Mixing Solutions and Separating Precipitate: Methods & Tips

THEORY:

Precipitation is a double decomposition reaction (also called metathesis) usually between two clear solutions, resulting in the formation of an insoluble substance. The insoluble solid which separates out is called the precipitate. Hence, only insoluble solids appear as precipitates, and soluble ones usually do not. For example, if the two given solutions of barium chloride and dil sulphuric acid are mixed, they react as follows with the formation of the insoluble barium sulphate as the precipitate.

BaCl₂ + H₂SO₄ —————> BaSO4₄+ 2HCl

APPARATUS:

1) Beakers

2) Filter paper and filter stand

3) Funnel

4) Wash bottle

5) Air oven

CHEMICALS:

1) BaCl₂ solution

2) H₂SO₄ solution

PROCEDURE:

i) Take about half a test tube of barium chloride solution in a beaker, and warm it to some extent.

ii) Stop heating, and then add the precipitant reagent (H₂SO₄) a little at a time stirring the mixture intermittently.

iii) Heat the mixture almost to boiling so as to coagulate the colloidal particles, and then allow the precipitate to settle down for sometime.

iv) Test for the completeness of the precipitation by pouring carefully a little precipitant along the side of the beaker down to the upper clear liquid.

v) When the precipitation is complete, filter off the clear liquid. Wash the precipitate two or three times with water by using the method of washing by decantation.

vi) Transfer the precipitate on to the filter paper with the help of a wash bottle, and wash it with warm water till the washing is free from the impurities of soluble sulphate (test with BaCl₂ solution) and soluble chloride (test with AgNO₃ solution)

vii) Remove the funnel together with the filter paper, and dry it inside the air oven.