Understanding the Process of Crystallisation

The question describes a process that occurs when a solid solute is added to a solvent, creating a solution. It specifically focuses on what happens when solute particles that are already dissolved in the solution interact with the undissolved solid solute particles.

Let's look at the options provided:

• Dissolution: This is the process where a solid solute dissolves in a solvent to form a solution. Solute particles separate from the solid and disperse throughout the solvent.
• Solubilisation: This term often refers to increasing the solubility of a substance, sometimes with the help of other agents.
• Crystallisation: This is the process where a solid crystalline phase separates from a solution, melt, or more rarely deposited directly from a gas. In the context of a solution, it's the process where dissolved solute comes out of the solution and forms a solid. This can happen when the solution is saturated or supersaturated, or when dissolved particles encounter existing solid surfaces (like undissolved solute) and deposit onto them.
• Deionisation: This is the removal of ions from a solution, typically using processes like ion exchange.

The question states that dissolved solute particles collide with the solid solute particles and "get separated out of solution". This means the dissolved particles are leaving the solution state and returning to a solid state. This process of a dissolved substance coming out of solution to form a solid is known as crystallisation (or precipitation, though crystallisation implies formation of a crystalline solid). The interaction with existing solid solute particles can act as nucleation sites, promoting the deposition of dissolved solute and its separation from the solution.

Therefore, the process where dissolved solute particles collide with solid solute and separate out of the solution is known as crystallisation.