🌟 Module 4: Strong and Weak Acids/Bases

🎯 Objective

To understand how strong and weak acids/bases differ based on their ionisation in water, and to distinguish strength from concentration. This module also explains why everyday acids like lemon juice behave the way they do.

🧪 1. The Concept of Ionisation in Water

When acids or bases dissolve in water, they ionise—that is, they split into ions.

• Acid → produces H⁺ ions

• Base → produces OH⁻ ions

🔹 Ionisation determines strength, not just concentration.

💪 2. Strong vs. Weak Acids

🔴 Strong Acid

• Completely ionises in water.

• Produces a large number of H⁺ ions.

• Highly corrosive and reactive.

🔍 Examples:

• Hydrochloric acid (HCl)

$$\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$$

• Nitric acid (HNO₃)

• Sulphuric acid (H₂SO₄)

📌 These acids are used in laboratories and industries and must be handled with care.

🟠 Weak Acid

• Partially ionises in water.

• Produces fewer H⁺ ions.

• Less corrosive.

🔍 Examples:

• Acetic acid (CH₃COOH)

$$\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$$

• Citric acid (from lemon)

• Carbonic acid (H₂CO₃) in soft drinks

📌 These are often found in food, fruits, and beverages.

🟦 3. Strong vs. Weak Bases

🔵 Strong Base

• Completely dissociates in water to give OH⁻ ions.

🔍 Examples:

• Sodium hydroxide (NaOH)

$$\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$$

• Potassium hydroxide (KOH)

📌 Used in soap making, drain cleaners—highly corrosive.

🟡 Weak Base

• Partially dissociates in water.

• Fewer OH⁻ ions released.

🔍 Examples:

• Ammonium hydroxide (NH₄OH)

$$\text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$$

• Calcium hydroxide (Ca(OH)₂) (lime water)

📌 Used in mild cleaning agents and neutralising soil.

🧂 4. Strength vs. Concentration – What’s the Difference?

These are often confused, but they are not the same:

Feature	Strength	Concentration
Definition	Degree of ionisation in water	Amount of acid/base dissolved in water
Related to	Nature of acid/base	Dilution of solution
Example	HCl (strong) vs. CH₃COOH (weak)	1M HCl vs. 0.1M HCl
Can vary?	No – inherent property	Yes – you can dilute or concentrate it

🧪 You can have:

• A dilute strong acid (e.g., 0.01 M HCl)

• A concentrated weak acid (e.g., pure acetic acid)

🧠 Critical Thinking: Why Does Lemon Juice Sting on a Wound?

✨ Answer:

• Lemon juice contains citric acid, a weak acid.

• But when you have a wound, your skin is broken, and nerve endings are exposed.

• Even the small number of H⁺ ions from a weak acid can irritate or stimulate nerve endings, causing a burning sensation.

🔍 It’s not the strength of the acid but the sensitivity of the wound that causes the pain!

📘 Real-life Applications

• Strong acids and bases are used in industry, but need safety precautions.

• Weak acids/bases are found in food, body fluids, and cleaning products.

• Understanding strength vs. concentration helps in medicine dosage and laboratory titrations.

🔁 Quick Recap

✅ Strong acids/bases = complete ionisation
✅ Weak acids/bases = partial ionisation
✅ Strength ≠ concentration
✅ Even weak acids can sting sensitive tissues
✅ Knowledge of strength helps in choosing the right chemical for the right task