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Acids, Bases and Salts — Class 9 Maharashtra SSC Science revision notes

A 3-page visual revision sheet for Acids, Bases and Salts, written for Class 9 Maharashtra SSC Science. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Acids, Bases and Salts in one look

the two families, the scale that ranks them, and the salts their meeting produces

What makes an acid or a base

  • an acid produces hydrogen ions when dissolved in water, and it tastes sour
  • a base produces hydroxide ions in water, tastes bitter and feels soapy
  • an alkali is a base that dissolves in water, so every alkali is a base but not every base is an alkali
  • acids turn blue litmus red and bases turn red litmus blue
  • strong acids and bases dissociate almost completely in water, weak ones only partly. Strength is not the same as concentration
  • concentration is how much is dissolved, strength is how completely it dissociates, and a dilute solution of a strong acid is perfectly possible

The pH scale

  • pH runs from 0 to 14 and measures the concentration of hydrogen ions
  • below 7 is acidic, exactly 7 is neutral, above 7 is basic
  • the lower the pH the more acidic, so pH 1 is a far stronger acid than pH 5
  • the scale is logarithmic: each step of one unit is a tenfold change, so pH 3 is a hundred times more acidic than pH 5
  • indicators change colour with pH: litmus, methyl orange, phenolphthalein, and universal indicator which gives a range of colours
  • olfactory indicators such as onion and vanilla change smell rather than colour, and are used to test with the nose

Why pH matters

  • our body works in a narrow range, and blood is held close to pH 7.4
  • tooth decay starts below pH 5.5, when bacteria in the mouth produce acid from sugar. Toothpaste is basic and neutralises it
  • acidity in the stomach is treated with an antacid, a mild base such as magnesium hydroxide
  • soil pH affects crops, and it is corrected with lime if too acidic or with organic matter if too basic
  • acid rain has a pH below 5.6 and damages crops, buildings and aquatic life
  • a bee sting is acidic and is treated with baking soda, while a wasp sting is basic and is treated with vinegar

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