Sometimes tested C16.2

Chemical Tests for Anions

This topic covers the standard chemical tests used to identify three key anions: carbonates, halides, and sulfates. Knowledge of the specific reagents, procedures, and the expected positive results (like a colour change, fizzing, or a precipitate) is essential.

Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Test for Carbonates (CO32-): Add any dilute acid, such as HCl. A positive test is effervescence (fizzing) as carbon dioxide gas is produced.
  • Test for Sulfates (SO42-): First add dilute HCl, then add aqueous barium chloride (BaCl2). A positive test is the formation of a dense white precipitate, which is barium sulfate (BaSO4).
  • Test for Halides (Cl-, Br-, I-): First add dilute nitric acid (HNO3), then add aqueous silver nitrate (AgNO3). A precipitate forms if a halide is present.
  • Halide Precipitate Colours: Silver chloride (AgCl) is white. Silver bromide (AgBr) is cream. Silver iodide (AgI) is yellow.
  • The initial addition of acid in the sulfate and halide tests is crucial to remove any carbonate ions that would interfere by also forming a precipitate.

Formulae

2H^+(aq) + CO32-(aq) → H2O(l) + CO2(g)

Represents the reaction of a carbonate with acid, producing carbon dioxide gas.

Ba2+(aq) + SO42-(aq) → BaSO4(s)

The ionic equation for the formation of the white precipitate in the test for sulfate ions.

Ag^+(aq) + X^-(aq) → AgX(s)

The general ionic equation for the test for halide ions (where X can be Cl, Br, or I).

Definitions

Anion
An ion that has gained one or more electrons, giving it an overall negative charge.
Precipitate
An insoluble solid that emerges from a liquid solution as the result of a chemical reaction.
Effervescence
The formation of gas bubbles in a liquid during a reaction, often seen as fizzing.

Worked example

An unknown aqueous solution is divided into two samples. To the first sample, dilute nitric acid is added, followed by silver nitrate solution, resulting in a white precipitate. To the second sample, dilute hydrochloric acid is added, and no change is observed. Which of these substances was dissolved in the original solution? A) Potassium Sulfate B) Sodium Carbonate C) Calcium Chloride D) Silver Bromide

  1. 1

    Step 1:

    Analyse the first test.

    The addition of acidified silver nitrate produces a white precipitate.

    This is the characteristic test for a chloride ion (Cl-), which forms insoluble white silver chloride (AgCl).

  2. 2

    Step 2:

    Analyse the second test.

    The addition of dilute hydrochloric acid produced no change.

    This indicates the absence of ions that would react, specifically carbonate ions (which would fizz).

  3. 3

    Step 3:

    Evaluate the options against the observations.

  4. 4

    A) Potassium Sulfate (K2SO4) contains sulfate ions, not chloride ions.

  5. 5

    B) Sodium Carbonate (Na2CO3) contains carbonate ions.

    This would have fizzed upon the addition of acid in both tests.

  6. 6

    C) Calcium Chloride (CaCl2) contains chloride ions, which would form a white precipitate with silver nitrate.

    It does not contain carbonate ions, so it would not fizz with acid.

    This matches both observations.

  7. 7

    D) Silver Bromide (AgBr) is a cream-coloured insoluble solid and would not form an aqueous solution to be tested.

Answer: C) Calcium Chloride

Common mistakes

  • ×Forgetting to add the initial acid. Nitric acid is added before silver nitrate to prevent false positives from carbonate ions. Likewise, hydrochloric acid is added before barium chloride for the same reason. Omitting this step is a common mistake.
  • ×Confusing the precipitate colours for the halides. The colours become progressively deeper down the group: AgCl is white, AgBr is cream, and AgI is yellow.
  • ×Mixing up the reagents and the ions they test for. Remember: Silver nitrate tests for Halides; Barium chloride tests for Sulfates.

No-calculator tips

  • ✓Use a mnemonic for the halide precipitate colours relative to their position in the periodic table. As you go down Group 17 (Cl → Br → I), the precipitate colour darkens (White → Cream → Yellow).
  • ✓Logically link the reagents to the products. The test for a SUlfate uses Barium ions to make Barium SUlfate (BaSO4). The test for a HAlide uses Silver ions to make a Silver HAlide (AgX).
  • ✓Always remember the 'acid first' rule for the sulfate and halide tests. Think of it as a cleaning step to remove interfering carbonate ions before performing the main test.

Read this topic in the official UAT-UK ESAT guide →

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