Consider the following equilibrium reaction:
`[CoCl_4]^(2+) + underset"Blue"(6H_2O) hArr [Co(H_2O)_6]^(2+) + underset"Pink"(4Cl^-)`
The colour of the solution is blue. What will be the colour change when water is added to the system at equilibrium ?

Answer

To solve the problem, we need to analyze the equilibrium reaction and apply Le Chatelier's Principle to understand the effect of adding water to the system.

### Step-by-Step Solution:

1. **Write the Equilibrium Reaction**:
The given equilibrium reaction is:
\[ [CoCl_4]^{2+} + 6H_2O \rightleftharpoons [Co(H_2O)_6]^{2+} + 4Cl^{-} \]
In this reaction, the left side (reactants) contains the complex ion \([CoCl_4]^{2+}\) and water, while the right side (products) contains the complex ion \([Co(H_2O)_6]^{2+}\) and chloride ions.

2. **Identify the Initial Color**:
It is stated that the color of the solution is blue. This indicates that the equilibrium is currently favoring the formation of \([CoCl_4]^{2+}\), which is blue in color.

3. **Effect of Adding Water**:
When water is added to the system, we are increasing the concentration of one of the reactants (water). According to Le Chatelier's Principle, if a change is made to a system at equilibrium, the system will adjust to counteract that change.

4. **Application of Le Chatelier's Principle**:
Since we are increasing the concentration of water, the equilibrium will shift in the direction that consumes water to reduce its concentration. In this case, that means the equilibrium will shift to the right (towards the products).

5. **Determine the New Color**:
As the equilibrium shifts to the right, more \([Co(H_2O)_6]^{2+}\) and \(Cl^{-}\) are produced. The complex ion \([Co(H_2O)_6]^{2+}\) is pink in color. Therefore, as the reaction moves forward, the color of the solution will change from blue to pink.

### Final Answer:
When water is added to the system at equilibrium, the color of the solution will change from blue to pink.

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