Description of experiment
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Needed compounds:
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sodium hydroxide : NaOH
sulphuric acid : H2SO4
sodium silicate : Na2SiO3
silver nitrate : AgNO3
Class:
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elem=Ag,Si
precipitation
Summary:
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Silver salts give a yellow precipitate with silicates. This precipitate
becomes white on addition of sulphuric acid. When a large quantity of
sodium hydroxide is added, then a dark, almost black, solid is created.
Description:
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Add a few drops of syrupy water glass (sodium silicate solution, appr. 35%)
to a solution of silver nitrate: A bright yellow precipitate is formed. The
pieces of the precipitate are fairly hard and brittle.
Add some sulphuric acid (appr. 1 mol/l): The precipitate does not dissolve,
but it becomes white. The structure also changes, it becomes more yelly like.
Probably the sulphuric acid causes the yellow compound (silver silicate) to
decompose, forming white sparingly soluble silver sulfate, combined with
sparingly soluble silicic acid.
Decant the liquid and keep part of the precipitate and add some solid sodium
hydroxide: A vigorous reaction occurs and a lot of heat is produced (reaction
between excess sulphuric acid and water with the NaOH). Besides this, a black
solid is produced in small quantities, this probably is metallic silver.