The pH increases when NaOH is added drop by drop to HCl because of the process of dissociation of both substances in water. HCl dissociates into H⁺ and Cl⁻ ions, while NaOH dissociates into Na⁺ and OH⁻ ions. Adding NaOH introduces a large concentration of OH⁻ ions, which react with the H⁺ ions from HCl. These H⁺ ions and OH⁻ ions combine to form water molecules (H₂O), reducing the concentration of H⁺ ions. As the H⁺ ion concentration decreases, the solution becomes less acidic, and the pH increases.
The reaction can be represented as:
H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O
Since the Na⁺ and Cl⁻ ions do not participate in the reaction, the net ionic equation is: H⁺ + OH⁻ → H₂O
When enough NaOH is added, all H⁺ ions are neutralized, and the solution contains only Na⁺ and Cl⁻ ions, forming a neutral solution of NaCl and water. The solution's only source of H⁺ and OH⁻ ions at this point is the slight dissociation of water.
This reaction is called a neutralization reaction, represented by:
HCl + NaOH → NaCl + H₂O
