✔ 最佳答案
The process of dissolving a solid in a solvent can be divided into two steps :
Step 1 :
The solid lattice of the solute is broken down into separated particles. For example, sodium chloride lattice is broken down into separated sodium ions and chloride ions.
Obviously, energy is needed in step 1. In other words, step 1 is endothermic.
Step 2 :
The separated particles of the solute mix with the molecules of the solvent. Due to the difference in amount, the separated particles of the solute are surrounded by the molecules of solvent. Attractions between the separate particles of the solute and the molecules of the solvent are formed. This step is known as solvation. (If water is used as the solvent, this step is also known as hydration.) In salvation, energy is released. In other words, step 2 is exothermic.
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Whether the dissolving of a solid in a solvent is endothermic or exothermic ?
1. When the energy needed to break the lattice of the solute (in step 1) is more than the energy released in solvaton (in step 2), the process of dissolving is endothermic.
2. When the energy needed to break the lattice of the solute (in step 1) is less than the energy released in solvation (in step 2), the process of dissolving is exothermic.