1. Introduction
The strength of an acid or base tells us how powerfully it behaves in water. Some acids react very strongly, while others react gently. The same is true for bases.
The strength depends on how many ions they release in water and how completely they dissociate (break apart) into ions.
2. What Do We Mean by Strength?
Strength is not the same as concentration. Strength tells us whether an acid or base ionises completely or only partially in water.
A strong acid produces more \( H^+ \) ions, while a strong base produces more \( OH^- \) ions.
2.1. Strong vs Weak Acids
Acids differ based on how easily they release \( H^+ \) ions.
2.1.1. Strong Acids
Strong acids completely dissociate in water. This means every acid molecule breaks to produce hydrogen ions.
Examples:
- Hydrochloric acid (HCl)
- Nitric acid (HNO3)
- Sulphuric acid (H2SO4)
General dissociation:
\( \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \)
2.1.2. Weak Acids
Weak acids partially dissociate in water. Only some molecules release \( H^+ \) ions.
Examples:
- Acetic acid (CH3COOH)
- Carbonic acid (H2CO3)
- Citric acid
Weak acid dissociation:
\( \text{CH}_3\text{COOH} \rightleftharpoons \text{H}^+ + \text{CH}_3\text{COO}^- \)
2.2. Strong vs Weak Bases
Bases differ based on how easily they release \( OH^- \) ions in water.
2.2.1. Strong Bases
Strong bases completely dissociate in water to produce many \( OH^- \) ions.
Examples:
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Calcium hydroxide (Ca(OH)2)
Dissociation example:
\( \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \)
2.2.2. Weak Bases
Weak bases partially dissociate in water and produce fewer \( OH^- \) ions.
Examples:
- Ammonium hydroxide (NH4OH)
- Magnesium hydroxide
Weak base dissociation:
\( \text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \)
3. How pH Shows Strength
The pH scale helps us understand how strong an acid or base is:
- Strong Acids: pH 0–3
- Weak Acids: pH 4–6
- Neutral: pH 7
- Weak Bases: pH 8–11
- Strong Bases: pH 12–14
The lower the pH, the stronger the acid. The higher the pH, the stronger the base.
4. Why Strength Matters
Knowing whether an acid or base is strong or weak helps us understand:
- how they react with metals, carbonates, and indicators
- their safety levels (strong ones are more corrosive)
- how to dilute them properly
- their uses in industries and households
For example, strong acids like sulphuric acid need careful handling, while weak acids like citric acid are safe enough to be present in fruits.
5. Difference Between Strength and Concentration
It is very important to remember:
Strength = how much an acid or base ionises.
Concentration = how much acid or base is dissolved in water.
So, a strong acid can be diluted, and a weak acid can be concentrated — strength does not change.