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    In a lesson today, we made magnesium sulphate by adding insoluble magnesium carbonate into sulphuric acid. We kept on adding the carbonate until no more dissolved, and then filtered it and so on. Then our teacher asked us why this process is not suitable for soluble carbonate.

    So my understanding is that any excess soluble carbonate will not remain as solid but instead it will dissolve in water. But I don't know how to give a proper explanation about why this method of reacting carbonate and acid is not suitable.
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    If you are trying to make a pure salt, you want to make sure that no acid or carbonate remains in the salty solution which you plan to evaporate.

    If you have an insoluble carbonate, e.g. MgCO3, then you can add and excess of carbonate so no acid remains and filter off the excess.

    If you have a soluble carbonate, you won't know when to stop adding the carbonate, so some will be in the salty solution, which won't be pure salt when evaporated.

    Titrate!
 
 
 
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