Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When metal ' \(\mathrm{M}\) ' is treated with \(\mathrm{NaOH}\), a white gelatinous precipitate \(' \mathrm{X}^{\prime}\) is obtained, which is soluble in excess of \(\mathrm{NaOH}\). Compound ' \(\mathrm{X}^{\prime}\) when heated strongly gives an oxide which is used in chromatography as an adsorbent. The metal ' \(\mathrm{M}\) ' is : \([\) Main \(\mathbf{2 0 1 8}]\) (a) \(\mathrm{Zn}\) (b) \(\mathrm{Ca}\) (c) \(\mathrm{Al}\) (d) \(\mathrm{Fe}\)

Step-by-Step Solution

Answer

The metal 'M' is \(\mathrm{Al}\) (aluminum).

Step 1: Understanding the chemical reaction

When the metal 'M' reacts with \(\mathrm{NaOH}\), a white gelatinous precipitate 'X' is formed. This formation suggests that the metal hydroxide, which is typical for amphoteric hydroxides, is formed. Compound 'X' is soluble in excess \(\mathrm{NaOH}\), which indicates the amphoteric nature of the compound.

Step 2: Identifying the amphoteric metals

Amphoteric metals react with both acids and bases to form salts. Common amphoteric metals include \(\mathrm{Al}\), \(\mathrm{Zn}\), and \(\mathrm{Sn}\). Aluminum hydroxide and zinc hydroxide typically form gelatinous precipitates and are soluble in excess \(\mathrm{NaOH}\).

Step 3: Analyzing heat decomposition of compound 'X'

The compound 'X', when heated strongly, decomposes to form an oxide used in chromatography. Aluminum oxide, \(\mathrm{Al_2O_3}\), and zinc oxide, \(\mathrm{ZnO}\), are both used as adsorbents, but \(\mathrm{Al_2O_3}\) is more common in chromatography.

Step 4: Making the final identification

Given that aluminum hydroxide, \(\mathrm{Al(OH)_3}\), forms a white gelatinous precipitate and dissolves in excess \(\mathrm{NaOH}\) to form an aluminate, and that \(\mathrm{Al_2O_3}\) is commonly used in chromatography, the metal 'M' fits the characteristics of aluminum, \(\mathrm{Al}\).

Key Concepts

Amphoteric Metals

Amphoteric metals have a unique characteristic. They can react with both acids and bases. This dual behavior allows amphoteric metals to form salts in various chemical reactions. The most common amphoteric metals include aluminum (Al), zinc (Zn), and tin (Sn). When these metals interact with a base like sodium hydroxide (\[\mathrm{NaOH}\]), they form metal hydroxides. For example, aluminum reacts to form aluminum hydroxide (\[\mathrm{Al(OH)_3}\]), which is a white gelatinous precipitate. This precipitate is soluble in excess \[\mathrm{NaOH}\], forming soluble aluminates.

Chromatography Adsorbents

Chromatography is a technique used to separate different components in a mixture. An essential part of this process is the adsorbent. An adsorbent is a substance that retains particles on its surface. Many metal oxides serve as adsorbents due to their high surface area and porosity.

• Aluminum oxide (\[\mathrm{Al_2O_3}\]) is a popular adsorbent in chromatography.
• It's commonly used to purify, separate, and identify compounds due to its effectiveness in adsorbing different molecules.
  
• Zinc oxide (\[\mathrm{ZnO}\]) can also act as an adsorbent, but it is second to aluminum oxide in terms of common use.

These oxides are pivotal in techniques like thin-layer chromatography and column chromatography, where separation based on adsorption is key.

Chemical Reactions with NaOH

When metals react with sodium hydroxide (\[\mathrm{NaOH}\]), they often produce hydroxides. These reactions are particularly interesting when involving amphoteric metals.

• Aluminum's reaction with \[\mathrm{NaOH}\] produces aluminum hydroxide (\[\mathrm{Al(OH)_3}\]). When excess \[\mathrm{NaOH}\] is present, it dissolves, forming a soluble complex called sodium aluminate (\[\mathrm{NaAlO_2}\]).
• This behavior indicates the amphoteric nature of aluminum.

Understanding these reactions is essential for analyzing and predicting the behavior of metals in different chemical environments. These reactions often involve significant changes in texture and solubility, such as the transformation from a precipitate to a soluble form.