12.5 Identification of Ions and Gases

2026 Syllabus Objectives

  1. Core: Describe tests to identify the anions:

    • (a) carbonate, CO32−CO_3^{2-}, by reaction with dilute acid and then testing for carbon dioxide gas
    • (b) chloride, Cl−Cl^-, bromide, Br−Br^-, and iodide, I−I^-, by acidifying with dilute nitric acid then adding aqueous silver nitrate
    • (c) nitrate, NO3−NO_3^-, reduction with aluminium foil and aqueous sodium hydroxide and then testing for ammonia gas
    • (d) sulfate, SO42−SO_4^{2-}, by acidifying with dilute nitric acid and then adding aqueous barium nitrate
    • (e) sulfite, SO32−SO_3^{2-}, by reaction with acidified aqueous potassium manganate(VII)

  2. Core: Describe tests using aqueous sodium hydroxide and aqueous ammonia to identify the aqueous cations:

    • (a) aluminium, Al3+Al^{3+}
    • (b) ammonium, NH4+NH_4^+
    • (c) calcium, Ca2+Ca^{2+}
    • (d) chromium(III), Cr3+Cr^{3+}
    • (e) copper(II), Cu2+Cu^{2+}
    • (f) iron(II), Fe2+Fe^{2+}
    • (g) iron(III), Fe3+Fe^{3+}
    • (h) zinc, Zn2+Zn^{2+}

  3. Core: Describe tests to identify the gases:

    • (a) ammonia, NH3NH_3, using damp red litmus paper
    • (b) carbon dioxide, CO2CO_2, using limewater
    • (c) chlorine, Cl2Cl_2, using damp litmus paper
    • (d) hydrogen, H2H_2, using a lighted splint
    • (e) oxygen, O2O_2, using a glowing splint
    • (f) sulfur dioxide, SO2SO_2, using acidified aqueous potassium manganate(VII)

  4. Core: Describe the use of a flame test to identify the cations:

    • (a) lithium, Li+Li^+
    • (b) sodium, Na+Na^+
    • (c) potassium, K+K^+
    • (d) calcium, Ca2+Ca^{2+}
    • (e) barium, Ba2+Ba^{2+}
    • (f) copper(II), Cu2+Cu^{2+}

Chemical Analysis in Chemistry

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Chemists analyse samples of water, soil and air to detect the presence of different substances quickly and accurately, particularly those that may be harmful to life. Analytical tests are routinely used across all areas of Chemistry, including in the analysis of bottled water.

🔑 Key Concepts

Qualitative Analysis is the process used to determine the presence or absence of a substance in a given sample. The tests can help scientists to understand what chemicals are either present (positive test) or not present (negative test).

The results of these tests can be:

  • Qualitative - giving a yes/no answer
  • Quantitative - giving the actual amount of something present

Example: Silver nitrate is added to domestic drinking water as a test for chloride ions. This is a qualitative test and the formation of a white precipitate (a positive result) would mean chloride ions were present in the water.

Importance of Mineral Water Testing: Bottled mineral water is a mixture containing water and several soluble metal compounds, known as minerals. These minerals dissolve as the water passes through different types of rock. Minerals are made up of a cation (the metal ion) and an anion (a non-metal ion). Some minerals are beneficial to health, but care is also needed as in high concentration some can cause illness. Chemical tests are necessary to determine the quality and safety of mineral water in order to ensure potentially harmful or 'fake' drinking water does not reach the consumer.

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