Question

The colour of \(Mg(OH)_2\) precipitate formed when it is precipitated in the presence of magneson reagent is

• A. Green
• B. Yellow
• C. Brown
• D. Blue

Hint:

Magneson reagent is commonly used for qualitative detection of magnesium ions due to the formation of a blue colored precipitate.

Answer 1

The Correct Option is D

Step 1: Understand magneson reagent.
Magneson (p-nitroaniline azoresorcinol) is a qualitative reagent used to detect \( Mg^{2+} \) ions in alkaline conditions.
Step 2: Describe the precipitate formation.
When \( Mg^{2+} \) is present in alkaline solution, \( Mg(OH)_2 \) precipitates: \[ Mg^{2+} + 2OH^- \rightarrow Mg(OH)_2 \downarrow \]
Step 3: Explain the colour change.
The magneson dye adsorbs on the surface of the \( Mg(OH)_2 \) precipitate. This adsorption changes the appearance of the precipitate.
Step 4: Identify the colour.
When magneson adsorbs on \( Mg(OH)_2 \), the precipitate turns blue. This blue colour is the positive indication for \( Mg^{2+} \).
Step 5: Eliminate other colours.
The colour is NOT green, yellow, or brown. The specific blue colour of the magneson-Mg(OH)2 complex is well established in qualitative inorganic analysis.
Step 6: State the answer.
The colour of \( Mg(OH)_2 \) precipitate with magneson reagent is blue. \[ \boxed{\text{Blue}} \]