6.3 Preparation, Separation, and Purification of Soluble Salts

Introduction

Soluble salts can be prepared in the laboratory by reacting an acid with a suitable base, metal, or carbonate. The method chosen depends on the nature of the reactants involved — specifically, whether the base or carbonate is soluble or insoluble, and whether the metal is reactive enough to react with the acid.

Once the salt is formed, it must be separated from the reaction mixture and purified to obtain pure crystals.

This section introduces the four common methods for preparing soluble salts:

• Reaction of acids with alkali (titration)
• Excess metal
• Excess insoluble base
• Excess insoluble carbonate

Definition

Preparation of Soluble Salts: The process of making a soluble salt by reacting an acid with a suitable base, metal, or carbonate, followed by separation and purification to obtain pure crystals.

In simple words: Making a soluble salt involves reacting an acid with something (a base, metal, or carbonate), then separating and purifying the salt.

Main Idea

• Soluble salts can be prepared through various methods, depending on the nature of the reactants involved.
• Four common methods are:
• After the reaction, the salt must be separated from the mixture and purified by crystallization.

Core Concept

Key point: The method chosen depends on whether the base or carbonate is soluble or insoluble, and whether the metal is reactive enough.

Detailed Explanation

Why Different Methods Are Needed

The method used to prepare a soluble salt depends on:

• Solubility of the base or carbonate:
• If the base or carbonate is soluble, it cannot be added in excess because it will not be easily removed by filtration.
• If the base or carbonate is insoluble, it can be added in excess and then filtered off.
• Reactivity of the metal:
• Only metals that are reactive enough will react with dilute acids to form salts and hydrogen gas.
• Metals like copper, silver, and gold do not react with dilute acids.

The Four Methods

1. Preparation by Titration (Acid and Alkali)

When to use: When the base (alkali) is soluble.

Example: Preparation of sodium chloride (NaCl) from hydrochloric acid (HCl) and sodium hydroxide (NaOH).

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

Key steps:

• Measure a known volume of acid using a pipette and transfer it to a conical flask.
• Add a few drops of indicator (e.g., phenolphthalein).
• Fill a burette with the alkali solution.
• Slowly add the alkali to the acid until the indicator changes colour (end point).
• Note the volume of alkali used.
• Repeat the experiment without indicator to obtain the pure salt solution.
• Evaporate the water gently to obtain a hot saturated solution.
• Allow the solution to cool and crystallize.
• Filter to collect the crystals.

Key point: Titration is used when both the acid and the base are soluble. An indicator is needed to find the end point.

2. Preparation with Excess Metal

When to use: When the metal is reactive enough to react with the acid.

Example: Preparation of zinc sulphate (ZnSO₄) from sulphuric acid (H₂SO₄) and zinc (Zn).

Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H_2(g)

Key steps:

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

Key point: The metal is added in excess and then filtered off. This method works only for metals that react with the acid.

3. Preparation with Excess Insoluble Base

When to use: When the base is insoluble in water.

Example: Preparation of copper(II) sulphate (CuSO₄) from sulphuric acid (H₂SO₄) and copper(II) oxide (CuO).

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

Key steps:

• Measure a known volume of acid into a beaker.
• Add the insoluble base in excess (more than needed).
• Stir the mixture until the reaction stops.
• Filter the mixture to remove the unreacted base.
• Evaporate the filtrate to obtain a hot saturated solution.
• Allow the solution to cool and crystallize.
• Filter to collect the crystals.

Key point: The insoluble base is added in excess and then filtered off. This method works only for insoluble bases.

4. Preparation with Excess Insoluble Carbonate

When to use: When the carbonate is insoluble in water.

Example: Preparation of calcium chloride (CaCl₂) from hydrochloric acid (HCl) and calcium carbonate (CaCO₃).

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

Key steps:

• Measure a known volume of acid into a beaker.
• Add the insoluble carbonate in excess (more than needed).
• 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 the solution to cool and crystallize.
• Filter to collect the crystals.

Key point: The insoluble carbonate is added in excess and then filtered off. This method works only for insoluble carbonates.

Summary Table

Intermediate Level

Why different methods are needed:

• If the base is soluble, it cannot be added in excess because it will not be easily removed by filtration. So titration is used.
• If the base or carbonate is insoluble, it can be added in excess and then filtered off.
• If the metal is reactive enough, it can be added in excess and then filtered off.

Steps for separation and purification:

• Filtration — to remove excess solid (metal, base, or carbonate).
• Evaporation — to obtain a hot saturated solution.
• Crystallization — to form pure crystals of the salt.
• Filtration — to collect the crystals.

Key point: The method chosen depends on the solubility of the base or carbonate and the reactivity of the metal.

Important Points

• - Soluble salts can be prepared through four common methods:
• 1. Titration — acid + soluble base (alkali)
• 2. Excess Metal — acid + metal
• 3. Excess Insoluble Base — acid + insoluble base
• 4. Excess Insoluble Carbonate — acid + insoluble carbonate
• - The method chosen depends on the nature of the reactants.
• - Titration is used when the base is soluble.
• - Excess metal is used when the metal is reactive enough.
• - Excess insoluble base is used when the base is insoluble.
• - Excess insoluble carbonate is used when the carbonate is insoluble.
• - After the reaction, the salt is separated by filtration and purified by crystallization.

Remember

• - Four methods for preparing soluble salts:
• 1. Titration (acid + soluble base)
• 2. Excess metal (acid + metal)
• 3. Excess insoluble base (acid + insoluble base)
• 4. Excess insoluble carbonate (acid + insoluble carbonate)
• - Method depends on solubility of base/carbonate and reactivity of metal.
• - Excess solid is removed by filtration.
• - Salt is purified by crystallization.
• - Titration uses an indicator.

Common Mistakes

Quick Revision

Preparation, Separation, and Purification of Soluble Salts:

Soluble salts can be prepared through various methods, depending on the nature of the reactants involved. Here, we'll describe four common methods: reaction of acids with alkali (titration), excess metal, excess insoluble base, and excess insoluble carbonate.

Four Methods:

• Used when the base (alkali) is soluble.
• Example: NaCl from HCl and NaOH.
• Indicator used to find end point.

• Used when the metal is reactive enough.
• Example: ZnSO₄ from H₂SO₄ and Zn.
• Metal added in excess; filtered off.

• Used when the base is insoluble.
• Example: CuSO₄ from H₂SO₄ and CuO.
• Base added in excess; filtered off.

• Used when the carbonate is insoluble.
• Example: CaCl₂ from HCl and CaCO₃.
• Carbonate added in excess; filtered off.

Steps for Separation and Purification:

• Filtration — to remove excess solid.
• Evaporation — to obtain a hot saturated solution.
• Crystallization — to form pure crystals of the salt.
• Filtration — to collect the crystals.

60-Second Revision

• Four methods for preparing soluble salts:
• Method depends on solubility of base/carbonate and reactivity of metal.
• Excess solid removed by filtration.
• Salt purified by crystallization.
• Titration uses an indicator.
• Soluble base → titration.
• Insoluble base/carbonate → excess method.
• Reactive metal → excess method.
• ⚠️ Warning: soluble bases cannot be added in excess — titration must be used instead.