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) in aqueous solutions by observing color changes, the formation of precipitates, or the release of specific gases.

Key Definitions

• Anion: A negatively charged ion (e.g., $Cl^-$, $SO_4^{2-}$).
• 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

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 dilute hydrochloric acid and warm gently. Test the gas produced ($SO_2$) with acidified aqueous potassium manganate(VII).
• Observation: The acidified potassium manganate(VII) changes color from purple to colorless.
• Explanation: Sulfite ions react with acid to produce sulfur dioxide gas, which is a reducing agent that decolorizes the purple $MnO_4^-$ ions.

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Extended Content (Extended Only)

There is no additional Supplement content for this specific sub-topic (12.5 Anions) beyond the Core requirements listed above.

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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.

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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) Describe the next step the student should take to confirm the identity of the anion in solution X. State the expected result. [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. The student should add dilute ammonia solution to the cream precipitate. [Stating the correct reagent]
2. The cream precipitate should dissolve slowly in dilute ammonia solution. [Stating the correct observation]

How to earn full marks:

• State 'add dilute ammonia solution'
• State 'precipitate dissolves slowly'

(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: Compound Y is heated strongly in a Bunsen burner flame. The flame turns green.

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) Describe a test to confirm the identity of the gas produced if dilute hydrochloric acid were added to solid compound Y. 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. The green flame colour in Test 2 confirms the presence of iron(II) ions. [Confirming the cation based on the flame test]
3. The precipitate being insoluble in excess NaOH is also consistent with $Fe^{2+}$. [Confirming the cation based on solubility]

How to earn full marks:

• State 'iron(II)' or 'Fe2+'
• State 'green precipitate with NaOH indicates iron(II)'
• State 'green flame indicates iron(II)'

(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 limewater (calcium hydroxide solution). [Stating the correct test]
2. If the gas is sulfur dioxide, the limewater will turn cloudy. [Stating the expected result]

How to earn full marks:

• State 'bubble through limewater'
• State 'limewater turns cloudy'

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 to use limewater to test for sulfur dioxide, not just any gas.

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.

(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 compound Z. [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, indicates the presence of aluminium ions ($Al^{3+}$). [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'

(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.