Strong and Weak Acids

Acids make hydrogen ions, $\text{H}^+$, when dissolved in water. Bases (alkalis when soluble) make hydroxide ions, $\text{OH}^-$. The number of $\text{H}^+$ ions in solution affects acidity and pH.

What “strong” and “weak” mean

Strength tells us how much an acid’s particles split into ions in water (dissociate), not how much acid is present.

• Strong acid: completely dissociated in water.
• Weak acid: only partially dissociated; many particles stay as molecules.

Symbol equations:

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

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

A single arrow shows complete dissociation (strong). A double arrow shows a reversible, partial dissociation (weak).

How to compare them

• pH and indicator: At the same concentration, strong acids have more $\text{H}^+$ and a lower pH. With universal indicator: strong acids are red (about pH 1–3); weak acids are orange–yellow (about pH 4–6). Neutral solutions are green (pH 7).
• Electrical conductivity: Strong acids conduct better because there are more ions to carry charge.
• Reaction speed: Both react with metals, carbonates, and alkalis, but strong acids of the same concentration react faster (more $\text{H}^+$ available). The overall products are the same type of salt, plus hydrogen (with metals) or carbon dioxide (with carbonates).

Neutralisation (for any acid + alkali) focuses on the ions:

$$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O(l)}$$

Examples

• Strong acids: hydrochloric acid (HCl), nitric acid (HNO$_3$), sulfuric acid (H$_2$SO$_4$).
• Weak acids: ethanoic acid (vinegar), citric acid (in citrus fruits), carbonic acid (in fizzy drinks).

Common misconceptions

• Strength ≠ concentration: A concentrated weak acid can be more corrosive than a dilute strong acid. Strength is about ion splitting; concentration is how much acid is in a given volume.
• Same products: Strong and weak acids make the same types of products in reactions; the difference is mainly speed and pH.