The process of dissolution can be considered to involve three steps. Which of the following is not one of these steps? (A) The separation of solvent–solute interactions. (B) The separation of solute–solute attractions. (C) The formation of solute–solvent interactions. (D) The separation of solvent–solvent intermolecular attractions.
This question is asking us to identify which of the steps listed in the answer choices is not a step in the process of dissolution. Dissolution is a process in which a solid solute dissolves into a liquid solvent forming a solution. But this is a simplified view of creating a solution. We are told in the question that dissolution actually involves three steps. Each step in the process involves an energy change. During the dissolution process, energy is absorbed to separate particles that are attracted to each other. In contrast, energy is released when new attractions form between particles. We’ll need to keep these two ideas in mind as we go through the three steps of dissolution.
The first step in the dissolution process is the separation of solvent particles. Let’s make the solvent in our example solution liquid water, and the particles shown here represent individual water molecules. In pure liquid water, there are strong intermolecular attractions between the water molecules, so the system must absorb energy to separate the water molecules. Answer choice (D) describes this first step of separating the solvent particles. Therefore, we can eliminate it as an incorrect answer choice.
The second step in the dissolution process is the separation of the solute particles. Let’s say that our solute is solid potassium chloride salt. The K+ and Cl− ions in the crystal lattice of solid KCl are held together by strong attractions. Therefore, the separation of these ions requires the input or absorption of energy. Answer choice (B) describes the separation of the particles in the solute. Therefore, we can eliminate it as an incorrect answer choice.
In the final step of the dissolution process, the solute and solvent particles mix together and interact with each other. The new attractions that form between the ions and the water molecules are made possible by the release of energy from the system. Answer choice (C) describes the formation of these new solute–solvent interactions. Therefore, we can eliminate it as an incorrect answer choice.
In our example, the diagram shown in step three represents the solution that formed when KCl dissolved completely in water. The new interactions between the water molecules and the K+ and Cl− ions are what make the formation of a solution possible, giving the solution its characteristic homogeneous composition.
Answer choice (A), the separation of solvent solute interactions, is not a part of the dissolution process. Therefore, answer choice (A) is the correct answer.