Physical or Chemical Change?

Question

Dissolving ammonium chloride in water is a physical or chemical change?

Answer 1

 Hi Liz B.,

It's a little complicated, b/c physical and chemical are absolute terms in a non-absolute world. Let's distinguish between covalent bonds (such as we can characterize them!), and ionic, hydrogen, dipole-dipole, and dipole-induced_dipole bonds. We all agree that ripping apart two covalently-bound atoms is a chemical change. But, what's with the rest? Have we done a chemical change, if we dissolve an ionic solid? We have removed an individual ion from its neighbors and coordinated it (non-covalently) with solvent molecules, a reversible process unless we precipitate it as something else! And that includes the temporary changes of equilibration NH₄⁺ = NH₃ + H⁺ . But, we are also removing it from its ion neighbors if we melt it, which we consider a physical process, don't we. And likewise with the rest of those kinds of bonds. Or suppose we dissolve polar in nonpolar (e.g., HCl in methanol->ethyl acetate->toluene) -- it may be dispersed as ions, single molecules, or clusters, and so on, in various proportions, depending on the medium. And it doesn't necessarily associate much with a very non-polar medium! It seems absurd to consider just one HCl molecule as the "gold standard" for chemical identity, and say a particular solution is a 10%physical and 90%chemical process! My preference would be to say that solution is a physical process, therefore. Until you produce a solid that re-associates the ions with other chemical partners. -- Cheers, --Mr. d.

Answer 2

The answer to whether dissolving salt (in this case, NH₄Cl) in water is a physical change or a chemical change depends on what criteria one uses to define these changes. Many would say that it is a chemical change because the products of the reaction are chemically different from the reactants (NH₄Cl <=> NH₄⁺ + Cl^-; or NH₄Cl + H₂O <=> NH₄OH + HCl). However, others would argue that it is a physical change because the original reactants can be recovered and so have not changed chemically. Just evaporate the water and you end up with NH4Cl again.

Answer 3

A physical change is when a molecule keeps it's chemical formula whereas a chemical change changes the chemical formula. When Ammonium chloride is dissolved in water, NH4Cl changes into something else as shown by the chemical reaction:

 NH4Cl+H2O→NH3+HCl

Therefore, it's a chemical change.