White precipitate, which dissolves in excess to give a colourless solution	White precipitate, which does not dissolve in excess
Ammonium ($NH_4^+$)	No precipitate; ammonia gas is given off when the mixture is warmed	Not used (the reagent already contains ammonia)
Calcium ($Ca^{2+}$)	White precipitate, which does not dissolve in excess	No precipitate, or only a very slight white one
Chromium(III) ($Cr^{3+}$)	Green precipitate, which dissolves in excess	Green precipitate, which does not dissolve in excess
Copper(II) ($Cu^{2+}$)	Light blue precipitate, which does not dissolve in excess	Light blue precipitate, which dissolves in excess to give a dark blue solution
Iron(II) ($Fe^{2+}$)	Green precipitate, which does not dissolve in excess and turns brown near the surface if left to stand	Green precipitate, which does not dissolve in excess and turns brown near the surface if left to stand
Iron(III) ($Fe^{3+}$)	Red-brown precipitate, which does not dissolve in excess	Red-brown precipitate, which does not dissolve in excess
Zinc ($Zn^{2+}$)	White precipitate, which dissolves in excess to give a colourless solution	White precipitate, which dissolves in excess to give a colourless solution

• White Precipitates: aluminium, calcium and zinc all give one with sodium hydroxide. Calcium's precipitate stays in excess, while aluminium's and zinc's dissolve. Aqueous ammonia then separates aluminium from zinc: only the zinc precipitate dissolves in excess ammonia. Calcium gives little or no precipitate with ammonia.
• Green Precipitates: chromium(III) and iron(II). Excess sodium hydroxide dissolves the chromium(III) precipitate but not the iron(II) one. The iron(II) precipitate turns brown at the surface on standing because oxygen in the air slowly oxidises the iron(II) to iron(III).
• Copper(II): the only cation in the table that gives a dark blue solution, when its light blue precipitate dissolves in excess aqueous ammonia.
• Ammonium: no precipitate forms. Warm the mixture with sodium hydroxide and hold damp red litmus paper at the mouth of the tube: the paper turns blue because ammonia gas is given off.

Example: Copper(II) Sulfate Solution with Sodium Hydroxide

• Word Equation:
  Copper(II) sulfate + Sodium hydroxide → Copper(II) hydroxide + Sodium sulfate
• Symbol Equation:
  $CuSO_4(aq) + 2NaOH(aq) \to Cu(OH)_2(s) + Na_2SO_4(aq)$

The state symbol $(s)$ marks the precipitate. The equations for iron(II) and iron(III) ions are on the Transition elements page (8.4).

Tests for Gases

Each gas in this topic has one standard test. When you describe a test, state what you do and what you see.

Gas	Test	Result if the gas is present
Ammonia ($NH_3$)	Hold damp red litmus paper in the gas	The paper turns blue
Carbon dioxide ($CO_2$)	Bubble the gas through limewater	The limewater turns milky (cloudy)
Chlorine ($Cl_2$)	Hold damp litmus paper in the gas	The paper is bleached (turns white)
Hydrogen ($H_2$)	Hold a lighted splint at the mouth of the test tube	The gas burns with a squeaky 'pop'
Oxygen ($O_2$)	Put a glowing splint into the gas	The splint relights
Sulfur dioxide ($SO_2$)	Bubble the gas through acidified aqueous potassium manganate(VII)	The solution turns from purple to colourless

• Damp Litmus: the paper must be damp for chlorine as well as for ammonia (see Exam Tips).
• Chlorine: damp blue litmus paper may turn red for a moment before it is bleached, because chlorine forms an acidic solution in the water on the paper. The bleaching is the result that identifies chlorine.
• Lighted or Glowing: use a lighted (burning) splint for hydrogen and a glowing splint (flame blown out, tip still glowing) for oxygen. The pop is the hydrogen burning quickly with oxygen from the air to form water.
• Links to the Anion Tests: carbonate ions give carbon dioxide with dilute acid (test (a)), and a sulfite warmed with dilute acid gives off sulfur dioxide. Keep the two reagents apart: limewater for carbon dioxide, acidified potassium manganate(VII) for sulfur dioxide. The same purple solution is also used on sulfite ions themselves (test (e)). Ammonia is the gas detected in the nitrate test (c) and in the ammonium test above.

Flame Tests

Some metal ions give a flame a strong colour when their compounds are heated. A flame test is usually done on a small amount of the solid. It is the test to use for lithium, sodium and potassium ions, because they give no precipitate with aqueous sodium hydroxide or aqueous ammonia.

