When anhydrous cobalt chloride paper is exposed to water, what color change is observed?

Analysis

The question asks for the color change that occurs when anhydrous cobalt(II) chloride ($CoCl_2$) reacts with water. This is a classic test for the presence of water in chemistry.

Concept

• Anhydrous Cobalt(II) Chloride ($CoCl_2$): The term "anhydrous" means "without water." In this state, cobalt(II) chloride exists as a blue crystalline solid.
• Hydrated Cobalt(II) Chloride ($CoCl_2 \cdot 6H_2O$): When water is added to the anhydrous salt, it absorbs the water molecules to form a hydrated complex. This hydration process changes the electronic environment of the cobalt ions, resulting in a distinct color change to pink.
• The Reaction: $CoCl_2(s) \text{ (blue)} + 6H_2O(l) \rightarrow CoCl_2 \cdot 6H_2O(s) \text{ (pink)}$

Step-by-Step Solution

1. Identify the starting state: Anhydrous cobalt chloride is blue.
2. Identify the process: Exposure to water causes the substance to become hydrated.
3. Identify the final state: Hydrated cobalt chloride is pink.
4. Determine the direction of change: The color shifts from the initial state (blue) to the final state (pink).

Conclusion

Comparing this to the provided options, the change is from blue to pink.

Exam Tip: Remember the mnemonic "Blue to Pink, Water's in the sink!" This helps you recall that the blue anhydrous paper turns pink when it detects moisture. Also, note that this process is reversible; if you heat the pink hydrated cobalt chloride, it will lose its water of crystallization and turn back to blue.