6.3.4 Preparation with Excess Insoluble Carbonate

Introduction

Some carbonates are insoluble in water — for example, calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), and copper(II) carbonate (CuCO₃). Because they are insoluble, they can be added in excess and then easily removed by filtration.

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

This section explains how calcium chloride (CaCl₂) is prepared from hydrochloric acid (HCl) and calcium carbonate (CaCO₃) using the excess insoluble carbonate method.

Definition

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

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

Main Idea

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

Core Concept

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

Detailed Explanation

Activity: Preparation of Calcium Chloride

Reaction:

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + CO_2(g) + H₂O_(l)

Materials required:

• Calcium carbonate (limestone) — insoluble carbonate
• Dilute hydrochloric acid (HCl)
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure

• Measure 25\ cm³ dilute hydrochloric acid into a beaker.
• Add excess calcium carbonate to dilute hydrochloric acid in a beaker.
• The reaction produces calcium chloride, carbon dioxide, and water.
• You will observe carbon dioxide has evolved with effervescence (bubbles).
• Continue adding CaCO₃ until no more effervescence (bubbles) is observed, indicating all the acid has reacted.
• The CaCO₃ is added in excess to ensure that all the acid is used up.

Separation

Filter the mixture to remove the unreacted calcium carbonate.

• The filtrate contains the calcium chloride solution.
• The residue is the unreacted calcium carbonate.

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 carbonate is used:

• Adding the carbonate 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 carbonate is then filtered off, leaving a pure salt solution.

Steps:

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

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

Important Points

• - Excess insoluble carbonate method is used when the carbonate is insoluble in water.
• - The carbonate is added in excess (more than needed).
• - The carbonate reacts with the acid to form salt, carbon dioxide, and water.
• - When the reaction stops (no more effervescence), the unreacted carbonate is filtered off.
• - The salt solution is then evaporated and crystallized to obtain pure salt crystals.
• - Example: Preparation of calcium chloride from hydrochloric acid and calcium carbonate.
• - Reaction: CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + CO_2(g) + H₂O_(l)
• - Materials: Calcium carbonate, dilute hydrochloric acid, beaker, glass rod, filter funnel, filter paper, evaporating dish, tripod stand, wire gauze, heating source.

Remember

• - Excess insoluble carbonate method used when carbonate is insoluble.
• - Carbonate added in excess.
• - Reaction stops when acid is used up.
• - Unreacted carbonate filtered off.
• - Salt solution evaporated and crystallized.
• - Example: CaCl₂ from HCl and CaCO₃.
• - Reaction: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.
• - Materials: CaCO₃, dilute hydrochloric acid, beaker, filter funnel, evaporating dish.
• - Effervescence indicates reaction.
• - No more effervescence = reaction complete.

Common Mistakes

Quick Revision

Preparation with Excess Insoluble Carbonate:

Activity: Preparation of Calcium Chloride

Reaction:

• CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + CO_2(g) + H₂O_(l)

Materials required:

• Calcium carbonate (limestone) — insoluble carbonate
• Dilute hydrochloric acid (HCl)
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure:

• Measure 25\ cm³ dilute hydrochloric acid into a beaker.
• Add excess calcium carbonate to dilute hydrochloric acid in a beaker. The reaction produces calcium chloride, carbon dioxide, and water. You will observe carbon dioxide has evolved with effervescence. Continue adding CaCO₃ until no more effervescence (bubbles) is observed, indicating all the acid has reacted.
• The CaCO₃ is added in excess to ensure that all the acid is used up.

• Separation:
• Filter the mixture to remove unreacted CaCO₃.

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 carbonate method used when carbonate is insoluble.
• Carbonate added in excess.
• Reaction stops when acid is used up.
• Unreacted carbonate filtered off.
• Salt solution evaporated and crystallized.
• Example: CaCl₂ from HCl and CaCO₃.
• Reaction: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.
• Materials: CaCO₃, dilute hydrochloric acid, beaker, filter funnel, evaporating dish.
• Effervescence indicates reaction.
• No more effervescence = reaction complete.
• ⚠️ Warning: the carbonate is added in excess to ensure all the acid is used up — the unreacted carbonate is then filtered off.