The PH Scale

• • The pH scale shows how acidic a substance is. It can be measured using a pH meter which gives a numerical value.
• • The pH scale ranges from 0 (very acidic) through 7 (neutral) to 14 (very alkaline)
• • pH can be also be measured using an indicator and comparing the colour with a comparison chart.
• Acidic solutions have pH values less than 7. Stomach acid turns universal indicator paper a strong red colour. This indicates a pH value of 0, the strongest value of acid.
• Alkaline solutions have pH values more than 7. Toothpaste will turn universal indicator paper blue indicating a pH value of 9, which is a weak alkali.
• There are 2 methods for testing for pH:

  • Using a pH meter
  • Using an indicator
• Using a pH meter

  A pH meter gives a numerical value of the pH. As long as the pH meter has been looked after properly, the measurement should be accurate. This means that the measurement is close to the true value.

  
• An indicator is a substance which will change colour depending on the pH of the solution it is mixed with. Some indicators are liquids, meaning we can add drops of the indicator to the solution being tested. Other indicators are strips of paper, and we can dip these into the solution.

  Litmus is an example of an indicator. It turns red in solutions that are acidic and it is blue in alkaline solutions.
• Red cabadge indicator

  1. Chop up the red cabbage into small pieces and place it in a mug.
  2. Add freshly boiled water from the kettle.
  3. Stir the cabbage and boiling water until a deep purple solution is made.
  4. Put drops of your substance sample onto a white plate and then add your red cabbage solution.
  5. Use the colour comparison chart to find the pH of your substances.