How does the formation of a solution involve energy?

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During the formation of a solution energy is either released ( $\textbf{exothermic reaction}$ ) or absorbed ( $\textbf{endothermic reaction}$ ).

In order for a solution to be formed the attraction between solute particles and the attraction between solvent particles must be broken. Breaking these attractions requires energy.

When these attractions are broken, new attractions form between solute and solvent particles. The formation of these new attractions releases energy.

The difference between these energies will dictate whether the reaction will be exothermic or endothermic.

For example when sodium hydroxide ( $NaOH$ ) is dissolved in water, the energy it releases when the new attractions between solvent and solute particles form is greater than the energy required to break the attractions between individual solvent and individual solute particles. This means that this reaction will be exothermic (the solution will become warmer).

When ammonium nitrate ( $NH_{4}NO_{3}$ ) dissolves in water, the energy required to break the attractions between individual solvent and individual solute particles is greater than the energy it releases when the new attractions between solvent and solute particles form. This means that this reaction will be endothermic (the solution will become colder).