It is the measure of hydrogen ion activity in aqueous solutions.

• A. Conductivity
• B. Viscosity
• C. pH
• D. Solubility

Answer: (C)

pH is the measure of hydrogen ion activity in aqueous solutions. It is defined as the negative base-10 logarithm of the activity of H+ ions: pH = -log10(a_H+). In dilute solutions, activity closely tracks the hydrogen ion concentration, so pH ≈ -log10[H+]. A lower pH means higher hydrogen ion activity (more acidic), while a higher pH means lower activity (more basic). This makes pH the direct and practical way to express how acidic or basic a solution is. Conductivity, viscosity, and solubility describe other properties—how well a solution conducts electricity, how thick or resistant to flow it is, and how much solute can dissolve—not the hydrogen ion activity itself.