Making Salts
Prepare a pure, dry sample of a soluble salt from an insoluble base and an acid.
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Method
- Measure a fixed volume of the acid and pour it into a beaker.
- Warm the acid gently.
- Add the insoluble base in small masses.
- Continue adding until the base is in excess.
Aim
- Make pure, dry crystals of a soluble salt from an insoluble base and an acid.
Equipment
- Acid
- Insoluble base
- Beaker, tripod, gauze and Bunsen burner
- Filter funnel and filter paper
- Evaporating basin
- Crystallising dish
- Glass rod
Safety
- Wear eye protection because acids are irritants.
- Use gentle heating and keep the Bunsen flame controlled.
- Handle hot glassware with care.
Examiner tips
- Use the word excess to explain why all acid is neutralised.
- Filtration removes the insoluble reactant, not the dissolved salt.
- Evaporate to the point of crystallisation rather than boiling to dryness.
Common mistakes
- Adding too little base so acid remains in the final solution.
- Forgetting that copper sulfate is soluble and passes through the filter.
- Writing evaporate all the water instead of crystallise the solution.
Variables
- Independent
- Mass of base added, or type of acid/base depending on investigation.
- Dependent
- Mass and quality of salt crystals produced.
- Control
- Volume and concentration of acid, Temperature, Evaporation time, Cooling conditions
Common exam questions
- Why is excess base added?
- Why is the mixture filtered?
- Why should the solution not be heated to dryness?
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