Indicators + ph scale

• Indicators

  Substances used to indicate if a substance is acidic, alkaline or neutral by showing a change in colour when added to a solution
• Indicators

  • Red cabbage
  • Beetroot
• Common laboratory indicators

  • Litmus (paper or solution)
  • Phenolphthalein
  • Methyl orange
  • Universal indicator
• Litmus paper

  A type of indicator paper made by treating the paper with natural dyes from lichens
• Litmus test


  1. Place a drop of the sample onto the litmus paper

  2. Blue litmus paper turns red in acid

  3. Red litmus paper turns blue in alkali/base

  4. Red/blue litmus paper that does not change colour indicates the sample is acidic/alkaline
• Litmus paper


  • Red litmus stays red in acid
  • Red litmus turns blue in alkali/base
  • Blue litmus turns red in acid
  • Blue litmus stays blue in alkali/base
• Phenolphthalein indicator


  Gives a pink colour in alkaline solutions and remains colourless in acidic solutions
• Methyl orange indicator


  Changes to red-orange in strong acids, but shows yellow in other solutions (weak acids, neutral and alkaline)
• Colour changes of methyl orange and phenolphthalein

  • Methyl orange: red-orange in acidic, yellow in neutral/alkaline
  • Phenolphthalein: colourless in acidic, pink in alkaline
• Universal indicator


  A mixture of different indicators that change colour with different strengths of acids or alkalis, used to measure the pH of a substance
• pH scale


  Ranges from 1 to 14, with 7 indicating neutrality, below 7 indicating acidity, and above 7 indicating alkalinity
• Red cabbage indicator


  • Made by boiling shredded red cabbage in water and using the juice, provides a wide range of colours for different pH values
• Red cabbage indicator provides a wider range of colours than the universal indicator
• Red cabbage indicator changes to more colours and yields more information
• Other indicators


  • Phenolphthalein
  • Methyl orange
• Phenolphthalein indicator


  Gives a pink colour in alkaline solutions and remains colourless in acidic solutions
• Methyl orange indicator


  Changes to red-orange in strong acids, but shows yellow in other solutions (i.e. in weak acids, neutral and alkaline solutions)