• The pH scale is a numerical scale that shows how?acidic?or?alkaline?a solution is
• The values on the pH scale go from 1-14 (extremely acidic substances have values of below 1)
• All acids have pH values of?below?7, all alkalis have pH values?above?7
• The?lower?the pH then the?more acidic?the solution is
• The?higher?the pH then the?more alkaline?the solution is

The pH scale showing acidity, neutrality and alkalinity

pH of water

• An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions

H₂O(l) ? H⁺(aq) + OH^-(aq)

• The equilibrium constant for this reaction is:

Kc?x [H₂O] = [H⁺] [OH^-]

• Since the concentration the H+?and OH-?ions is very small, the concentration of water is considered to be a constant, such that the expression can be rewritten as:

Kw?= [H⁺] [OH^-]

Where?Kw?(ionic product of water)????? = ????????Kc?x [H₂O]

=???????? 10^-14?mol²?dm^-3?at 298K

• Water at 298K has?equal amounts?of?OH^-?and?H⁺?ions?with concentrations of?10^-7?mol dm^-3
• To calculate the pH of water, the following formula should be used:

pH?? ??? = -log (10-7)

= 7

• Thus, water has a pH of 7

pH of acids

• Acidic?solutions (strong or weak)?always?have more H⁺?than OH^-?ions
• Since the concentration of?H⁺?is always?greater?than the concentration of?OH^-?ions, [H⁺] is always?greater?than 10^-7?mol dm^-3
• Using the pH formula, this means that the?pH of acidic solutions?is always?below?7
• The higher the [H⁺] of the acid, the lower the pH

pH of bases

• Basic?solutions (strong or weak)?always?have more?OH^-than H⁺?ions
• Since the concentration of?OH^-is always?greater?than the concentration of?H⁺?ions, [H⁺] is always?smaller?than 10^-7?mol dm^-3
• Using the pH formula, this means that the?pH of basic solutions?is always?above?7
• The higher the [OH^-] of the base, the higher the pH