Welcome to Preparation of Salts!

Hello future chemists! This chapter is like learning a few essential recipes. Salts are incredibly important in chemistry and everyday life—they are not just the table salt (sodium chloride) we eat! Understanding how to make them accurately in the lab is a fundamental skill.

Don't worry if this seems tricky at first; we will break down the methods step-by-step. The key is knowing one thing first: Is the salt soluble (dissolves in water) or insoluble (does not dissolve in water)?


The Critical First Step: Solubility Rules

Before we can prepare a salt, we must know if it will dissolve in water. This determines which preparation method we must use.

Quick Review of Essential Solubility Rules

Memorizing these general rules helps you decide your method instantly:

  • ALWAYS Soluble: All salts containing Sodium (Na), Potassium (K), or Ammonium (\(\text{NH}_4\)) ions. (The "S.P.A." salts – Sodium, Potassium, Ammonium).
  • ALWAYS Soluble: All Nitrates (\(\text{NO}_3\)).
  • Mostly Soluble: Most Chlorides, Bromides, and Iodides (Halides), except those of Silver (\(\text{Ag}^+\)) and Lead (\(\text{Pb}^{2+}\)).
  • Mostly Soluble: Most Sulfates (\(\text{SO}_4^{2-}\)), except those of Barium (\(\text{Ba}^{2+}\)), Calcium (\(\text{Ca}^{2+}\)), and Lead (\(\text{Pb}^{2+}\)).
  • Mostly Insoluble: Most Carbonates (\(\text{CO}_3^{2-}\)) and Hydroxides (\(\text{OH}^-\)), except those containing S.P.A. (Sodium, Potassium, Ammonium).
🔑 Key Takeaway: Soluble vs. Insoluble

If the salt is Soluble, we use Method 1 (Reaction of Acid with Excess Solid, followed by Crystallisation).
If the salt is Insoluble, we use Method 2 (Precipitation).


Method 1: Preparation of Soluble Salts (Using Excess Reactant)

This method is used when the desired salt is soluble. Since we are dealing with acids, we must ensure all the acid is used up to produce a pure, neutral salt solution. We do this by adding an excess of an insoluble solid.

Who are the Reactants?

We react a suitable acid with one of the following insoluble solids:

  1. Metal: (If reactive enough, e.g., Zinc or Magnesium).
    Example: Acid + Zinc Metal \(\rightarrow\) Zinc Salt + Hydrogen gas.
  2. Insoluble Base: (Metal Oxide or Hydroxide).
    Example: Acid + Copper(II) Oxide \(\rightarrow\) Copper Salt + Water.
  3. Insoluble Carbonate:
    Example: Acid + Calcium Carbonate \(\rightarrow\) Calcium Salt + Water + Carbon Dioxide gas.
🛑 Common Mistake to Avoid:

We do NOT use a soluble base (alkali) like Sodium Hydroxide in this method, because it would be impossible to filter out the excess base later! We use an insoluble reactant so we can easily separate the excess.

Step-by-Step Procedure (The "Excess Solid" Method)

Step 1: Reaction and Excess Addition

Add the insoluble solid (e.g., copper oxide powder) to the warm acid (e.g., sulfuric acid) slowly while stirring. Continue adding the solid until no further reaction occurs (no more fizzing, or the solid stops disappearing) and some solid remains at the bottom. This confirms the acid is completely used up (neutralised).

Analogy: Think of making strong tea. You add an excess of tea leaves (the solid reactant) to ensure all the water (the acid) is fully flavoured.

Step 2: Filtration

Filter the mixture to remove the excess, unreacted insoluble solid. The substance left in the filter paper is the excess solid. The liquid passing through the filter paper (the filtrate) is a pure, neutral solution of the desired salt.

Step 3: Evaporation (Removing Water)

Heat the filtrate gently (often in an evaporating basin) to evaporate most of the water, but not all of it. Stop heating when small crystals start to form around the edge of the solution. This means the solution is now saturated.

