Pure water has a pH of

Pure water has a pH of

• A. 1
• B. 7
• C. 10
• D. 4

Answer: (B)

pH measures how acidic or basic a solution is by looking at the hydrogen ion concentration. Pure water undergoes a tiny self-ionization: H2O ⇌ H+ + OH−. At room temperature, the concentrations of H+ and OH− are equal, each about 1.0 × 10^-7 M. The pH is the negative logarithm of the hydrogen ion concentration, so pH = -log10(1.0 × 10^-7) = 7. This makes pure water neutral on the pH scale. Temperature can shift this neutrality a bit because the water ionization constant Kw changes with temperature. Values like 1 or 4 would mean a lot more H+ (acidic), while 10 would mean more OH− (basic).

pH measures how acidic or basic a solution is by looking at the hydrogen ion concentration. Pure water undergoes a tiny self-ionization: H2O ⇌ H+ + OH−. At room temperature, the concentrations of H+ and OH− are equal, each about 1.0 × 10^-7 M. The pH is the negative logarithm of the hydrogen ion concentration, so pH = -log10(1.0 × 10^-7) = 7. This makes pure water neutral on the pH scale. Temperature can shift this neutrality a bit because the water ionization constant Kw changes with temperature. Values like 1 or 4 would mean a lot more H+ (acidic), while 10 would mean more OH− (basic).