$$\ce{PbCl2 + H2O ->[\Delta] Pb(OH)Cl + HCl}\tag{1}$$ $$\ce{2ZnCO3 + H2O ->[\Delta] Zn2(OH)2CO3 + CO2}\tag{2}$$ $$\ce{2NiS + 2H2O → Ni2(OH)2S + H2S}\tag{3}$$ $$\ce{Hg2SO4 + H2O → HgO + Hg + H2SO4}\tag{4}$$

The above salts are all insoluble in water. Even in the saturated solution, their concentration of salts is very low. They are expected to undergo complete dissociation in water due to their low concentration.

$$\ce{insoluble~ salt <=>[water] constituent~ ions}$$

For e.g. let's take lead chloride:

$$\ce{PbCl2 <=> Pb^{2+} + 2Cl^-}$$

$\ce{K_{sp}}$ = $\ce{[Pb^{2+}][Cl^{-}]^2}$ = $\mathrm{1.6} \times 10^{-5}$

From the solubility product value, we can determine that lead chloride is insoluble in water. But what about the reaction above with water? Since, they do not dissolve in water, they do not react with water in any way except dissociation. So, can the reactions be neglected? Or should it be considered occurring while the salt is poured in water?

source :- reaction 1, reaction 2, reaction 3, reaction 4

  • $\begingroup$ Where have you got those reactions from? None of them describes something that would really happen in the way it is expressed there. And generally there are three possibilities: 1. (strong) reaction of the solid with water, 2. slow reaction because only the dissolved ions react, 3. nothing at all. $\endgroup$ – Karl Mar 25 '16 at 20:02
  • $\begingroup$ I think you're missing the point that soluble and insoluble are relative terms. Lead Chloride does have a very small amount of lead and chloride in solution. So you could think of the lead chloride dissolving and then lead hydroxide chloride precipitating. // The other thing is that the different salts have different crystal sizes/structures. Think of iron rust. The rust spalls off the iron. $\endgroup$ – MaxW Mar 27 '16 at 18:35

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