State the expression which attains a constant value in each of the following equilibria.

Urea (s) `hArr ` Urea (solution)

Answer

To find the expression that attains a constant value in the equilibrium between solid urea and urea in solution, we can follow these steps:

### Step 1: Understand the Equilibrium
In this equilibrium, we have solid urea (s) and urea dissolved in water (aqueous solution). The process can be represented as:
\[ \text{Urea (s)} \rightleftharpoons \text{Urea (solution)} \]

### Step 2: Identify the Processes
The two opposing processes occurring here are:
1. **Dissolution of Urea**: The process where solid urea dissolves in water to form urea in solution.
2. **Crystallization of Urea**: The process where urea in solution crystallizes back into solid urea.

### Step 3: Write the Equilibrium Expression
At equilibrium, the rate of dissolution equals the rate of crystallization. The equilibrium constant expression (K) for this physical equilibrium can be written as:
\[ K = \frac{[\text{Urea (solution)}]}{[\text{Urea (s)}]} \]

### Step 4: Consider the Concentration of Solids
In equilibrium expressions, the concentration of pure solids and liquids is not included in the expression because their activity is considered to be 1. Therefore, the concentration of solid urea is taken as 1.

### Step 5: Final Expression
Thus, the equilibrium expression simplifies to:
\[ K = [\text{Urea (solution)}] \]

### Summary
The expression that attains a constant value in the equilibrium between solid urea and urea in solution is:
\[ K = [\text{Urea (solution)}] \]

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