The Characteristic Properties of Acids & Bases

Revise The Characteristic Properties of Acids & Bases for Chemistry 0620 (O Level) — revision notes and instant AI marking.

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What You'll Learn

What defines acids and bases chemically
How to write and balance acid-base reactions
Understanding pH and hydrogen ion concentration
The difference between strong and weak acids
How to classify oxides as acidic or basic
Using indicators to test solutions

Properties of Acids & Bases

What Are Acids?

Think of an acid as a hydrogen ion donor. When you dissolve an acid in water, it releases H⁺ ions (also written as H₃O⁺) into the solution. These free-floating hydrogen ions are why the solution tastes sour and feels corrosive.

Definition: An acid is a substance that produces hydrogen ions (H⁺) in aqueous solution. It can neutralise a base, forming a salt and water.

Characteristic Properties of Acids:
• pH less than 7 (acidic)
• Sour taste (when edible)
• Corrosive — can damage skin and materials
• Turn blue litmus paper red
• Can react with metals, bases, and carbonates

Example: When hydrochloric acid (HCl) dissolves in water:

Equation

HCl(aq) → H⁺(aq) + Cl⁻(aq)

💡 Key Insight

It's the H⁺ ions that make the solution acidic, not the chloride ions. Any acid producing more H⁺ ions will be stronger.

Acids Reacting with Metals

Only metals above hydrogen in the reactivity series will react with dilute acids. This is because hydrogen ions are more "eager to grab electrons" than the metal can prevent.

General Equation:
acid + metal → salt + hydrogen gas

The name of the salt depends on which acid you use:

Acid Used	Salt Name Contains	Example Equation
Hydrochloric acid (HCl)	Chloride	Mg + 2HCl → MgCl₂ + H₂
Sulfuric acid (H₂SO₄)	Sulfate	Mg + H₂SO₄ → MgSO₄ + H₂
Nitric acid (HNO₃)	Nitrate	Mg + 2HNO₃ → Mg(NO₃)₂ + H₂

⚡ Practice: Write the equation for zinc reacting with sulfuric acid.

What Are Bases?

A base is the opposite of an acid — it's a hydrogen ion acceptor. Bases are substances that can neutralise acids. Most bases are metal oxides or metal hydroxides.

What's an Alkali?

Characteristic Properties of Bases & Alkalis:

Example: When sodium hydroxide (NaOH) dissolves in water:

Equation

NaOH(s) → Na⁺(aq) + OH⁻(aq)

⚠️ Common Confusion

Base alkali

What to Memorise

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Also in the full note

The Ions in Acids & Alkalis
Proton Transfer & Strong vs Weak Acids
Classifying Oxides
Concepts Checklist
Exam Tips & Common Mistakes

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