# 8.1.7 Interpreting pH

### Interpreting pH

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is
• The values on the pH scale go from 0-14 (extremely acidic substances have values of below 0)
• 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 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

#### The pH of water

• 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 at 298 K
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