Step 4: Cooling and Drying (Crystallisation)

Allow the hot, saturated solution to cool slowly, usually overnight, in a cool place. As it cools, the salt becomes less soluble and crystals form.

Once crystals have formed, separate them from the remaining liquid (mother liquor) by decanting or filtration, and then dry them gently, perhaps between pieces of filter paper or in a low-temperature oven.

✅ Quick Review: Soluble Salts (4 Steps)
  1. Add Excess Insoluble Solid to Acid.
  2. Filter to remove excess solid.
  3. Evaporate (to saturation).
  4. Cool to crystallise.

Method 2: Preparation of Insoluble Salts (Precipitation)

This method is used when the desired salt is insoluble (i.e., it does not dissolve in water).

Since the salt product won't dissolve, we can make it by mixing two solutions that are soluble. When they meet, the ions rearrange and form an insoluble product that immediately "falls out" of the solution as a solid. This solid is called a precipitate.

The Double Decomposition Reaction

To make an insoluble salt, you must choose two soluble compounds that contain the necessary ions.

Example: To make Barium Sulfate (\(\text{BaSO}_4\)), which is insoluble:

  • You need Barium ions (\(\text{Ba}^{2+}\)). Use a soluble Barium salt, like Barium Chloride (\(\text{BaCl}_2\)).
  • You need Sulfate ions (\(\text{SO}_4^{2-}\)). Use a soluble Sulfate, like Sodium Sulfate (\(\text{Na}_2\text{SO}_4\)).

When you mix the two soluble solutions, the ions swap partners (double decomposition):

\[\text{Barium Chloride} + \text{Sodium Sulfate} \rightarrow \text{Barium Sulfate (solid)} + \text{Sodium Chloride}\]

The insoluble Barium Sulfate forms the precipitate.

Step-by-Step Procedure (The Precipitation Method)

Step 1: Mixing

Mix the two solutions (the two soluble reactants) together in a beaker. A cloudy solid (the precipitate) will immediately form.

Analogy: This is like adding milk to coffee. The soluble ingredients mix, but instantly a cloudy, solid-like material (the precipitate) appears.

Step 2: Filtration

Filter the mixture. The precipitate (the insoluble salt you want) will be caught on the filter paper. The liquid (filtrate) contains the other, soluble salt (the waste product) and any excess reactants.

Step 3: Washing

While the precipitate is still in the filter funnel, wash it several times with distilled water. This removes any traces of the unwanted soluble salt or residual soluble reactants trapped in the filter paper. This step is vital for purity.

Step 4: Drying

Carefully remove the filter paper and dry the pure insoluble salt, usually in a drying oven or by leaving it in a warm, dry place.

The Ionic Equation for Precipitation

In precipitation reactions, we often only write the ions that are actively forming the precipitate. These are the spectator ions are left out.

For the example above (Barium Sulfate formation): \[\text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{BaSO}_4(\text{s})\]


Did you know? Precipitation reactions are used in water treatment to remove harmful ions, making the water safer to drink!

✅ Quick Review: Insoluble Salts (4 Steps)
  1. Mix two soluble solutions (to cause precipitation).
  2. Filter to collect the precipitate.
  3. Wash the precipitate with distilled water (for purity).
  4. Dry the pure salt.

Summary Table of Salt Preparation Methods

To summarize everything we've covered, the method you choose depends entirely on the solubility of the final product and the solubility of your starting materials.

Salt Type Reactants Used Key Process Steps Purpose
Soluble Salts
(Most common lab prep)		 Acid + Excess Insoluble Solid (Metal, Base, or Carbonate) Reaction, Filtration, Evaporation, Crystallisation. Ensure all acid is neutralised and excess solid is filtered out.
Insoluble Salts
(Precipitates)		 Two Soluble Solutions containing necessary ions. Mixing, Filtration, Washing, Drying. Form the salt instantaneously as a solid, then separate the solid from the liquid.

You’ve done a great job mastering these essential laboratory techniques! Remember, practice identifying solubility first, and the rest is just following the recipe!