Which of the following elements would produce coloured ions in aqueous solution?

Step 1: Analyze the question We need the element whose ions give a coloured solution in water.

Step 2: The concept Coloured ions are typical of transition metals. They have partially filled d-orbitals. When light hits the ion, an electron can jump between d-orbitals (a d-d transition). This absorbs some wavelengths of visible light, so the solution appears coloured.

Ions with no d-electrons, or with a completely filled d-subshell, have no d-d transitions. Their solutions are colourless.

Step 3: Check each option

• A. Calcium (Ca²⁺): configuration [Ar] after losing 2 electrons. It has no d-electrons, so it is colourless.
• B. Iron (Fe²⁺ / Fe³⁺): a d-block element. • Fe²⁺ is [Ar] 3d⁶ and gives a pale green solution. • Fe³⁺ is [Ar] 3d⁵ and gives a yellow-brown solution. Both have partially filled d-orbitals, so they are coloured.
• C. Magnesium (Mg²⁺): like neon, with no d-electrons. It is colourless.
• D. Sodium (Na⁺): like neon, with no d-electrons. It is colourless.

Step 4: Answer Only iron is a transition element, so it is the only one that forms coloured ions in aqueous solution.

Correct option: B (Iron)

Exam tip: Calcium, magnesium and sodium are s-block metals, and their ions are almost always colourless in WASSCE questions. If you see Fe, Cu, Mn, Co, Ni, Cr or Zn's neighbours, think "transition metal, so coloured". Common colours to memorise:

• Cu²⁺: blue
• Fe²⁺: green
• Fe³⁺: yellow/brown
• MnO₄⁻: purple

(Zn²⁺ is a catch: it is in the d-block but has a full 3d¹⁰ shell, so it is colourless.)