* Solubility: Barium sulphate (BaSO4) is practically insoluble in water. This means that when barium chloride (BaCl2) is added to a solution containing sulphate ions (SO4^2-), a reaction occurs, forming a white precipitate of barium sulphate.
* Driving the reaction to completion: By adding excess barium chloride, we ensure that all the sulphate ions in the solution react with barium ions to form barium sulphate. This is due to Le Chatelier's principle, which states that a system at equilibrium will shift to relieve stress. The addition of excess barium chloride puts stress on the equilibrium, driving the reaction to favor the formation of more barium sulphate.
* Filtration and washing: After the reaction is complete, the precipitate is filtered and washed to remove any remaining impurities, including excess barium chloride. This ensures that the final sample contains only pure barium sulphate.
In summary: Adding excess barium chloride is a crucial step in the preparation of a dry sample of sulphate because it:
* Ensures complete precipitation of all sulphate ions.
* Allows for effective removal of impurities through filtration and washing.
* Leads to a pure and accurate sample of barium sulphate.
