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Acids, Bases and Salts - Preparation of soluble and insoluble salts

Grade 11IGCSEChemistry

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

🔑Concepts

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A salt is a compound formed when the H+H^+ ion of an acid is replaced by a metal ion or an ammonium (NH4+NH_4^+) ion.

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Solubility Rules: All nitrates (NO3−NO_3^-) and Group 1 salts are soluble. Most sulfates (SO42−SO_4^{2-}) are soluble except BaSO4BaSO_4, PbSO4PbSO_4, and CaSO4CaSO_4. Most chlorides (Cl−Cl^-) are soluble except AgClAgCl and PbCl2PbCl_2. Carbonates (CO32−CO_3^{2-}) are generally insoluble except for Group 1 and NH4+NH_4^+.

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Preparation of Soluble Salts (Method 1 - Excess Solid): Used for reacting an acid with an insoluble metal, base, or carbonate. The solid is added in excess to ensure all acid is neutralized, then filtered.

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Preparation of Soluble Salts (Method 2 - Titration): Used when both reactants are soluble (e.g., acid + alkali). An indicator is used to find the exact end-point, and the process is repeated without the indicator to obtain a pure salt solution.

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Preparation of Insoluble Salts (Precipitation): Two soluble salt solutions are mixed together to form an insoluble precipitate. The precipitate is filtered, washed with distilled water to remove impurities, and dried.

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Crystallization: The process of heating a salt solution until the 'point of crystallization' is reached, then cooling it slowly to form crystals.

📐Formulae

Acid+Metal→Salt+H2(g)Acid + Metal \rightarrow Salt + H_2(g)

Acid+Base→Salt+H2O(l)Acid + Base \rightarrow Salt + H_2O(l)

Acid+Carbonate→Salt+H2O(l)+CO2(g)Acid + Carbonate \rightarrow Salt + H_2O(l) + CO_2(g)

AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)

H+(aq)+OH−(aq)→H2O(l)H^+(aq) + OH^-(aq) \rightarrow H_2O(l)

💡Examples

Problem 1:

Describe the preparation of pure, dry crystals of copper(II) sulfate (CuSO4⋅5H2OCuSO_4 \cdot 5H_2O) starting from copper(II) oxide (CuOCuO) and sulfuric acid (H2SO4H_2SO_4).

Solution:

  1. Add excess CuOCuO to warm H2SO4H_2SO_4 while stirring until no more dissolves. 2. Filter the mixture to remove the unreacted CuOCuO. 3. Heat the filtrate (CuSO4CuSO_4 solution) in an evaporating dish until the point of crystallization. 4. Allow to cool so crystals form. 5. Filter crystals and dry them between filter papers.

Explanation:

This is the 'Excess Solid' method because CuOCuO is an insoluble base. The reaction is: CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)

Problem 2:

Identify a suitable pair of reagents to prepare the insoluble salt barium sulfate (BaSO4BaSO_4) and write the ionic equation.

Solution:

Reagents: Barium nitrate (Ba(NO3)2Ba(NO_3)_2) and Sodium sulfate (Na2SO4Na_2SO_4). Ionic equation: Ba2+(aq)+SO42−(aq)→BaSO4(s)Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)

Explanation:

To prepare an insoluble salt, you must react two soluble salts. All nitrates and all Group 1 salts are soluble, making Ba(NO3)2Ba(NO_3)_2 and Na2SO4Na_2SO_4 ideal choices for the precipitation reaction.

Preparation of soluble and insoluble salts Revision - Grade 11 Chemistry IGCSE