What Are Acids and Bases?

Acids and bases are important types of chemical substances that are commonly studied in O Level Chemistry.

Acids and bases have different chemical properties and behaviours, and they react with each other in reactions called neutralisation reactions.

Understanding acids and bases helps explain many processes in chemistry, biology, and everyday life.

What Is an Acid?

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

Acids usually have a sour taste and can react with metals and bases.

Examples of common acids include:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Ethanoic acid (CH₃COOH)

These acids are widely used in laboratories and industrial processes.

Properties of Acids

Acids have several characteristic properties.

Acids:

• Produce hydrogen ions in aqueous solutions
• Turn blue litmus paper red
• React with metals to produce hydrogen gas
• React with bases to form salt and water
• Have pH values less than 7

These properties help chemists identify acidic substances.

What Is a Base?

A base is a substance that neutralises acids.

Bases may produce hydroxide ions (OH⁻) when dissolved in water.

Examples of common bases include:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)
• Ammonia solution (NH₃)

Many bases are used in cleaning products and chemical manufacturing.

What Is an Alkali?

An alkali is a base that dissolves in water and produces hydroxide ions.

Examples of alkalis include:

• Sodium hydroxide
• Potassium hydroxide
• Aqueous ammonia

All alkalis are bases, but not all bases are soluble in water.

The pH Scale

The pH scale measures how acidic or alkaline a solution is.

The scale usually ranges from 0 to 14.

pH ValueType of Solution0–6Acidic7Neutral8–14Alkaline

Examples:

• Lemon juice → pH about 2
• Pure water → pH 7
• Sodium hydroxide solution → pH about 13

The pH scale helps chemists measure strength of acids and alkalis.

Neutralisation Reaction

A neutralisation reaction occurs when an acid reacts with a base to form salt and water.

General equation:

Acid + Base → Salt + Water

Example reaction:

HCl + NaOH → NaCl + H₂O

Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water.

Importance of Acids and Bases

Acids and bases are important in many areas such as:

• Industrial chemical production
• Agriculture and soil treatment
• Food and beverages
• Biological processes in the body

Understanding acids and bases helps chemists design useful chemical reactions.

Exam Tip (5070)

Students are often asked to:

• Define acid, base and alkali
• Identify substances using the pH scale
• Write neutralisation reactions

Example exam question:

What is the product formed when an acid reacts with a base?

Answer:

A salt and water are formed.

Practice Question

What is the pH of a neutral solution?

Answer

The pH of a neutral solution is 7.

Example: Pure water.

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