Question

Water turns blue cobalt chloride paper:
A) No change
B) Red
C) Pink
D) orange

Answer 1

Hint: Cobalt chloride paper is used to test for the presence of water which may be leaking through tiny cracks carrying water. In presence of the cobalt chloride forms the hexahydrate compound which is pink in colour.

Complete step by step answer:
Cobalt (III) chloride $\text{(CoC}{{\text{l}}_{\text{3}}}\text{)}$ is an inorganic compound of cobalt and chlorine. It is a sky blue colour compound. It is one of the commonly used compounds in the lab. It exists in the form of several hydrates. The general formula is $\text{ CoCl }\text{. n}{{\text{H}}_{\text{2}}}\text{O }$. It easily undergoes hydration and dehydration causes the colour change and thus it is used as the indicator for water in desiccants.

Cobalt chloride changes the colour in response to the humidity. As the humidity increases the cobalt chloride changes the colour from sky blue to the purple and then to the pink.
On exposure to the water or the water vapours, the cobalt chloride absorbs the water molecule. The cobalt chloride structure rearranges itself to make a room for water molecules.
First, the two water molecules surround each cobalt atom and result in the formation of dehydrate. Cobalt dehydrate is purple. The reaction for the hydration of cobalt chloride is represented as follows:

$\text{ }\begin{matrix}
   \text{CoC}{{\text{l}}_{\text{2}}} & \text{+ 2}{{\text{H}}_{\text{2}}}\text{O} & \to & \text{CoC}{{\text{l}}_{\text{2}}}\text{.2 }{{\text{H}}_{\text{2}}}\text{O} \\
   \text{(Blue)} & {} & {} & \text{(Purple)} \\
\end{matrix}$

As the crystal is further exposed to the water the crystal structure changes. This time the four more water molecules surround the cobalt resulting in the formation of hexahydrate cobalt chloride.
The reaction for the formation of hexahydrate as follows:
$\text{ }\begin{matrix}
   \text{CoC}{{\text{l}}_{\text{2}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O} & \text{+ }{{\text{H}}_{\text{2}}}\text{O} & \to & \text{CoC}{{\text{l}}_{\text{2}}}\text{.6 }{{\text{H}}_{\text{2}}}\text{O} \\
   \text{(Purple)} & {} & {} & \text{(Pink)} \\
\end{matrix}$

The hexahydrate complex of cobalt chloride is pink in colour. Such distinct change in colour makes cobalt chloride useful as the humidity indicator in weather instruments.
Therefore, the water turns the blue cobalt chloride paper into the pink.
So, the correct answer is “Option C”.

Note: On heating, the hydrates lose its water molecules and thus cobalt chloride turns to blue which is water free form. Metal hydrates have a distinct property to form coloured hydrates. For example, copper sulfate pentahydrate is a blue colour solution.