Question

In the Lake test of $A{{l}^{3+}}$ ion, there is the formation of coloured floating. It is due to:
(A) Adsorption of litmus by ${{H}_{2}}O$
(B) Adsorption of litmus by $Al{{\left( OH \right)}_{3}}$
(C) Adsorption of litmus by $Al\left( OH \right)_{4}^{-}$
(D) None of the above

Answer 1

Hint: Think about the inorganic qualitative analysis. List down the steps of Lake’s test. Write down all the reagents used in the lake's test and try to think about why the coloured compound formed floats and what must be the phenomena behind it.


Complete step by step solution:
- Lake’s test is the confirmatory test in inorganic qualitative analysis used to detect the presence of aluminium cation.
- In the lake’s test the following steps are present.
- Take a spatula of salt mixture and make a solution with water. To this solution, add aluminium chloride, $AlC{{l}_{3}}$ and then add ammonium hydroxide till the solution is alkaline. A white gelatinous precipitate is observed which shows the presence of aluminium cation, $A{{l}^{3+}}$.
- Now, to this white gelatinous precipitate, hydrochloric acid is added. The above reaction gets reversed and aluminium chloride is formed which is soluble in water and a colourless solution is obtained.
\[Al{{\left( OH \right)}_{3}}+3HCl\to AlC{{l}_{3}}+3{{H}_{2}}O\]
- Now, to this solution, two drops of blue litmus solution is added which turns red. Then, to make the medium alkaline, ammonium hydroxide is added which leads to precipitation of aluminium hydroxide and the solution turns to blue.
- Aluminium hydroxide forms adsorbs the blue litmus and changes its colour to blue and floats on the solution.
- Aluminium hydroxide precipitate floats in the solution due to its gelatinous nature and less density.
- Therefore, in the Lake test of $A{{l}^{3+}}$ ion, there is the formation of coloured floating. It is due to adsorption of litmus by $Al{{\left( OH \right)}_{3}}$.

- Therefore, the answer is option (B).


Note: The Lake’s test is also known as blue Lake’s test and is the principle confirmatory test used to confirm the presence of aluminium cation in a given unknown sample. Aluminium hydroxide formed has less density and gelatinous nature because of which it adsorbs blue litmus on its surface and becomes blue coloured and floats at the surface of the solution.