🧪 Acids, Bases & Salts

1. Introduction

Acids, bases and salts are important groups of chemical substances. They are involved in neutralisation, industrial processes, agriculture and everyday life.

Learning Objectives

• Define acids, bases, alkalis and salts.
• Describe properties of acids and bases.
• Use indicators and the pH scale.
• Explain neutralisation.
• Describe methods of preparing salts.
• Write balanced chemical equations.

2. Acids

An acid is a substance that produces hydrogen ions, H⁺, when dissolved in water.

Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

Properties of acids

• Have pH values below 7.
• Turn blue litmus paper red.
• React with some metals to produce hydrogen.
• React with bases to form salt and water.
• React with carbonates to produce carbon dioxide.

3. Bases and Alkalis

A base is a substance that neutralises an acid. An alkali is a soluble base that produces OH⁻ ions in water.

Examples of alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH) and aqueous ammonia.

Properties

• Have pH values above 7.
• Turn red litmus paper blue.
• Neutralise acids.
• Many alkalis feel soapy, but chemicals should never be tasted.

4. The pH Scale

The pH scale is used to indicate how acidic or alkaline a solution is.

Universal indicator gives different colours across the pH scale and is useful for estimating pH.

5. Strong and Weak Acids

A strong acid ionises almost completely in water, while a weak acid ionises only partially.

Important: Strong does not mean concentrated. Strength refers to ionisation, while concentration refers to the amount of solute per volume.

6. Neutralisation

Neutralisation occurs when an acid reacts with a base to form salt and water.

The net ionic equation is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

Applications

• Antacids neutralise excess stomach acid.
• Farmers can use lime to reduce soil acidity.
• Industrial wastewater may be neutralised before disposal.

7. Salts

A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or another positive ion.

Examples:

• Sodium chloride — NaCl
• Copper(II) sulfate — CuSO₄
• Calcium nitrate — Ca(NO₃)₂

8. Preparing Soluble Salts

A soluble salt can be prepared by reacting an acid with an excess insoluble base, oxide or carbonate.

The excess solid is filtered off. The solution is concentrated and then cooled so crystals form.

9. Titration

Titration is used to determine the concentration of an acid or alkali by reacting it with a solution of known concentration.

Basic procedure

1. Fill the burette with one solution.
2. Measure a known volume of the other solution using a pipette.
3. Add indicator.
4. Add the burette solution carefully until the end point.
5. Repeat until concordant titres are obtained.

10. MANEB Examination Focus

• Define acid, base, alkali and salt.
• Identify acids and alkalis using indicators.
• Explain neutralisation.
• Write balanced acid-base equations.
• Distinguish strong/weak from concentrated/dilute.
• Describe preparation and crystallisation of salts.
• Perform simple acid-base calculations.

11. Practice Exercises

1. What colour does blue litmus turn in an acid?
2. What is the pH of a neutral solution?
3. Define an alkali.
4. Write the equation for HCl reacting with NaOH.
5. Name the gas produced when an acid reacts with a carbonate.
6. Give one use of neutralisation.
7. Explain the difference between a strong acid and a weak acid.

Answers

12. Common Mistakes

• Do not confuse a base with an alkali.
• Do not confuse acid strength with concentration.
• Remember that neutralisation produces salt and water.
• Carbonates reacting with acids produce CO₂.

13. Exam Tip

When writing an acid reaction, first identify the type of reaction. This helps you predict the products before balancing the equation.