📖 Lesson 4: Methods of Salt Preparation

👩🏽‍🏫 Introduction

Hey science explorer! 👋🏽

Ever wondered how the salt in your food, the medicine you take, or the fertilizer on a farm is made? 🧂💊🌾 Well, they’re all types of salts, and in this lesson, you’ll learn the different methods of preparing salts—some bubbling, some neutral, and some forming beautiful crystals!

By the end of this lesson, you’ll be able to describe, carry out (in a lab or on paper), and explain how various salts are prepared from acids, bases, metals, and even other salts.

📘 Core Concepts

🔹 What Are Salts?

Salts are chemical compounds formed when the hydrogen ion (H⁺) of an acid is partially or completely replaced by a metal or ammonium ion (NH₄⁺).

$$\text{Acid}+\text{Base}\to \text{Salt}+\text{Water}$$

But salt can also be prepared through other reactions. Let’s explore them all!

🔹 Methods of Preparing Salts

1. Neutralization (Acid + Base)

This is the most common method. It involves reacting an acid with a base to form a salt and water.

HCl+NaOH→NaCl+H₂O

Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

2. Acid + Metal

Some reactive metals (like zinc or magnesium) can react with acids to produce salt and hydrogen gas.

Zn+H₂SO₄→ZnSO₄+H₂

Example: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen gas

🧯 Safety Note: This produces a flammable gas—do in a well-ventilated lab or simulate in class.

3. Acid + Carbonate

Carbonates react with acids to form salt, water, and carbon dioxide gas.

CaCO₃+HCl→CaCl₂+H₂O+CO₂

Example: Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

🎈 The fizzing is from CO₂ gas!

4. Precipitation Reaction (Double Displacement)

This method involves mixing two soluble salts to produce an insoluble salt (precipitate).

AgNO₃+NaCl→AgCl (precipitate)+NaNO₃

Example: Silver nitrate + Sodium chloride → Silver chloride (white solid) + Sodium nitrate

⚗️ This is used in qualitative analysis and salt purification.

5. Direct Combination (Rare)

Some salts are formed by directly combining elements or compounds, especially gases.

NH₃+HCl→NH₄Cl

Example: Ammonia gas + Hydrogen chloride gas → Ammonium chloride

This method is often used in controlled industrial settings.

🔹 Crystallization (Getting Salt Crystals)

After preparing a salt in solution, we often need to crystallize it by:

1. Filtering to remove impurities or undissolved solids.

2. Evaporating some of the water to concentrate the solution.

3. Cooling the solution to form crystals.

4. Drying the crystals with filter paper.

🧪 Real-Life Example

🔹 Salt in Medicine: Antacids like magnesium hydroxide react with stomach acid to form magnesium salts, which relieve heartburn.

🔹 Fertilizers: Salts like ammonium nitrate and potassium chloride are prepared in factories using acid + base or acid + salt methods to support plant growth.

🧩 Sample Problem Walkthrough

🧠 Problem:
You are asked to prepare sodium chloride in the lab using hydrochloric acid and sodium hydroxide.

✅ Step-by-Step Solution:

• Step 1: Identify the method – this is neutralization.

• Step 2: Write balanced equation:

  HCl+NaOH→NaCl+H2O

• Step 3: Procedure:

  • Mix equal volumes of dilute HCl and NaOH in a beaker.

  • Test with litmus until it turns purple (neutral).

  • Heat slightly to concentrate, then leave to cool.

  • Crystals of NaCl will form.

• Step 4: Filter and dry the salt crystals.

✔️ Answer:
Sodium chloride is prepared by neutralizing hydrochloric acid with sodium hydroxide, then crystallizing the resulting solution.

✍🏽 Practice Exercises

1. Fill in the blanks:

(a) Acid + carbonate → ______ + ______ + ______
(b) In a precipitation reaction, the salt formed is ______ in water.

Answers:
(a) Salt, water, carbon dioxide
(b) Insoluble

2. Match the reaction to the salt formed:

Answers:

• H₂SO₄ + Zn → B. ZnSO₄

• HCl + NaOH → A. NaCl

• AgNO₃ + NaCl → C. AgCl

3. Short Answer:

Why must you evaporate a salt solution before crystals can form?

Sample Answer:
Evaporation reduces the water content, making the solution more concentrated so that the salt begins to form solid crystals when cooled.

🌀 Recap

Today you’ve learned:

• Salts can be made using neutralization, acid + metal, acid + carbonate, and precipitation methods.

• Some salts require crystallization to collect them as solids.

• These reactions are used in making medicine, fertilizers, and cleaning agents.

💭 Reflection Prompt

Visit your kitchen or garden and list two things that might contain salts. How do you think those salts were made? What method—neutralization, precipitation, or something else—was likely used? Record your guess in your science journal.