Question

Most transition metal ions are colored. For example, a solution of \(\mathrm{CuSO}_{4}\) is blue. How would you show that the blue color is due to the hydrated \(\mathrm{Cu}^{2+}\) ions and not the \(\mathrm{SO}_{4}^{2-}\) ions?

Short answer

The blue color is due to \(\mathrm{Cu}^{2+}\) ions, not \(\mathrm{SO}_{4}^{2-}\) ions.

Step-by-step solution

Observing the color of each component

To identify the source of the color, observe the color of individual components. Here, \\(\mathrm{CuSO}_4\) is composed of \(\mathrm{Cu}^{2+}\) ions and \(\mathrm{SO}_{4}^{2-}\) ions. Consider the fact that \(\mathrm{Cu}^{2+}\) ions in aqueous solutions typically appear blue, while \(\mathrm{SO}_{4}^{2-}\) ions are colorless.

Dissolving each component independently

Dissolve \(\mathrm{CuSO}_4\) in water and also obtain a solution of \(\mathrm{Na}_2\mathrm{SO}_4\) separately. \(\mathrm{Na}_2\mathrm{SO}_4\) contains \(\mathrm{SO}_4^{2-}\) ions but no transition metal ions.

Comparing the colors of the solutions

Compare the colors of the two solutions. The \(\mathrm{CuSO}_4\) solution appears blue, while the \(\mathrm{Na}_2\mathrm{SO}_4\) solution is colorless. This suggests that \(\mathrm{SO}_4^{2-}\) ions do not impart any color to the solution.

Conclusion

Conclude that the blue color observed in the \(\mathrm{CuSO}_4\) solution is due to the presence of \(\mathrm{Cu}^{2+}\) ions, as the \(\mathrm{SO}_4^{2-}\) ions, when isolated in a \(\mathrm{Na}_2\mathrm{SO}_4\) solution, do not contribute any color.

Key concepts

Hydrated Ions

When certain metal ions dissolve in water, they form what are known as hydrated ions. This occurs because water molecules are polar, with slight positive and negative charges. These water molecules attract and surround the ion, creating a sort of "hydration shell." For transition metals like copper, this often results in the formation of complex ions where water acts as a ligand, tightly bound around the metal ion.
This process is crucial because the hydrated form often exhibits different properties, including color, compared to its dry or "anhydrous" form.

• The effect of water molecules not only stabilizes the ions but can also affect their electronic structure.
• This is why compounds that seem colorless as solids can show vivid colors when dissolved.
• In essence, the interaction between the ion and water molecules can result in the absorption of specific wavelengths of light leading to specific color appearances.

Color of Solutions

The color of solutions containing transition metal ions like \(\mathrm{Cu}^{2+}\) depends largely on electronic transitions within the d-orbitals of the metal ion. Transition metals have partially filled d-orbitals which allow electrons to absorb specific wavelengths of light. This absorbed light removes certain colors from the visible spectrum, and what we see is the complementary color.
Let's consider the case of a copper ion in a solution where the blue color observed corresponds to the absorption of light in the red-orange part of the spectrum.

• These absorbed wavelengths are a direct result of the energy difference between specific d-orbital electron transitions.
• Each transition and corresponding energy difference are unique to the metal ion involved, resulting in various colors with different metals.
• This phenomenon underscores the distinct and visually striking appearances transition metal solutions often exhibit.

Copper Sulfate

Copper sulfate is a common chemical compound with the formula \(\mathrm{CuSO}_{4}\). It typically exists as a blue crystalline solid, often in its hydrated form \(\mathrm{CuSO}_{4} \, \cdot \, 5\mathrm{H}_2\mathrm{O}\), also known as blue vitriol. Hydration plays a significant role in its color.
In an aqueous solution, the \(\mathrm{Cu}^{2+}\) ions dissociate and become hydrated, leading to the characteristic blue hue.

• Copper sulfate solutions are notably used to demonstrate the principle of hydration and the effects on color.
• In its solid form, without hydrating water, copper sulfate is generally colorless or pale gray.
• This highlights the importance of hydration in revealing the typical blue color associated with copper solutions.

Sulfate Ions

Sulfate ions \(\mathrm{SO}_{4}^{2-}\) are a polyatomic ion consisting of one sulfur atom surrounded by four oxygen atoms in a tetrahedral arrangement. These ions are quite common in many salts, especially those that incorporate divisively colored transition metals.
However, alone, sulfate ions do not impart any color to solutions, as demonstrated by the colorless appearance of solutions like sodium sulfate.

• The absence of unpaired d-electrons in the sulfate ion is a key reason for their lack of color.
• This is why, when examining a solution of \(\mathrm{CuSO}_4\), it's clear that the blue color cannot be due to the \(\mathrm{SO}_{4}^{2-}\) ions.
• By comparing similar sulfate compounds, such as \(\mathrm{Na}_2\mathrm{SO}_4\), which are clear, we further confirm the absence of color from sulfate ions.