Another method of preparing salts depends on the solubility of certain salts. Salts that are insoluble in water can be prepared by a precipitation method. That is when two clear liquids are mixed together and a reaction between them gives a solid precipitate, insoluble in water. An example of this process is the preparation of lead iodide, method below.
Place 50 cm3 of dilute nitric acid in a 100 ml beaker and warm it gently over a Bunsen burner, on a tripod and gauze, to about 50oC. DO NOT BOIL THE ACID. Remove the beaker containing the acid from the tripod, and add small spatula measures of lead(II) oxide, PbO, stirring constantly, until no more will dissolve. Allow the solution to cool. Filter off the excess lead(II) oxide and allow the filtrate (the aqueous solution) to pass into a clean conical flask.
Take about half the filtrate (the liquid filtered through the funnel) and evaporate some of the water off, as with the preparation of copper(II) sulphate crystals before.
To the other 25 cm3 of the solution add an equal volume of dilute potassium iodide(aq), KI. Filter off the solid lead(II) iodide produced. Wash the residue with distilled water and leave it to dry.
(i) Write up all your method and observations as they occur in your reaction.
(ii) Why cannot lead iodide be prepared by the reaction of
|(a) lead metal with dilute hydroiodic acid?|
|(b) lead carbonate with dilute hydroiodic acid?|
(iii) What is the intermediate lead compound formed in this reaction?