Not all acids behave the same in solution. Some acids fully ionise, while others only partially ionise. This is the difference between strong and weak acids.

What is a strong acid?

A strong acid is one that completely ionises in water.

• All of the acid molecules release H⁺ ions
• This creates a high concentration of H⁺ ions in the solution

Examples of strong acids:

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

What is a weak acid?

A weak acid is one that only partially ionises in water.

• Only some molecules release H⁺ ions
• The rest stay as whole acid molecules
• This creates a lower concentration of H⁺ ions compared to a strong acid of the same concentration

Examples of weak acids:

• Ethanoic acid (CH₃COOH)
• Citric acid
• Carbonic acid (H₂CO₃)

Key difference: ionisation

💡 This is different from concentration, which is about how much acid is dissolved, not how much ionises.

Effect on pH

• Strong acids have a lower pH (closer to 0–1)
• Weak acids have a higher pH (closer to 4–6)
• Even if concentration is the same, a strong acid will have a lower pH than a weak one

Effect on reactions

Strong acids:

• React faster and more vigorously with metals, bases, and carbonates
• Produce more bubbles and a quicker temperature rise

Weak acids:

• React slower, as fewer H⁺ ions are available at any one time