# How is dissolving a physical change and not a chemical change?

When salt dissolves in water it dissociates and it is no longer in a lattice structure. The electrostatic forces are no longer there and are replaced by water-ion interactions. To me that sounds like old bonds breaking and new bonds forming. Shouldn't the reaction look something like this? $$\ce{NaCl + H2O -> Na+ + Cl- + H2O}$$

• A process where bonds are broken and made is classified chemical, a process where no chemical bonds are involved is classified as physical. What about processes like the one above where there is only breaking or making? Maybe the issue is in the classification of hydration. – Brinn Belyea Jun 24 '15 at 14:21

This is indeed a bit of a grey area. Consider the following: chemical or physical?

$$\ce{Na+ Cl- (aq) + Ag+ NO3- (aq) -> Ag+Cl- (s) + Na+ NO3- (aq)}$$

There is a tendency in more advanced chemistry to view the formation / breaking of ionic lattices as a physical change, because there are no differences in the electron shells of the atoms. Ions are considered as independent species. As such we could consider the sodium and nitrate irrelevant and reduce the above to simply

$$\ce{Ag+ (aq) + Cl- (aq) -> Ag+ Cl- (s)}$$

Note that I have shown the charges all the way through, to indicate that there are no covalent, dative, or metallic bonds made or broken.

Reactions such as these are definitely "chemical":

$$\ce{Na + \frac{1}{2}Cl2 -> Na+ Cl-}$$
(metallic bonds in $\ce{Na}$ and covalent bonds in $\ce{Cl2}$ broken, electron transferred from $\ce{Na}$ to $\ce{Cl}$.)

$$\ce{C2H4 + H2 -> C2H6}$$ (double bond of $\ce{C2H4}$ opened up, bond of $\ce{H2}$ broken, new covalent bonds formed.)

$$\ce{SOCl2 + 2H2O -> H2SO3 + 2HCl}$$ (covalent bonds broken, new bonds made.)

This last reaction sets up another point. It is usually done with excess water, so the products (which are themselves acids) will then dissolve in water, thereby protonating it:

$$\ce{H2SO3 + H2O -> H3O+ + SO3H-}$$ $$\ce{HCl + H2O -> H3O+ + Cl-}$$

These particular dissolutions are more likely to be seen as a chemical change than the dissolution of $\ce{Na+Cl-}$, because there is a rearrangement of covalent bonds (even though it's only a $\ce{H+}$ that moves from one molecule to the other.)

Finally, $\ce{H2SO3}$ has another equilibrium, again involving covalent bonds being broken and made, and therefore a chemical change: $$\ce{H2SO3 -> H2O + SO2 (g)}$$

Conclusion

Making or breaking of metallic, covalent or dative bonds, or the formation/destruction of new ions or molecules, is always considered a chemical change.

The simple dissolution of an ionic salt, where none of the above mentioned bonds are made or broken, and no new ions or molecules are formed/destroyed, is frequently considered a physical change.

Note the care taken to always write $\ce{Na+Cl-}$ (and never $\ce{NaCl}$) in this answer.

• I have improved the formatting of your post using $\LaTeX$. For more information on how to do this yourself please see here and here. – bon Jun 24 '15 at 17:20
• When solvating $\ce{Na+}$, you create dative bonds from water’s oxygen to sodium. That kind of counteracts your conclusion. Otherwise I like your answer. – Jan Jun 24 '15 at 18:04
• @Jan Another grey area. Whereas H+ (which has extremely high charge density) definitely exists in aqueous solution strongly associated with one water molecule, forming the molecular ion H3O+, Na+ exists in aqueous solution surrounded by several water molecules, forming a weak association with each of them. A better example of what you are referring to would be a small multiply charged ion such as Zn2+ or Al3+ with higher charge density, which form closer to dative bonds with water, and undeniably form dative with bonds with OH- ions when an acidic solution is basified. – Level River St Jun 24 '15 at 18:29
• @LevelRiverSt What about $\ce{CO2 (aq) \rightleftharpoons CO2(g)}$? Is this a reaction (it has also an equilibrium constant)? – ado sar Oct 13 '20 at 16:54

The reaction you have written is correct. The physical vs. chemical change issue has to do with the presence/absence of substances with new compositions not previously present appearing after the change, and I would argue that the dissolution reaction you present represents both a physical and chemical change: Physical due to the change in properties/state of NaCl, and chemical due to the appearance of aqueous species not present initially.

The dissolution of NaCl in water is a physical change. Because, it's a reversible process. If water evaporates, you get back the salt.

The dielectric constant $\epsilon_r$ of water is 80 compared with 1 for air. So, the attractive electrostatic forces between ions are weakened by a factor of 80 when salt is added to water. We say the salt NaCl is dissociated. The molecules of water are rearranged around the dissociated ions and we get the hydrated ions, $\ce{Na^+_{aq}}$, $\ce{Cl^{-}_{aq}}$

• What if we have a solution that can't be separated? How would the dissolution be considered a physical change in that case? – tkhanna42 Jun 24 '15 at 16:23
• Let's consider the dissolution of sugar in water. Here, the molecules of sugar do not dissociate. But, they are stabilized in the solvent by intermolecular forces (Hydrogen bonds and Van der Waals forces). The process is always reversible. It's a physical process. – Yomen Atassi Jun 24 '15 at 16:45
• So is your reason ‘reversable = physical process’? – Jan Jun 24 '15 at 18:03
• yes. Physical change: a reversible process+formation of new substances. – Yomen Atassi Jun 24 '15 at 18:08
• Some chemical changes are reversible. For example any acid base reaction is reversible(although the forward is preferred over the reverse). The formation of an alkyl halide is another reversible chemical change – Caters Jun 24 '15 at 20:52