Question

Lake test of \[{\text{A}}{{\text{l}}^{{\text{3 + }}}}\] ion is based on adsorption of blue litmus on solid surface of:
A. \[{\text{Al}}\]
B. \[{\text{Al}}{\left( {{\text{OH}}} \right)_{\text{3}}}\]
C. \[{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\]
D. \[{\text{AlC}}{{\text{l}}_{\text{3}}}\]

Answer 1

Hint: In this question we will apply Lake test reaction which confirms the presence of Aluminium ions in the given sample. It is a kind of confirmatory test. Confirmatory test is done to ensure that other interfering cations or ions have been removed or if one has to check the presence of specific ions in a particular sample.

Complete step by step answer:
To the given solution, add water and Ammonium Chloride and then add Ammonium Hydroxide.
White precipitates are observed which confirms the presence of Aluminium ions.
To the above solution add \[{\text{HCl}}\], a colorless solution is obtained (as precipitates are soluble in \[{\text{HCl )}}\]
\[{\text{Al(OH}}{{\text{)}}_{\text{3}}}{\text{ + 3HCl }} \to {\text{AlC}}{{\text{l}}_{\text{3}}}{\text{ + 3}}{{\text{H}}_{\text{2}}}{\text{O}}\]
Add blue litmus solution to the above solution, red color is obtained.
Now add \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}\]to make the solution alkaline and Aluminium Hydroxide is formed along with blue precipitates.
\[{\text{AlC}}{{\text{l}}_{\text{3}}}{\text{ + 3N}}{{\text{H}}_{\text{4}}}{\text{OH }} \to {\text{3N}}{{\text{H}}_{\text{4}}}{\text{Cl + Al(OH}}{{\text{)}}_{\text{3}}}\]
Blue litmus solution gets adsorbed on Aluminium hydroxide as it acts as an adsorbent.

So, the correct answer is Option B.

Additional information:
As we know that aluminium is a versatile metal with various important properties like light in weight, highly flexible that is it can be moulded in different shapes easily, highly resistant to corrosion, good conductor of electricity and heat, odourless and most important it can be recycled without change in its properties. Most important example of aluminium as an alloy in day-to-day life is Alum (Potash Alum) which is a sulphate salt and is widely used as a water filter, for teeth pain etc. Aluminium foils are also used in packing food items to keep it soft and warm for a long time.

Note: Adsorption is a surface phenomenon where the particles of solid or liquid or gas get adhered to the solid surface. Adsorbate and adsorbent are two terms used in adsorption where adsorbate defined as a substance which is adsorbed and adsorbent is the substance may be solid or liquid on which adsorbate gets accumulated. Litmus is a mixture of water soluble dyes which is used as a pH indicator to check the acidity and basicity on the scale from \[0\]to \[14\] where \[7\] indicates neutral.