• Method:
  1. Clean a flame test wire (made of nichrome or platinum) by dipping it in hydrochloric acid and holding it in a hot Bunsen flame until it no longer colours the flame.
  2. Dip the clean wire in the acid again, then into a small amount of the solid so that some sticks to it.
  3. Hold the wire at the edge of a blue (non-luminous) Bunsen flame and note the colour.

Cation	Flame colour
Lithium ($Li^+$)	Red
Sodium ($Na^+$)	Yellow
Potassium ($K^+$)	Lilac
Calcium ($Ca^{2+}$)	Orange-red
Barium ($Ba^{2+}$)	Light green
Copper(II) ($Cu^{2+}$)	Blue-green

• Colours That Look Alike: lithium (red) and calcium (orange-red) are easy to confuse, and so are barium (light green) and copper(II) (blue-green). A second test helps: with aqueous sodium hydroxide, calcium ions give a white precipitate and copper(II) ions a light blue one.
• A Clean Wire Matters: a trace of an earlier sample left on the wire colours the flame and gives a false result. Sodium is the worst offender, because its yellow is bright enough to hide other colours.

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Extended Content (Extended Only)

There is no additional Supplement content for this sub-topic (12.5) beyond the Core requirements listed above: the tests for anions, aqueous cations and gases, and the flame tests, are all Core.

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Key Equations

Ion	Ionic Equation for Test
Carbonate	$CO_3^{2-}(aq) + 2H^+(aq) \to CO_2(g) + H_2O(l)$
Chloride	$Ag^+(aq) + Cl^-(aq) \to AgCl(s)$
Bromide	$Ag^+(aq) + Br^-(aq) \to AgBr(s)$
Iodide	$Ag^+(aq) + I^-(aq) \to AgI(s)$
Sulfate	$Ba^{2+}(aq) + SO_4^{2-}(aq) \to BaSO_4(s)$

• $Ag^+$ = Silver ion
• $Ba^{2+}$ = Barium ion
• $(aq)$ = Aqueous (dissolved in water)
• $(s)$ = Solid (the precipitate)

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Common Mistakes to Avoid

• ❌ Wrong: Using Sulfuric acid to acidify the Sulfate test.
• ✓ Right: Always use Nitric acid. Sulfuric acid contains sulfate ions which will react with the barium and give a false positive result.
• ❌ Wrong: Describing the Potassium Manganate(VII) change as "going clear."
• ✓ Right: Use the word colorless. (A solution can be clear but still have a color, like pale yellow).
• ❌ Wrong: Testing for Nitrate by just adding Aluminium.
• ✓ Right: You must add Sodium Hydroxide AND Aluminium and then heat it.
• ❌ Wrong: Testing for hydrogen with a glowing splint, or for oxygen with a lighted splint.
• ✓ Right: Hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint.
• ❌ Wrong: Naming a cation from the colour of its precipitate alone, e.g. "white precipitate, so it is zinc".
• ✓ Right: Also say what happens in excess. Aluminium, calcium and zinc all give white precipitates with sodium hydroxide; only their behaviour in excess sodium hydroxide and in aqueous ammonia tells them apart.

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Exam Tips

• Command Words: If a question asks you to "Describe a test," you must include the reagents added AND the result (e.g., "Add silver nitrate, a white precipitate forms").
• Litmus Paper: Always state that litmus paper must be damp when testing for ammonia gas, otherwise the gas cannot dissolve to show its alkaline nature.
• The "Acidify" Step: If asked why acid is added before silver nitrate or barium nitrate, the answer is "to react with and remove any carbonate ions" which would otherwise form their own precipitates and confuse the result.
• Colors: Memorizing the "White-Cream-Yellow" sequence for halides is essential. They look very similar, so in a practical exam, always compare them if possible.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student performs tests to identify ions present in an unknown solution, solution X.

(a) The student adds dilute nitric acid, followed by aqueous silver nitrate, to a sample of solution X. A cream precipitate forms. Identify the anion present in solution X. [1]

(b) Explain why the student uses dilute nitric acid, and not dilute hydrochloric acid, to acidify solution X before adding the aqueous silver nitrate. [2]

(c) The student adds dilute hydrochloric acid to another sample of solution X. A gas is produced which turns limewater cloudy. Identify the anion present in solution X. [1]

(d) Write the ionic equation for the reaction between the anion identified in (c) and dilute hydrochloric acid. Include state symbols. [1]

Worked Solution:

(a)

