Q.Why sodium hydroxide is much more water soluble than sodium chloride?
Step 1. Whether an ionic solid dissolves readily in water depends on the balance between the lattice enthalpy (energy needed to break the solid's ionic lattice apart) and the hydration enthalpy (energy released as the separated ions are surrounded by water molecules) -- a compound dissolves more readily when its hydration enthalpy outweighs its lattice enthalpy by a larger margin.
Step 2. Both NaOH and NaCl share the same cation, Na⁺, so the difference between them comes down to the anion: OH⁻ is a considerably smaller ion than Cl⁻.
Step 3. Because OH⁻ is smaller, it carries a more concentrated negative charge and is hydrated by water molecules much more strongly (larger-magnitude hydration enthalpy) than the larger, more diffuse Cl⁻ ion.
Step 4. This stronger hydration of OH⁻ makes the overall dissolution of NaOH considerably more energetically favourable than that of NaCl, which is why sodium hydroxide is a highly deliquescent solid that dissolves readily (with evolution of heat, as noted in Section 5.3) while sodium chloride's solubility is comparatively modest and barely temperature-dependent.
NaOH's smaller, more strongly hydrated hydroxide ion gives it a much more favourable hydration-vs-lattice energy balance than NaCl's larger, more weakly hydrated chloride ion, making NaOH far more water-soluble than NaCl.
Unlock everything free for 14 days
- Full step-by-step solutions
- Concept-first explanations
- Methods, shortcuts & mistakes
- PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.