Do ionic solids retain their inherent properties in their aqueous solutions?

Question

I am a highschool student and lately we were studying ionic equilibrium where I came across the concept of salt hydrolysis. We first started with salts of strong acids and strong bases and concluded that they have no effect on the ionic product of water and that they do not hydrolyse. Why?

What my teacher said was that since strong electrolytes completely dissociate in their aqueous solution, there must be a fixed number of ions present in the solution. For example, 1 mole of NaCl in water must dissociate to give 1 mole each of Na+ and Cl-, and there mustn't be separate existences of NaOH or HCl (formed by hydrolysis of NaCl), or NaCl, only the ions, and hence we can conclude that strong electrolytes or salts of strong acids and strong bases do not hydrolyse.

So, my question is, if the salts of SA and SB do not hydrolyse and exist as ions in their aqueous solution, do they retain their ionic-solid properties (I am phrasing it sooo poorly 😭) If we mix common salt in water, does it still retain it's properties as common salt, is it still common salt or is it just Na+ and Cl- ions now?

Answer 1

The physical and chemical properties of separate ions are not the same as that of the ions in a crystalline solid. Here are a few examples:

• State: At STP, $\ce{NaCl}$ is a crystalline solid. At STP, $\ce{Na+}$ and $\ce{Cl-}$ ions are a (solvated) liquid.
• Conductivity: At STP, $\ce{NaCl}$ is not a conductor of electricity (except above breakdown voltage, where crystal defects are introduced). At STP, $\ce{Na+}$ and $\ce{Cl-}$ ions conduct electricity well.
• Reactivity: Crystalline $\ce{NaCl}$ mixed with crystalline $\ce{AgNO3}$ does nothing. Add water, and you're left with $\ce{NaNO3 (aq)}$ and $\ce{AgCl ↓}$.

Perhaps the solvated $\ce{Na+}$ and $\ce{Cl-}$ ions have more in common with a plasma, as might be formed with $\ce{NaCl}$ applied to carbon electrodes, and heated in an arc. In water and in plasma, both respond to electromagnetic fields, both conduct electricity, and both can be used to make a magnetohydrodynamic generator (MHD generator) or MHD drive.

• Plasma-based MHD
• Salt-water MHD-propelled boat (at 5 min, see the boat) and salt-water MHD generator

Answer 2

Below the solubility limit of a salt there is no trace of the original ionic solid. Such a solution is homogeneous. However above the solubility limit undissociated salt coexists in equilibrium with the electrolyte. Such a saturated solution is heterogeneous.

Electrolyte solutions are in dynamic equilibrium. Hydrolysis reactions that convert sodium ions and water into $\ce{NaOH +H^+}$ or chloride ions and water into $\ce{HCl +OH-}$ might happen, but the products $\ce{NaOH}$ and $\ce{HCl}$ then quickly dissociate and hydroxyl ($\ce{OH-}$) and hydronium ($\ce{H+}$) quickly recombine to form water again*.

$\ce{NaOH}$ and $\ce{HCl}$, if present, are solvated species, which means they are surrounded by water molecules that interact non-covalently. Interactions with the solvent are indicated by writing (for instance) $\ce{HCl(aq)}$, where "aq" is shorthand for aqueous (solvated by water). But again, such species are highly unstable in water and will rapidly dissociate into ions, mainly because oxygen and chlorine are highly electronegative and readily remove an electron from sodium and hydrogen respectively, and the separated ions are strongly stabilized by interactions with polar water molecules which form a solvation sphere (or hydration shell).

Water itself dissociates into hydronium and hydroxyl ions (by reacting with other water molecules) but the equilibrium again strongly favors formation of water rather than the persistence of the ions above a limiting ion concentration (or rather product of concentrations, as determined by the dissociation constant of water). Above the limit the reverse reaction between available $\ce{OH-}$ and $\ce{H+}$ occurs with a very high likelihood and therefore faster than the forward self-dissociation.

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*Sodium ion might be seen here acting as a catalyst in the autodissociation of water, as indicated by writing $$\ce{H2O + Na+ -> NaOH +H^+ -> Na+ + OH- +H^+}$$, but these are speculations on my part. However catalysis of water autodissociation, for instance in bipolar membranes, is an active area of study. And given the importance of charge separation during autodissociation, the possible role of stabilizing charges would not be surprising.

Answer 3

Once sodium chloride is dissolved in water, you do not have the solid sodium chloride anymore, and strictly speaking you do not have monatomic sodium and chloride ions either. You have hydrated sodium and chloride ions, in which the ionic bonds in the original solid have been reloaced by multiple electrostatic interactions between tje ions and water molecules.

However, there are situations where you do have "traces of the solid" because the ions can become associated into ion pairs or clusters, as is the case with magnesium sulfate[1]. Such species may be regarded as incipient formation of a solid-phase structure (in this case, hydrated magnesium sulfate), but without the long-range order that would be present in a the fully solid salt.

Reference

1. Federico Sebastiani, Ana Vila Verde, Matthias Heyden, Gerhard Schwaab, and Martina Havenith (2020). "Cooperativity and ion pairing in magnesium sulfate aqueous solutions from the dilute regime to the solubility limit". Phys. Chem. Chem. Phys., 22, 12140-12153.