# 1.7.10 pH Scale

### The pH Scale

• 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 H+ than OH 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

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
Close

# Join Save My Exams

## Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.