Acids and pH

• pH scale


  Measure of how acidic or alkaline a solution is
• pH scale range


  • 0
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
• Acidic solution


  pH less than 7
• Alkaline solution


  pH greater than 7
• Neutral substance


  pH 7
• Indicator


  • Dye that changes colour depending on pH
  • Some indicators contain a mixture of dyes that gradually change colour over a broad range of pH (wide range indicators)
• pH meter


  Probe that measures pH electronically and displays a numerical value
• Acid


  Substance that forms an aqueous solution with a pH less than 7, acids form H+ ions in water
• Base


  Substance that will react with an acid to form a salt
• Alkali


  Base that dissolves in water to form a solution with a pH greater than 7, alkalis form OH- ions in water
• Neutralisation


  Reaction between acids and alkali to form a salt and water
• Skin is slightly acidic (pH 5.5)
• Neutralisation reaction


  1. Hydrogen ions from acid react with hydroxide ions from alkali to make water

  2. Leftover bits of acid and alkali make a salt
• Strong acid


  Acid that ionises completely in water, all acid particles dissociate to release H+ ions
• Weak acid


  Acid that does not fully ionise in solution, only a small proportion of acid particles dissociate to release H+ ions
• Ionisation of weak acid

  Reversible reaction that sets up an equilibrium between undissociated and dissociated acid
• pH


  Measure of the concentration of H+ ions in a solution
• Decrease in pH by 1


  Concentration of H+ ions increases by a factor of 10
• pH of strong acid is always lower than pH of weaker acid at same concentration
• Acid strength (strong/weak) is different from acid concentration
• Metal oxide


  Some dissolve in water to form alkalis
• Metal hydroxide


  Some dissolve in water to form alkalis
• Reaction of acid and metal oxide/hydroxide


  Forms a salt and water
• Reaction of acid and metal carbonate

  Forms a salt, water and carbon dioxide
• Making a soluble salt from an insoluble base

  1. Choose right acid and insoluble base

  2. Gently warm dilute acid

  3. Add insoluble base until no more reacts

  4. Filter out excess solid

  5. Evaporate solution to get pure salt crystals