Which of the following ionic compounds forms a distinctly colored aqueous solution due to the presence of transition metal ions?

Introduction / Context:
Many aqueous solutions of ionic compounds appear colorless, but some show vivid colors because of the presence of transition metal ions. Recognizing which salts give colored solutions is useful in qualitative analysis and helps students connect electronic structure to observed color. This question asks you to identify which of the listed compounds forms a colored aqueous solution.

Given Data / Assumptions:

• Listed compounds: CrCl3, KBr, CaCl2, and NaOH.
• All are assumed to be dissolved in water to form aqueous solutions.
• We rely on the usual behavior of transition metal ions and simple main group ions in solution.

Concept / Approach:
Transition metal ions often form colored solutions because d electron transitions absorb specific wavelengths of light. Chromium is a transition metal, so its ions typically produce colored solutions. In contrast, ions from s block metals such as potassium, sodium, and calcium usually produce colorless solutions in water. Therefore, the compound that contains a transition metal cation is the most likely to give a colored aqueous solution.

Step-by-Step Solution:
Step 1: Identify the cations in each compound: CrCl3 has Cr3 plus, KBr has K plus, CaCl2 has Ca2 plus, and NaOH has Na plus. Step 2: Recognize that chromium, Cr, is a transition metal with partially filled d orbitals. Step 3: Recall that solutions of chromium(III) salts such as CrCl3 are characteristically colored, often green or violet depending on ligands. Step 4: Note that potassium, calcium, and sodium are main group metals whose simple ions usually yield colorless aqueous solutions. Step 5: Conclude that CrCl3 is the compound that forms a colored aqueous solution.

Verification / Alternative check:
You can verify this by thinking about flame tests or laboratory observations. Salts of sodium and potassium typically show characteristic flame colors but their aqueous solutions are nearly colorless. Calcium salts may show a faint effect in flame tests but their solutions appear clear. However, when chromium(III) chloride is dissolved in water, the resulting solution is clearly colored due to d d transitions. Laboratory manuals and inorganic chemistry texts confirm that transition metal ions like Cr3 plus are responsible for solution color.

Why Other Options Are Wrong:
Option B, KBr, yields potassium and bromide ions, both of which lead to a colorless solution under normal conditions. Option C, CaCl2, produces calcium ions, which do not give a strong solution color. Option D, NaOH, dissociates to sodium and hydroxide ions, forming a colorless solution although it is strongly basic. These options lack transition metal ions, so they do not produce distinctly colored aqueous solutions like CrCl3 does.

Common Pitfalls:
Students may confuse flame colors with solution colors and think that any metal giving a colored flame will also give a colored solution. Another pitfall is to overlook the special role of transition metals and instead focus only on the anions. Remember that color in ionic solutions is often due to transition metal cations with partially filled d subshells. Keeping this idea in mind helps you quickly identify compounds like CrCl3 as sources of colored solutions.

Final Answer:
The compound that forms a colored aqueous solution is CrCl3 because it contains the transition metal ion Cr3 plus.