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Chemistry - Identification of Ions and Gases

Grade 9IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Flame Tests: Used to identify metal cations by the color they emit in a Bunsen flame. Li+Li^{+} (Red), Na+Na^{+} (Yellow), K+K^{+} (Lilac), Ca2+Ca^{2+} (Orange-red), Cu2+Cu^{2+} (Blue-green).

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Identification of Aqueous Cations: Using aqueous Sodium Hydroxide (NaOHNaOH) and aqueous Ammonia (NH3NH_{3}). For example, Cu2+Cu^{2+} forms a light blue precipitate, while Fe2+Fe^{2+} forms a green precipitate and Fe3+Fe^{3+} forms a red-brown precipitate.

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Distinguishing Al3+Al^{3+}, Zn2+Zn^{2+}, and Ca2+Ca^{2+}: All form white precipitates with NaOHNaOH. However, Ca2+Ca^{2+} precipitate does not dissolve in excess NaOHNaOH, while Al3+Al^{3+} and Zn2+Zn^{2+} do. Zn2+Zn^{2+} is the only one that dissolves in excess NH3(aq)NH_{3}(aq).

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Anion Tests - Halides: To test for Cl−Cl^{-}, Br−Br^{-}, or I−I^{-}, add dilute Nitric Acid (HNO3HNO_{3}) followed by Silver Nitrate (AgNO3AgNO_{3}). Cl−Cl^{-} gives a white precipitate, Br−Br^{-} gives cream, and I−I^{-} gives yellow.

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Anion Tests - Carbonates and Sulfates: Carbonates (CO32−CO_{3}^{2-}) react with dilute acids to produce CO2CO_{2}. Sulfates (SO42−SO_{4}^{2-}) are tested using Barium Nitrate (Ba(NO3)2Ba(NO_{3})_{2}) under acidic conditions to form a white precipitate (BaSO4BaSO_{4}).

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Gas Tests: H2H_{2} (squeaky pop with lighted splint), O2O_{2} (relights glowing splint), CO2CO_{2} (turns limewater milky), Cl2Cl_{2} (bleaches damp litmus paper), and NH3NH_{3} (turns damp red litmus paper blue).

📐Formulae

Cu2+(aq)+2OH−(aq)→Cu(OH)2(s)Cu^{2+}(aq) + 2OH^{-}(aq) \rightarrow Cu(OH)_{2}(s)

Fe3+(aq)+3OH−(aq)→Fe(OH)3(s)Fe^{3+}(aq) + 3OH^{-}(aq) \rightarrow Fe(OH)_{3}(s)

Ag+(aq)+Cl−(aq)→AgCl(s)Ag^{+}(aq) + Cl^{-}(aq) \rightarrow AgCl(s)

Ba2+(aq)+SO42−(aq)→BaSO4(s)Ba^{2+}(aq) + SO_{4}^{2-}(aq) \rightarrow BaSO_{4}(s)

CO32−(s)+2H+(aq)→CO2(g)+H2O(l)CO_{3}^{2-}(s) + 2H^{+}(aq) \rightarrow CO_{2}(g) + H_{2}O(l)

💡Examples

Problem 1:

A colorless solution XX produces a white precipitate when aqueous NaOHNaOH is added. This precipitate dissolves when excess NaOHNaOH is added. When aqueous NH3NH_{3} is added to a fresh sample of XX, a white precipitate forms but does not dissolve in excess. Identify the cation in XX.

Solution:

The cation is Aluminium (Al3+Al^{3+}).

Explanation:

Both Al3+Al^{3+} and Zn2+Zn^{2+} form white precipitates that dissolve in excess NaOHNaOH. However, only Zn2+Zn^{2+} dissolves in excess NH3NH_{3}. Since the precipitate from solution XX did not dissolve in excess NH3NH_{3}, it must be Al3+Al^{3+}.

Problem 2:

Identify an unknown gas that turns damp red litmus paper blue.

Solution:

NH3NH_{3} (Ammonia)

Explanation:

Ammonia is the only common alkaline gas. It reacts with the water on the damp litmus paper to form hydroxide ions (OH−OH^{-}), which turn the red litmus indicator blue.

Problem 3:

A solid salt gives a lilac flame test and produces bubbles of gas when dilute HClHCl is added. The gas turns limewater milky. Name the salt.

Solution:

Potassium Carbonate (K2CO3K_{2}CO_{3})

Explanation:

The lilac flame indicates the presence of Potassium ions (K+K^{+}). The reaction with acid to produce CO2CO_{2} (which turns limewater milky) confirms the presence of Carbonate ions (CO32−CO_{3}^{2-}).