1. The anion that produces a cream precipitate with silver nitrate is the bromide ion.
   [Identifying the correct anion based on the observation]

How to earn full marks:

• State 'bromide' or 'Br-'

(b)

1. Dilute hydrochloric acid contains chloride ions.
   [Stating what hydrochloric acid adds to the solution]
2. Those chloride ions would react with the silver nitrate to give a white precipitate of silver chloride whatever anion solution X contained, so the result could not be trusted.
   [Explaining how this spoils the test]

How to earn full marks:

• State 'hydrochloric acid contains chloride ions'
• State 'chloride ions give a white precipitate with silver nitrate, so the test would give a false result'

(c)

1. The anion that produces carbon dioxide gas (turns limewater cloudy) when reacted with acid is the carbonate ion.
   [Identifying the correct anion based on the observation]

How to earn full marks:

• State 'carbonate' or 'CO32-'

(d)

1. $CO_3^{2-}(aq) + 2H^+(aq) \rightarrow CO_2(g) + H_2O(l)$
   [Correct reactants and products]

How to earn full marks:

• Correct chemical formulae for all reactants and products
• Correct balancing and state symbols

Common Pitfall: Remember to include state symbols in your ionic equations. Also, be careful to distinguish between the different halide precipitates formed with silver nitrate, as they can look quite similar.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A scientist is analysing a sample of river water to check for pollution.

(a) The scientist adds acidified aqueous potassium manganate(VII) to a sample of the river water. The purple colour disappears. Identify a possible anion that could be present in the river water and explain why the purple colour disappears. [2]

(b) The scientist suspects the presence of nitrate ions. Describe the test for nitrate ions, including the reagents used and the expected observation if nitrate ions are present. [4]

Worked Solution:

(a)

1. A possible anion is sulfite ($SO_3^{2-}$).
   [Identifying a suitable anion]
2. The sulfite ions reduce the manganate(VII) ions, causing the purple colour to disappear as the $Mn^{7+}$ ions are reduced to $Mn^{2+}$ ions.
   [Explaining the colour change in terms of reduction]

How to earn full marks:

• State 'sulfite'
• State 'sulfite reduces manganate(VII)' OR 'manganate(VII) is reduced'

(b)

1. Add aqueous sodium hydroxide and aluminium foil to the sample.
   [Stating the correct reagents]
2. Heat the mixture gently.
   [Stating the heating step]
3. Test any gas produced with damp red litmus paper.
   [Stating how to test for the gas]
4. If nitrate ions are present, the litmus paper will turn blue, indicating the presence of ammonia.
   [Stating the correct observation and conclusion]

How to earn full marks:

• State 'add NaOH and aluminium foil'
• State 'heat the mixture'
• State 'test gas with damp red litmus paper'
• State 'litmus paper turns blue (ammonia present)'

Common Pitfall: When testing for nitrate ions, make sure you add both sodium hydroxide and aluminium foil. Heating is also crucial to speed up the reaction. Remember that ammonia gas is alkaline, so it turns damp red litmus paper blue.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student investigates a metal compound, compound Y. Compound Y is known to contain one cation and one anion. The student performs the following tests:

Test 1: Compound Y is dissolved in water to form a solution. Aqueous sodium hydroxide is added dropwise to the solution until in excess. A green precipitate forms, which is insoluble in excess sodium hydroxide.

Test 2: Aqueous ammonia is added to a fresh sample of the solution until in excess. A green precipitate forms, which is insoluble in excess aqueous ammonia.

Test 3: Dilute nitric acid is added to a solution of Compound Y, followed by aqueous barium nitrate. A white precipitate forms.

(a) Identify the cation present in compound Y, based on Test 1 and Test 2. Explain your reasoning. [3]

(b) Identify the anion present in compound Y, based on Test 3. Explain your reasoning. [2]

(c) Write the chemical formula of compound Y. [1]

(d) A sulfite gives off a gas when it is warmed with dilute hydrochloric acid. Describe a test to confirm the identity of this gas. State the expected result. [2]

Worked Solution:

(a)

1. The green precipitate in Test 1 suggests the presence of iron(II) ions ($Fe^{2+}$).
   [Identifying the cation based on the precipitate colour]
2. Chromium(III) ions also give a green precipitate, but theirs dissolves in excess sodium hydroxide. The precipitate in Test 1 does not dissolve, so the cation is iron(II), not chromium(III).
   [Using the result in excess to rule out chromium(III)]
3. Test 2 agrees: iron(II) ions give a green precipitate that does not dissolve in excess aqueous ammonia.
   [Checking the result with the second reagent]

