Final answer:
When NaCl is added to pure water, the pH remains unchanged because it dissociates into Na+ and Cl- ions, both of which are from a strong base and a strong acid respectively, and therefore do not hydrolyze to affect the pH.
Step-by-step explanation:
When NaCl is added to pure water, the pH will remain unchanged. The reason is that when NaCl dissolves in water, it dissociates into Na+ (aq) and Cl- (aq) ions. Na+ is the conjugate acid of a strong base (NaOH), and it does not hydrolyze because it is completely ionized in solution. On the other hand, Cl- is the conjugate base of a strong acid (HCl), and it has no basic character since HCl is also completely ionized in solution. Therefore, neither ion affects the pH significantly, and the solution stays neutral.
In contrast, other ionic compounds called salts can sometimes alter the pH of the solution through a process called hydrolysis, if the ions formed are weak acids or bases themselves. For example, NH4Cl can make the solution slightly acidic because the NH4+ ion, which is the conjugate acid of NH3, can donate a proton to the water to form NH3 and H3O+. However, since NaCl is made of ions from strong acid and strong base, the pH of the water does not change. This principle helps to understand that dissolution and dissociation do not always lead to changes in pH.