Acids and Alkalis 01
The acid found in lemons is citric acid.

Acids and Alkalis 01

As a KS3 science student, you will be familiar with using indicators to tell whether a substance is acidic, alkaline or neutral. Indicators are usually made from plants, for example, you can use beetroot juice as a simple indicator. You have probably used the two types of litmus paper. Litmus is a mixture of dyes that come from lichens, very slow growing organisms that are made from algae or bacteria living in symbiosis (look it up if you don't know what that means) with a fungus. Blue litmus turns red in an acid and red litmus turns blue in an alkali.

Another mixture of dyes is called universal indicator. This is more useful than litmus as it can be used to measure how strong an acid or an alkali is using the pH scale. A pH of 7 is neutral (like for example water); a pH of 1 or 2 is a strong acid (for example sulphuric acid); weak acids like vinegar have a pH of 3 to 6. Weak alkalis (like soap) have pH values of 8 to 10 and a strong alkali like sodium hydroxide will have a pH of 13 or 14. Universal indicator comes as a liquid or as a roll of universal indicator paper.

  1. Which of the following pH numbers indicates a neutral substance?
    Pure water has this pH value
  2. Which of the following substances contain an alkali?
    It is weak enough to be used in food
  3. What colour is universal indicator in strong acid?
    Red indicates a pH of 1 or 2
  4. Which of the following pH numbers indicates a strong acid?
    Be careful not to confuse a strong acid with a concentrated acid - a strong acid will always give a pH value of 1 or 2 until it is extremely dilute
  5. Which of the following substances contain acid?
    Citrus fruit (oranges, lemons etc) contain citric acid
  6. What colour is universal indicator in a strong alkali?
    For weak alkalis, the colour is more of a blueish-green. It is often easier to see the colour using universal indicator paper
  7. Which of the following pH numbers indicate a strong alkali?
    Hydrangea flowers can be blue or pink depending on the pH of the soil
  8. Strong acids and strong alkalis are.......
    We use the word corrosive for acids and caustic for alkalis, either way, if you get any on your skin, they are both harmful and will cause a chemical burn
  9. Which of the following acids do we have in our stomach?
    The acid kills bacteria and helps to begin the process of digesting proteins
  10. What colour is universal indicator in a neutral solution?
    When using univeral indicator liquid, you only ever need to add a few drops to see the colour

Author: Sue Davison

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