Chemistry 2002 JAMB Past Questions - Question 25: Steam changes the colour of anhydrous cobalt (II) chloride frm

Correct Answer

A

Explanation

Anhydrous cobalt(II) chloride (CoCl2) is a compound that can exhibit different colors depending on its oxidation state and the presence of water molecules. Cobalt(II) chloride is known for its ability to undergo a reversible color change between blue and pink, depending on its hydration state.

1. **Anhydrous Cobalt(II) Chloride (CoCl2):**
  - **Color:** Anhydrous cobalt(II) chloride is typically blue in color.

2. **Hydrated Cobalt(II) Chloride (CoCl2·nH2O):**
  - **Color Change:** When anhydrous cobalt(II) chloride reacts with water or absorbs moisture from the air, it forms a hydrated complex. The most common hydrated form is cobalt(II) chloride hexahydrate (CoCl2·6H2O).
  - **Color:** In its hydrated form, cobalt(II) chloride is pink.

The color change is due to the alteration of the crystal structure and electronic configuration of the cobalt ions upon hydration. The hydrated form has a different coordination environment around the cobalt ions, leading to a change in the absorption of light and, consequently, a change in color.

This property of cobalt(II) chloride is often used as a moisture indicator in chemistry laboratories. The reversible color change makes it useful for detecting the presence or absence of water.