In chemistry, we often need to find out what is in a sample without necessarily measuring how much of it is there. This is the realm of qualitative analysis.

Qualitative tests must be quick, reliable, and produce distinct visual changes—such as color changes, gas evolution, or the formation of an insoluble solid. In this guide, you will learn the exact chemical tests and the strict sequence required to identify key anions and cations under the OCR A specification.

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What you'll learn

• How to test for carbonate, sulfate, and halide ions on a test-tube scale.
• Why a strict sequence of testing is absolutely vital to prevent false-positive results.
• How to identify the ammonium cation ($N{H}_4^{+}$) using a simple chemical test.

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Defining Qualitative Analysis

Before diving into the specific test procedures, we must define our key terms.

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Anion Testing: The Individual Tests

Anions are negatively charged ions. For your A-Level exams, you must know the tests for three main classes of anions: carbonates (${CO}_3^{2-}$), sulfates (${SO}_4^{2-}$), and halides (${Cl}^{-}$, ${Br}^{-}$, and $I^{-}$).

1. The Carbonate Test

To test for the presence of a carbonate ion (${CO}_3^{2-}$):

• Method: Add a dilute strong acid, such as dilute nitric acid (${HNO}_3(aq)$), to the solid or aqueous sample at room temperature.
• Observation: If carbonate ions are present, you will observe immediate effervescence (bubbling) as carbon dioxide gas (${CO}_2$) is released.
• Confirmation: To prove the gas is ${CO}_2$, bubble it through aqueous calcium hydroxide (limewater). The limewater will turn cloudy due to the formation of a white precipitate of calcium carbonate (${CaCO}_3$).

The ionic equation for the primary test is:

$${CO}_3^{2-}(aq)+2H^{+}(aq)\to {CO}_2(g)+H_{2}O(l)$$

The confirmation reaction in limewater is:

$${Ca(OH)}_2(aq)+{CO}_2(g)\to {CaCO}_3(s)+H_{2}O(l)$$

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2. The Sulfate Test

To test for the presence of a sulfate ion (${SO}_4^{2-}$):

• Method: Add aqueous barium ions (${Ba}^{2+}(aq)$) to the sample. This is typically added as barium chloride (${BaCl}_2(aq)$) or barium nitrate (${Ba(NO}_3{)}_2(aq)$).
• Observation: If sulfate ions are present, a dense white precipitate of barium sulfate (${BaSO}_4$) forms immediately.

The ionic equation for this precipitation reaction is:

$${Ba}^{2+}(aq)+{SO}_4^{2-}(aq)\to {BaSO}_4(s)$$

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3. The Halide Test

To test for halide ions—chloride (${Cl}^{-}$), bromide (${Br}^{-}$), and iodide ($I^{-}$):

• Method: First, add dilute nitric acid (${HNO}_3(aq)$) to remove any interfering carbonate ions. Then, add aqueous silver nitrate (${AgNO}_3(aq)$).
• Observation: Silver ions react with halide ions to form colored silver halide precipitates:
  • Chloride (${Cl}^{-}$): Forms a white precipitate ($AgCl$).
  • Bromide (${Br}^{-}$): Forms a cream precipitate ($AgBr$).
  • Iodide ($I^{-}$): Forms a yellow precipitate ($AgI$).

The general ionic equation is:

$${Ag}^{+}(aq)+X^{-}(aq)\to AgX(s)$$

Confirming Halides with Ammonia

Because white, cream, and yellow can look highly subjective and similar under test-tube conditions, you must confirm the halide using aqueous ammonia (${NH}_3(aq)$):

• Silver chloride ($AgCl$): Precipitate dissolves in dilute ammonia solution to form a colorless solution.
• Silver bromide ($AgBr$): Precipitate is insoluble in dilute ammonia, but dissolves in concentrated ammonia solution.
• Silver iodide ($AgI$): Precipitate is completely insoluble in both dilute and concentrated ammonia.

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The Crucial Sequence: Carbonate $\to$ Sulfate $\to$ Halide

If you are handed an unknown solution that could contain a mixture of ions, you cannot simply perform these tests in any random order. Doing so will lead to chemistry-induced errors called false positives.

Why is this sequence necessary?

Let us break down the chemistry of what happens if you do these tests in the wrong order:

• Why Carbonate must come before Sulfate: If you add barium ions (${Ba}^{2+}$) to an unknown solution that contains carbonate ions, you will get a white precipitate of barium carbonate (${BaCO}_3(s)$) because barium carbonate is also insoluble in water!

$${Ba}^{2+}(aq)+{CO}_3^{2-}(aq)\to {BaCO}_3(s)$$

You would mistake this for a positive sulfate result. Because the carbonate test is done first (by adding acid), any carbonate ions are destroyed and bubbled away as ${CO}_2$ before you move on to the sulfate test.

• Why Sulfate must come before Halide: If you add silver ions (${Ag}^{+}$) to a solution containing sulfate ions, you can form a precipitate of silver sulfate (${Ag}_2{SO}_4(s)$) because silver sulfate is only sparingly soluble in water.

$$2{Ag}^{+}(aq)+{SO}_4^{2-}(aq)\to {Ag}_2{SO}_4(s)$$

This white precipitate would easily be confused with a white silver chloride ($AgCl$) precipitate, giving you a false positive for chloride. Doing the sulfate test first allows you to precipitate out and filter off all sulfate ions before adding silver nitrate.

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Testing for a Cation: The Ammonium Ion (${NH}_4^{+}$)

The main cation test specified in this section of the OCR A syllabus is the test for the ammonium ion (${NH}_4^{+}$).

• Method: Add aqueous sodium hydroxide ($NaOH(aq)$) to the solid or solution in a test tube, and warm the mixture gently in a water bath or over a Bunsen burner.
• Observation: No precipitate forms, but ammonia gas (${NH}_3$) is evolved.
• Detection: You can detect the presence of ${NH}_3$ gas in two ways:
  1. It has a highly pungent, choking smell.
  2. Hold damp red litmus paper near the mouth of the test tube. The alkaline ammonia gas dissolves in the water on the paper, turning the litmus paper blue.

The ionic equation for this reaction is:

$${NH}_4^{+}(aq)+{OH}^{-}(aq)\to {NH}_3(g)+H_{2}O(l)$$

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Worked Example: Deductive Qualitative Analysis

Here is how you apply this logical sequential testing to an unknown mixture in a real laboratory scenario.

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