Acids and Bases

The pH scale goes from 0 to 14

• The pH scale is a measure of how acidic or alkaline a solution is
• The lower the pH of a solution, the more acidic it is
• The high the pH of a solution, the more alkaline it is
• A neutral substance has pH 7

You can measure the pH of a solution

• An indicator is a dye that changed colour depending on whether it’s above or below a certain pH
• Some indicators contain a mixture of dyes meaning they gradually change colour
  • Called wide range indicators for example universal indicator
• A pH probe attached to a pH meter can also be used to measure pH electronically
• The probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value meaning its more accurate

Acids and bases neutralise each other

• An acid is a substance that forms aqueous solution with a pH of less than 7
  • Acids form H+ions in water
• A base is a substance with a pH greater than 7
• An alkali is a base that dissolves in water to form a solution with a pH greater than 7
  • Alkalis form OH-ions in water
• The reaction between acids and bases is called neutralisation:
  • Acids + base - salt + water
• Neutralisation between acids and alkalis can be seen in terms on H+ and OH-ions:
  • H+ + OH- - H20
• When an acid neutralises a base the products are neutral
• An indicator can be used too show that a neutralisation reaction is over
• Neutralisation reactions of strong acids and alkalis can be used to calculate the concentration of an acid or alkali by titration, there is more about this technique