While revising Thermodynamics and Thermochemistry from revision material published by Arihant publications I encountered the following mentioned statement.

During dissolution, the physical state of the compound changes while during hydration, there is no change in the physical state of the compound.

  • How does dissolution result in a change in physical state?

  • Should not hydration (or solvation in general) be the same as dissolution as in both the bonds of solid salt to be dissolved or solvated weaken and a new bond is formed between constituents of salt and the solvent? Could someone please explain what I'm missing?


1 Answer 1


Before dissolution, the substance is usually a solid and forms crystals. Its physical state is solid. After dissolution, the substance is not visible any more : it is not a solid any more. Its physical state has changed.

Before hydration, the substance is usually solid. After hydration, the color and the volume may have changed, but the physical state has not changed : it is still a solid –

  • $\begingroup$ Hydration has 2 meanings. One is the hydration provided in this answer, e.g. $\ce{CaCl2(s) + 2 H2O(g) -> CaCl2 . 2H2O(s)}$. The other one is a part of dissolution, mainly of ionic compounds, wrapping "naked" ions by oriented layers of water, being a special case of solvation. $\endgroup$
    – Poutnik
    May 10, 2021 at 16:20
  • $\begingroup$ I indeed agree with @Poutnik 's comment. For example, differential hydration energy indeed refers to the energy release in solvating 1 mole of salt in excess of water. So in such a case is there a way ionic salt can stay a solid, or be even capable of maintaining a strong bond between both the ions? $\endgroup$
    – SteelCubes
    May 10, 2021 at 17:30

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