Auto-ionisation of water

1. Key Concept

• Water $(H₂O)$ is amphoteric, meaning it can act as both an acid (donating $H⁺$) and a base (accepting $H⁺$).
• The process where two water molecules react to form ions is called auto-ionisation or autoprotolysis.

2. Auto-ionisation Reaction

• One water molecule acts as an acid, donating an $H⁺$ ion: $H_2O \rightleftharpoons H^+ + OH^-$

• Another water molecule acts as a base, accepting the $H⁺$ ion: $H_2O + H^+ \rightleftharpoons H_3O^+$

• Net reaction: $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$

3. Properties of Water Related to Auto-ionisation

• Water is a weak acid and a weak base.
• Only a small fraction of water molecules ionise at a time.
• Pure water is a weak electrical conductor due to the presence of $H₃O⁺$ and $OH⁻$ ions.

4. Key Takeaways

• Auto-ionisation is a reversible reaction, maintaining a balance between $H₃O⁺$ and $OH⁻$ ions.
• This reaction plays a crucial role in pH calculations and the acid-base properties of aqueous solutions.
• The concentration of ions in pure water at 25°C is $H₃O⁺ = OH⁻ = 1.0 × 10^{-7} M$.