How to identify whether H2SO4 is a dilute or concentrated???
Answers
Answer:
A quicker and easier method is to use pure (solid) potassium iodide or sodium iodide.
When a little iodide salt is heated gently with concentrated H2SO4, purple vapours of iodine (I2) will evolve, along with colorless sulphur dioxide (SO2). The contents of the test tube also turns purple-grey.
2KI + H2SO4 = K2SO4 + 2HI
2HI + H2SO4 = SO2 + I2 + 2H2O
The hydrogen iodide (HI) initially formed by double decomposition is readily oxidised to free iodine by excess concentrated H2SO4.
Dilute H2SO4 does not produce any visible chemical change with the iodide salt as it is non-oxidising.
In place of an iodide salt, sodium bromide or potassium bromide can also be used. The bromide salt will give off orange-brown vapours of free bromine on heating with concentrated H2SO4 by similar reaction.
Both HBr and HI are strong reducing agents and so they are easily oxidised to free bromine and iodine respectively by hot concentrated H2SO4.
A chloride salt is of no use here as HCl, a mild reducing agent, is not easily oxidised by concentrated H2SO4.