Making salts

Making salts

  • Most chlorides, sulphates and nitrates (salts) are soluble in water – notable exceptions are lead chloride, lead sulphate and silver chloride
  • Most oxides and hydroxides are insoluble
  • The method to make a soluble salt depends are whether the base is soluble or not

Making soluble salts from a metal or insoluble base

  • You need to pick the right acid – pick a metal or insoluble base
  • You add the metal or insoluble base to the acid
  • If it a reactive enough metal then it will dissolve – the experiment will finish when the excess metal drops to the bottom – this means all the acid has been neutralised
  • You can then filter off the excess solid – to get a salt solution
  • You can then get pure crystals of the salt by evaporating off some of the solution and leaving the rest to evaporate slowly

Making soluble salts from an alkali

  • Use the same method as above but because what you adding is not a solid you can’t tell when the reaction is finished
  • This means you need to add the exact right amount of alkali – the way you can tell it is finished is to put an indicator in the solution and see if it is neutral
  • Then you repeat the experiment using those exact amounts but without the indicator so it isn’t contaminated
  • You would repeat the crystallisation method as above

Making insoluble salts (precipitation reactions)

  • Pick two solutions (that are both soluble) that contain what you need – e.g. to make lead chloride (insoluble) you would pick lead nitrate and sodium chloride
  • You mix the two solutions then the salt will precipitate out and will lie on the bottom of the flask
  • You then filter the solid out – then wash it and dry it
  • Often this precipitation reaction is used to remove poisonous ions from things like drinking water
  • This is the way that water softeners work as they remove the hard metals from it
  • It is also what they use to clean sewage