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