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I have book that contains the following paragraph:

Due to high dielectric constant, it (water) has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water.

Is this statement self-contradictory? Doesn't 'hydolysed in water' mean dissolved in water?

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  • $\begingroup$ Absolutely not. $\ce{NaCl}$ dissolves, but never hydrolyzes. $\ce{Al2S3}$ hydrolyzes, but never dissolves. $\endgroup$ – Ivan Neretin Dec 30 '15 at 8:43
  • $\begingroup$ What's the difference ? $\endgroup$ – user22729 Dec 30 '15 at 8:44
  • $\begingroup$ I think that under the IUPAC definition of dissolution, hydrolysis is (or at least can be) a type of dissolution. $\endgroup$ – Nicolau Saker Neto Dec 30 '15 at 13:31
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Hydrolysis is the chemical breakdown of a compound due to reaction with water.

Dissolution is a process by which the original states of matter turn into a liquid or other solvent and become solutes - dissolved components in solution.

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  • $\begingroup$ Thanks for the response , but I don't think it answered my question effectively. It will be a great help for me if you could explain the statement given in the book(written in the blockquotes). $\endgroup$ – user22729 Dec 30 '15 at 8:43
  • $\begingroup$ OK wait for a while.. $\endgroup$ – solanki... Dec 30 '15 at 8:53
  • $\begingroup$ Water has high dielectric constant because due its polar nature, it has more tendency to decrease the attraction between the ions in an ionic compound. It means that water is dissociating or ionising the ionic compound into its constituent particles i.e. it is known as hydrolysis which is done on some ionic and covalent compounds (not for all ionic compounds). $\endgroup$ – solanki... Dec 30 '15 at 9:07
  • $\begingroup$ Dissolution simply means mixing. As all ionic compounds can mixed in water to from a solution which my be homogenous or heterogenous. IF you like my answer, please up vote me. $\endgroup$ – solanki... Dec 30 '15 at 9:09
  • $\begingroup$ On hydrolysis , the ionic compound is split into its constituent ions but on dissolution , the ions are mixed with water so what forms is not the original ions...Am I right ? $\endgroup$ – user22729 Dec 30 '15 at 9:12
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Hydrolysis is a broad chemical term that means reacting with the solvent water in some way. So for examples esters can be hydrolysed according to the following reaction:

$$\ce{H3CCOO-C2H5 + H2O -> H3CCOOH + HO-C2H5}$$ (Ethyl acetate and water react to form acetic acid and ethanol.)

Dissolution is the process of something being dissolved in water, intermixing with the water molecules so that each atom/ion/molecule of the substance is more or less completely surrounded by water molecules. This process is usually understood to happen without any reaction, thus the term hydrolysis is not used.

However, the above is often a simplification. For example, some water-free salts such as $\ce{AlCl3}$ or $\ce{CuSO4}$ will react with the water to form the corresponding hexaaquacomplexes $\ce{[Al(H2O)6]^3+ + 3 Cl-}$ or $\ce{[Cu(H2O)6]^2+ + SO4^2-}$. These are technically reactions, best exemplified in the aluminium example where the metal was originally coordinated by chlorido ligands and after dissolution is coordinated by aqua ligands.

The only thing that changes is the immediate vicinity of the metal ion in these cases, so speaking of dissolution rather than hydrolysis (essentially the metal cation remains the same as do the corresponding anions) is often tolerated.

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  • $\begingroup$ So, putting it in a simple language, dissolution involves no reaction, just the constituent atom/molecule/ion of the substance is surrounded bu water molecules, whereas, hydrolysis involves the reaction. This is what I got from your answer. Am I correct ? $\endgroup$ – user22729 Jan 9 '16 at 7:14
  • $\begingroup$ @user3503781 yeah. $\endgroup$ – Jan Jan 18 '16 at 13:17

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