How to earn full marks:

• State 'iron(II)' or 'Fe2+'
• State 'green precipitate with sodium hydroxide and with aqueous ammonia'
• State 'insoluble in excess sodium hydroxide, so not chromium(III)'

(b)

1. The white precipitate with barium nitrate suggests the presence of sulfate ions ($SO_4^{2-}$).
   [Identifying the anion based on the precipitate colour]
2. Barium ions react with sulfate ions to form insoluble barium sulfate.
   [Explaining the formation of the precipitate]

How to earn full marks:

• State 'sulfate' or 'SO42-'
• State 'white precipitate with barium nitrate indicates sulfate'

(c)

1. $\boxed{FeSO_4}$
   [Correctly combining the identified ions]

How to earn full marks:

• Correct chemical formula: $FeSO_4$

(d)

1. Bubble the gas produced through acidified aqueous potassium manganate(VII).
   [Stating the correct test]
2. If the gas is sulfur dioxide, the solution will turn from purple to colourless.
   [Stating the expected result]

How to earn full marks:

• State 'bubble through acidified aqueous potassium manganate(VII)'
• State 'turns from purple to colourless'

Common Pitfall: Make sure you link the observations directly to the ions they indicate. For example, don't just say "green precipitate" – say "green precipitate indicates iron(II) ions." Also, remember that the test for sulfur dioxide is acidified aqueous potassium manganate(VII); limewater is the test for carbon dioxide.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A chemical company is investigating a new compound, compound Z, which is a white solid. The company's chemists perform several tests to identify the ions present.

Test 1: Compound Z is dissolved in distilled water to form a solution.

Test 2: A flame test is performed on solid compound Z. The flame turns lilac.

Test 3: Dilute nitric acid is added to the solution from Test 1, followed by aqueous silver nitrate. A yellow precipitate forms.

Test 4: Aqueous sodium hydroxide is added dropwise to the solution from Test 1, until in excess. A white precipitate forms, which dissolves in excess sodium hydroxide. With aqueous ammonia instead, a white precipitate forms that does not dissolve in excess.

(a) Based on Test 2, identify the cation present in compound Z. [1]

(b) Based on Test 3, identify the anion present in compound Z. [1]

(c) Based on Test 4, identify the other cation present in compound Z. [2]

(d) Suggest the formula of the compound formed between the cation in (c) and the anion in (b). [1]

(e) Write an ionic equation, including state symbols, for the formation of the yellow precipitate in Test 3. [2]

(f) Explain why dilute nitric acid is added before adding silver nitrate in Test 3. [2]

Worked Solution:

(a)

1. The lilac flame indicates the presence of potassium ions ($K^+$).
   [Identifying the cation based on the flame test colour]

How to earn full marks:

• State 'potassium' or 'K+'

(b)

1. The yellow precipitate with silver nitrate indicates the presence of iodide ions ($I^-$).
   [Identifying the anion based on the precipitate colour]

How to earn full marks:

• State 'iodide' or 'I-'

(c)

1. The white precipitate with sodium hydroxide, which dissolves in excess, shows aluminium or zinc ions. The precipitate with aqueous ammonia does not dissolve in excess, so the ion is aluminium ($Al^{3+}$), not zinc.
   [Identifying the cation based on the precipitate behaviour]
2. Aluminium hydroxide is amphoteric.
   [explaining precipitate behavior]

How to earn full marks:

• State 'aluminium' or 'Al3+'
• State 'white precipitate dissolves in excess NaOH but not in excess aqueous ammonia'

(d)

1. $\boxed{AlI_3}$
   [Correctly combining the identified ions]

How to earn full marks:

• Correct chemical formula: $AlI_3$

(e)

1. $Ag^+(aq) + I^-(aq) \rightarrow AgI(s)$
   [Correct reactants and products]

How to earn full marks:

• Correct chemical formulae for all reactants and products
• Correct balancing and state symbols

(f)

1. The dilute nitric acid is added to remove any carbonate ions that might be present.
   [Stating the purpose of adding acid]
2. Carbonate ions would react with silver ions to form a precipitate, which would interfere with the test for halide ions.
   [Explaining the potential interference]

How to earn full marks:

• State 'to remove carbonate ions'
• State 'carbonate ions interfere with the test'

Common Pitfall: Remember that aluminium hydroxide is amphoteric, meaning it dissolves in excess sodium hydroxide. Also, always add nitric acid before silver nitrate to avoid false positives from carbonate ions. Be careful not to confuse the colours of the halide precipitates.