Why do some substances dissolve spontaneously while others do not?
Some substances dissolve spontaneously because dissolving leads to a lower-energy, more disordered state, while others do not dissolve because doing so would require too much energy or would decrease overall stability. Whether a substance dissolves is determined by two key factors: enthalpy change (energy absorbed or released) and entropy change (change in disorder). The balance between these determines if dissolving is thermodynamically favorable.
For dissolution to occur spontaneously, the overall energy change must be favorable. Dissolving involves breaking solute–solute forces, breaking some solvent–solvent forces and forming solute–solvent interactions. If the new interactions formed are strong enough to compensate for the energy required to break the old ones, dissolving can occur on its own. This is why ionic salts like NaCl dissolve easily in water: hydration energy from strong ion–dipole interactions offsets the energy required to separate ions from the crystal.
Entropy, the measure of disorder, also plays a crucial role. When a structured solid dissolves in a liquid, the particles disperse and become more randomly distributed, increasing entropy. Systems naturally tend toward greater disorder, so this increase often drives spontaneous dissolving even when the energy change is not ideal. This explains why some endothermic dissolutions—like dissolving ammonium nitrate—still occur spontaneously.
However, not all substances dissolve spontaneously. If breaking solute–solute forces requires too much energy and the resulting solute–solvent interactions are weak, dissolving becomes unfavorable. For example, oil does not dissolve in water because polar water molecules cannot form strong enough attractions with nonpolar oil to compensate for disrupting their hydrogen-bonded structure. This produces both a poor enthalpy outcome and little entropy gain, making dissolving nonspontaneous.
Additionally, gases often do not dissolve spontaneously in warm water because doing so decreases entropy. Gas particles lose freedom when trapped in a liquid, and if the system does not gain enough stabilizing energy, dissolving becomes unfavorable.
In summary, substances dissolve spontaneously when the combination of energy changes and entropy increase makes the process thermodynamically favorable.
Frequently Asked Questions
Can a substance dissolve even if the process absorbs heat?
Yes. If the entropy increase is large enough, endothermic dissolutions can still be spontaneous.
Why doesn’t oil dissolve in water?
Because water cannot form sufficiently strong interactions with oil molecules, making the process energetically and entropically unfavorable.
Do all spontaneous processes release energy?
No. Spontaneity depends on free energy, not just enthalpy, so some spontaneous processes absorb heat.
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