Chemistry 2008 JAMB Past Questions - Question 14: When a few drops of water is added to a blue anhydrous cobalt (II) chloride, the colour changes to

Correct Answer

B

Explanation

When a few drops of water are added to blue anhydrous cobalt(II) chloride, the color will change to **pink**. This fascinating transformation happens due to the compound's **hydration.**

Here's the breakdown:

- **Anhydrous:** This means the cobalt(II) chloride is free of water molecules. In this state, its crystal structure allows specific light absorption, resulting in the **blue color.**
- **Hydration:** When water is added, the water molecules interact with the cobalt(II) chloride, forming a **hydrated salt** (typically CoCl2·6H2O). This changes the arrangement of atoms and the way light interacts with them, leading to a **shift in the absorbed wavelengths.**
- **Pink color:** This new interaction causes the hydrated cobalt(II) chloride to primarily absorb **green and blue light**, while reflecting **red and violet**. Our eyes perceive the reflected colors, hence the observed **pinkish hue.**

This **reversible reaction** makes cobalt(II) chloride a valuable **moisture indicator.** It has practical applications in various fields, like:

- **Desiccants:** Cobalt(II) chloride is incorporated into some desiccants like silica gel, changing color from blue to pink as they absorb moisture, indicating their "spent" state.
- **Invisible ink:** When dissolved in water, it can be written invisibly. Heating the paper causes it to turn blue, revealing the message.

Remember, the exact shade of pink may vary depending on the concentration of water and the surrounding environment.