During dissolution when solute is added to the solvent, some solute particles separate out from the solution as a result of crystallisation . At the stage of equilibrium, the concentration of solute in the solution at given temperature and pressure

Options

A. increases
B. decreases
C. remains constant
D. keeps changing

Explanation

To solve the question, we need to understand the concepts of dissolution and crystallization and how they relate to equilibrium in a solution.

### Step-by-Step Solution:

1. **Understanding Dissolution and Crystallization**:
- **Dissolution** is the process where solute particles dissolve in a solvent, leading to an increase in the concentration of solute in the solution.
- **Crystallization** is the reverse process where solute particles come out of the solution and form solid crystals, which decreases the concentration of solute in the solution.

**Hint**: Remember that dissolution adds solute to the solution, while crystallization removes solute from it.

2. **Equilibrium Condition**:
- At equilibrium, the rate of dissolution (the process of solute entering the solution) equals the rate of crystallization (the process of solute leaving the solution to form crystals).
- This means that the number of solute particles dissolving into the solvent is equal to the number of solute particles crystallizing out of the solution.

**Hint**: Think of equilibrium as a balance where the processes of adding and removing solute are happening at the same rate.

3. **Concentration at Equilibrium**:
- Since the rates of dissolution and crystallization are equal, the overall concentration of solute in the solution does not change.
- Therefore, at equilibrium, the concentration of solute remains constant.

**Hint**: If the rates of two opposing processes are equal, the overall effect on concentration will be stable.

4. **Conclusion**:
- Based on the explanation above, at the stage of equilibrium, the concentration of solute in the solution at a given temperature and pressure remains constant.

**Final Answer**: The correct option is that the concentration of solute "remains constant".