Question

Which one of the following when dissolved in water gives coloured solution in nitrogen atmosphere ?

• A. \(ZnCl_2\)
• B. \(CuCl_2\)
• C. \(Cu_2Cl_2\)
• D. \(AgCl\)

Hint:

To find if an ion is coloured, calculate its oxidation state and check its d-electron count. \(d^0\) and \(d^{10}\) = Colourless; \(d^1\) to \(d^9\) = Coloured.

Answer 1

The Correct Option is B

To determine which compound gives a colored solution when dissolved in water, we need to analyze the chemical nature of each option:

1. \(ZnCl_2\): Zinc chloride in aqueous solution forms colorless solutions. Zinc ions (\(Zn^{2+}\)) do not exhibit color because they have fully filled d-orbitals (\(d^{10}\) configuration) and no d-d transitions that absorb visible light.
2. \(CuCl_2\): Copper(II) chloride dissolves in water to form a blue-green colored solution. This is due to the presence of copper(II) ions (\(Cu^{2+}\)) that have partially filled d-orbitals (\(d^9\) configuration) and can absorb certain wavelengths from the visible spectrum, thus appearing colored.
3. \(Cu_2Cl_2\): Copper(I) chloride generally results in a white or colorless solution because copper(I) ions (\(Cu^+\)) have a \(d^{10}\) configuration, similar to zinc, leading to no color.
4. \(AgCl\): Silver chloride remains largely insoluble in water, and even when it partially dissolves, the solution formed is colorless. Silver ions also have a \(d^{10}\) configuration.

Considering the above explanations, the compound \(\(CuCl_2\)\), when dissolved in water, gives a colored solution due to the d-d transition of copper(II) ions. This makes option \(B: CuCl_2\) the correct answer.