12.5 Identification of Ions and Gases: Anions

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.