Question

How do you identify the following cations in their aqueous solutions by using caustic soda solution? (i) \(\mathrm{Fe}^{+2}\) (ii) \(\mathrm{Zn}^{+2}\) (iii) \(\mathrm{Al}^{+3}\)

Short answer

Answer: When caustic soda solution is added to an aqueous solution containing Iron(II) ions, a greenish-white precipitate of iron(II) hydroxide is formed. When added to Zinc(II) ions, a white precipitate of zinc(II) hydroxide is formed, which dissolves in excess caustic soda solution to form a colorless solution containing the complex ion [Zn(OH)4]²⁻. When added to Aluminum(III) ions, a white gelatinous precipitate of aluminum hydroxide is formed, which dissolves in excess caustic soda solution to form a colorless solution containing the complex ion [Al(OH)4]⁻.

Step-by-step solution

Identification of Iron(II) (\(\mathrm{Fe}^{+2}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Fe}^{+2}\) ions, a greenish-white precipitate of iron(II) hydroxide, \(\mathrm{Fe(OH)_2}\), forms. The reaction is given as follows: $$\mathrm{Fe}^{+2} + 2 \mathrm{NaOH} \rightarrow \mathrm{Fe(OH)_2} + 2 \mathrm{Na}^{+}$$

Identification of Zinc(II) (\(\mathrm{Zn}^{+2}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Zn}^{+2}\) ions, a white precipitate of zinc(II) hydroxide, \(\mathrm{Zn(OH)_2}\), forms. The reaction is given as follows: $$\mathrm{Zn}^{+2} + 2 \mathrm{NaOH} \rightarrow \mathrm{Zn(OH)_2} + 2 \mathrm{Na}^{+}$$ The precipitate of \(\mathrm{Zn(OH)_2}\) dissolves in excess caustic soda solution, forming a colorless solution containing the complex ion \(\mathrm{[Zn(OH)_4]}^{2-}\): $$\mathrm{Zn(OH)_2} + 2 \mathrm{NaOH} \rightarrow \mathrm{[Zn(OH)_4]}^{2-} + 2 \mathrm{Na}^{+}$$

Identification of Aluminum(III) (\(\mathrm{Al}^{+3}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Al}^{+3}\) ions, a white gelatinous precipitate of aluminum hydroxide, \(\mathrm{Al(OH)_3}\), forms. The reaction is given as follows: $$\mathrm{Al}^{+3} + 3 \mathrm{NaOH} \rightarrow \mathrm{Al(OH)_3} + 3 \mathrm{Na}^{+}$$ The precipitate of \(\mathrm{Al(OH)_3}\) will dissolve in excess caustic soda solution, forming a colorless solution containing the complex ion \(\mathrm{[Al(OH)_4]}^{-}\): $$\mathrm{Al(OH)_3} + \mathrm{NaOH} \rightarrow \mathrm{[Al(OH)_4]}^{-} + \mathrm{Na}^{+}$$

Key concepts

Caustic Soda in Cation Identification

Cation identification using caustic soda, or sodium hydroxide (NaOH), is a common method for analyzing metal ions in solutions. This approach helps determine the presence of cations such as

• Iron(II)
• Zinc(II)
• Aluminum(III)

In this process, NaOH is added to the solution containing the unknown cation. Depending on the cation, a precipitate may form, indicating the presence of a specific metal ion.
For example, when NaOH is mixed with Iron(II) ions, a distinct greenish-white precipitate forms. This is the iron(II) hydroxide, ext{Fe(OH)}_2. Although direct observation of color and texture changes can already provide clues, additional reactions in certain cases play a crucial role in confirming results.

Ionic Reactions with NaOH

Ionic reactions involving NaOH provide a pathway for identifying cations by forming insoluble hydroxides. These reactions occur when the metal ions combine with the hydroxide ions from NaOH.
For instance:

• With zinc ions ( ext{Zn}^{+2}), a white precipitate of ext{Zn(OH)}_2 is formed.
• Similarly, Iron(II) ions ( ext{Fe}^{+2}) react to form a greenish-white precipitate of ext{Fe(OH)}_2.
• Aluminum ions ( ext{Al}^{+3}) produce a white gelatinous precipitate of ext{Al(OH)}_3.

These results help identify the specific cations in a mixture.
This ionic exchange involves:

• Replacement of the sodium ions in the NaOH with the metal ions from the aqueous solution.
• The formation of insoluble hydroxides, leading to precipitation.

The nature of the precipitate further reinforces the identity of the cations involved.

Complex Ion Formation

Complex ion formation occurs when precipitates dissolve in excess NaOH, resulting in complex ions. For some metal ions, such as zinc and aluminum, this transformation indicates the possible formation of:

• Complex ions like [Zn(OH)_4]^{2-} and [Al(OH)_4]^{-}

In the case of zinc:

• ext{Zn(OH)}_2 dissolves when extra NaOH is added, forming ext{[Zn(OH)_4]}^{2-}.

Similarly for aluminum:

• ext{Al(OH)}_3 becomes soluble in excess NaOH, leading to ext{[Al(OH)_4]}^{-}.

These complex ions are soluble, unlike the original precipitates.
This behavior can confirm the presence of certain cations, as only specific metal ions will form these complex ions when reacting with excess NaOH.

Precipitation Reactions in Chemistry

Precipitation reactions are foundational in cation identification. They involve the formation of an insoluble solid, known as a precipitate, from a solution.
This chemistry concept is used extensively in identifying cations such as:

• Iron(II) with a greenish-white precipitate of ext{Fe(OH)}_2
• Zinc(II) with a white precipitate of ext{Zn(OH)}_2
• Aluminum(III) yielding a gelatinous ext{Al(OH)}_3

Precipitates typically form due to the low solubility of certain compounds in water when certain ions are present.
This process is useful for chemists:

• To isolate specific substances from mixtures.
• To visually confirm the presence of specific ions through known color changes and physical properties of precipitates.

Each metal ion has characteristic reactions that lead to distinct and observable precipitates, making cation identification efficient and clear.