Metal hydroxides and identifying negative ions C12

What solution can be used to identify some metal ions (cations)?

Sodium hydroxide solution

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How do we distinguish aluminium ions using sodium hydroxide solution?

Solutions of aluminium, calcium and magnesium ions form white precipitates when sodium hydroxide solution is added but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.

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How do we distinguish calcium and magnesium ions using sodium hydroxide solution?

Calcium and magnesium ions form white precipitates when sodium hydroxide solution is added. Then use flame tests

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What are the reactions for aluminium, calcium and magnesium ions with hydroxide solution?

Ca²⁺ (aq) + 2OH⁻ (aq) → Ca(OH)₂ (s)
Mg²⁺ (aq) + 2OH⁻ (aq) → Mg(OH)₂ (s)
Al³⁺ (aq) + 3OH⁻ (aq) →Al(OH)₃ (s)

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What happens to solutions of copper(II), iron(II) and iron(III) ions when sodium hydroxide solution is added?

They form coloured precipitates

copper(II) = blue
iron (II) = green
iron (III) = brown

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What are the reactions for copper(II), iron(II) and iron(III) ions with sodium hydroxide solution?

CuCl₂ (aq)+ 2NaOH (aq) → Cu(OH)₂ (s) + 2 NaCl
FeI₂ (aq)+ 2NaOH (aq) → Fe(OH)₂ (s) + 2 NaI
FeBr₃ (aq) + 3NaOH (aq) → Fe(OH)₃ (s) + 3NaBr

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Test for the carbonate ion

Add dilute acid with a carbonate to make carbon dioxide gas. There will be fizzing. Bubble the gas with lime water and if the lime water turns cloudy, carbon dioxide is present (which means we started with a carbonate ion)

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Equation for carbonate test

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

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Test for the halide ion

Add dilute nitric acid and dilute silver nitrate solution. Halide ions produce precipitates with silver nitrate solution.
Chloride ions - white precipitate of silver chloride
Bromide ions - cream precipitate of silver bromide
Iodide ions - yellow precipit

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Equations for halide ions tests

Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s)
Ag⁺ (aq) + Br ⁻ (aq) → AgBr (s)
Ag⁺ (aq) +I ⁻ (aq) → AgI(s)

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Test for sulfate ions

Add dilute hydrochloric acid to the samples and add barium chloride solution If sulphide ions are present, a white precipitate will be produced

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Equation for sulfate ions test

Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)

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Compound A produced a lilac flame test. A sample of compound A was reacted with silver nitrate solution and dilute nitric acid. A yellow precipitate was formed. Identify compound A

Lilac flame test - contains potassium ion
Yellow precipitate - contains iodide ion

Compound A is potassium iodide

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Metal hydroxides and identifying negative ions C12

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