Which of the following elements would produce coloured ion in agueous solution?

Analyze the question: We need the element whose ion gives a coloured aqueous solution. The options are Calcium, Copper, Magnesium and Sodium.

Concept: Coloured ions are usually formed by transition metals. Their ions have partially filled d-orbitals. Electrons can absorb visible light and jump between d-orbital energy levels (d-d transitions). The light that is not absorbed is what we see, so the solution looks coloured.

Ions of s-block metals (Groups 1 and 2) have no d-electrons. They have a noble-gas configuration, so they give colourless solutions.

Solution:

• A. Calcium (Ca²⁺): s-block, configuration [Ar]. No d-electrons, so colourless.
• B. Copper (Cu²⁺): d-block transition metal, configuration [Ar] 3d⁹. It has a partially filled d-subshell, so the solution is blue. This is the familiar colour of copper(II) sulphate solution.
• C. Magnesium (Mg²⁺): s-block, configuration [Ne]. Colourless.
• D. Sodium (Na⁺): s-block, configuration [Ne]. Colourless.

Answer: Copper, option B.

Exam tip: Remember the common colours of transition metal ions:

• Cu²⁺ is blue
• Fe²⁺ is pale green
• Fe³⁺ is yellow or brown
• Mn²⁺ is pale pink
• MnO₄⁻ is purple
• Cr³⁺ is green

If the options include a Group 1 or Group 2 metal next to a transition metal, the transition metal is almost always the answer for "coloured ions".