6.3.3 Preparation with Excess Insoluble Base

Introduction

Some bases are insoluble in water — for example, copper(II) oxide (CuO), magnesium oxide (MgO), and iron(II) oxide (FeO). Because they are insoluble, they can be added in excess and then easily removed by filtration.

This method is called the excess insoluble base method. It is used when the base is insoluble in water.

This section explains how copper(II) sulphate (CuSO₄) is prepared from sulphuric acid (H₂SO₄) and copper(II) oxide (CuO) using the excess insoluble base method.

Definition

Preparation with Excess Insoluble Base: A method of preparing a soluble salt by reacting an acid with an insoluble base, where the base is added in excess and then filtered off after the reaction is complete.

In simple words: Add more insoluble base than needed, let it react with the acid, then filter off the leftover base to get a pure salt solution.

Main Idea

• This method is used when the base is insoluble in water.
• The insoluble base is added in excess (more than needed).
• After the reaction stops, the unreacted base is filtered off.
• The salt solution is then evaporated and crystallized to obtain pure salt crystals.

Core Concept

Key point: The insoluble base is added in excess and then filtered off, leaving a pure salt solution.

Detailed Explanation

Activity: Preparation of Copper(II) Sulphate

Reaction:

CuO_(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O_(l)

Materials required:

• Copper(II) oxide (CuO) — insoluble base
• Dilute sulphuric acid (H₂SO₄)
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure

• Measure a known volume of dilute sulphuric acid into a beaker.
• Add excess copper(II) oxide to the dilute sulphuric acid in the beaker.
• The reaction produces copper(II) sulphate and water. You will observe the blue colour of copper(II) sulphate forming in the solution.
• Continue adding CuO until no more CuO dissolves (some black CuO remains at the bottom), indicating all the acid has reacted.
• The CuO is added in excess to ensure that all the acid is used up.

Separation

Filter the mixture to remove the unreacted copper(II) oxide.

• The filtrate contains the copper(II) sulphate solution (blue colour).
• The residue is the unreacted copper(II) oxide (black solid).

Crystallization

• Evaporate the filtrate to obtain a hot saturated solution.
• Allow the solution to cool and crystallize.
• Filter to collect the crystals.

Summary Table

Intermediate Level

Why excess insoluble base is used:

• Adding the base in excess ensures that all the acid is used up.
• This means the final solution contains only the salt and water — no leftover acid.
• The unreacted base is then filtered off, leaving a pure salt solution.

Steps:

• Measure a known volume of acid into a beaker.
• Add the insoluble base in excess.
• Wait until the reaction stops (no more base dissolves).
• Filter the mixture to remove the unreacted base.
• Evaporate the filtrate to obtain a hot saturated solution.
• Allow to cool and crystallize.
• Filter to collect the crystals.

Key point: The excess insoluble base method is used when the base is insoluble in water. The base is added in excess and then filtered off.

Important Points

• - Excess insoluble base method is used when the base is insoluble in water.
• - The base is added in excess (more than needed).
• - The base reacts with the acid to form salt and water.
• - When the reaction stops (no more base dissolves), the unreacted base is filtered off.
• - The salt solution is then evaporated and crystallized to obtain pure salt crystals.
• - Example: Preparation of copper(II) sulphate from sulphuric acid and copper(II) oxide.
• - Reaction: CuO_(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O_(l)
• - Materials: Copper(II) oxide, dilute sulphuric acid, beaker, glass rod, filter funnel, filter paper, evaporating dish, tripod stand, wire gauze, heating source.

Remember

• - Excess insoluble base method used when base is insoluble.
• - Base added in excess.
• - Reaction stops when acid is used up.
• - Unreacted base filtered off.
• - Salt solution evaporated and crystallized.
• - Example: CuSO₄ from H₂SO₄ and CuO.
• - Reaction: CuO + H₂SO₄ → CuSO₄ + H₂O.
• - Materials: CuO, dilute sulphuric acid, beaker, filter funnel, evaporating dish.
• - Blue colour indicates copper(II) sulphate formation.

Common Mistakes

Quick Revision

Preparation with Excess Insoluble Base:

Preparation of Copper(II) Sulphate:

Reaction:

• CuO_(s) + H₂SO_4(aq) → CuSO_4(aq) + H₂O_(l)

Materials required:

• Copper(II) oxide (CuO) — insoluble base
• Dilute sulphuric acid (H₂SO₄)
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure:

• Measure a known volume of dilute sulphuric acid into a beaker.
• Add excess copper(II) oxide to the dilute sulphuric acid in the beaker. The reaction produces copper(II) sulphate and water. You will observe the blue colour of copper(II) sulphate forming in the solution. Continue adding CuO until no more CuO dissolves (some black CuO remains at the bottom), indicating all the acid has reacted.
• The CuO is added in excess to ensure that all the acid is used up.

• Separation:
• Filter the mixture to remove unreacted copper(II) oxide.

Crystallization:

• Evaporate the filtrate to obtain a hot saturated solution.
• Allow the solution to cool and crystallize, then filter to collect the crystals.

60-Second Revision

• Excess insoluble base method used when base is insoluble.
• Base added in excess.
• Reaction stops when acid is used up.
• Unreacted base filtered off.
• Salt solution evaporated and crystallized.
• Example: CuSO₄ from H₂SO₄ and CuO.
• Reaction: CuO + H₂SO₄ → CuSO₄ + H₂O.
• Materials: CuO, dilute sulphuric acid, beaker, filter funnel, evaporating dish.
• Blue colour indicates copper(II) sulphate formation.
• No more base dissolves = reaction complete.
• ⚠️ Warning: the base is added in excess to ensure all the acid is used up — the unreacted base is then filtered off.