In the same way, the dissolution of any ionic species in water must also be regarded as chemical change. So the moral? On the criteria advanced, the dissolution of #NaCl# in water is quite clearly an example of chemical change. Note that there are many chemical reactions that are reversible. The formation of 6 such ion-dipole bonds is almost precisely equal to the lattice enthalpy of solid sodium chloride, and the reaction is readily reversible. Nevertheless, because there are 6 such ion-dipole interactions (and there is also aquation of the negative ion to give #^-#), dissolution in water is thermodynamically feasible. The interaction between the water molecule, and the sodium ion is an ion-dipole interaction, which are hardly as strong as ion-ion interactions.

The reaction is very slightly endothermic (we'll come to why in a minute), nevertheless it is clear that (iii) is certainly a bond-breaking reaction, in which very strong electrostatic forces between #Na^+# and #Cl^-# ions have been disrupted.īut what do we mean by #NaCl(aq)#? This is in fact the aquated sodium ion, which we could represent as, #^+#, that is a sodium ion coordinated to 6 water molecules, what an inorganic chemist would call a complex ion. Inorganic chemists, however, would routinely designate this reaction as an example of chemical change. It is tempting to also designate (iii), the dissolution of salt in water, as a physical change. #H_3COH(l) + H_2O(l) rarr H_3COH(aq)# (ii)Īs regards reaction (ii), we know that methanol is infinitely miscible with water (in fact if you rigorously dry methanol and then expose it to the air on a humid day, you can actually see the volume of the methanol expand as it absorbs air-borne moisture) nevertheless, this reaction is certainly physical inasmuch as no new bonds are formed (except for the very weak dispersion forces between methanol and water). The issue becomes much more ambiguous when we consider dissolution reactions.

The designation of this reaction as a physical change is quite unambiguous. We could also write the equation in this form to show how energy transfers in the reaction:Ĭondensation of steam is thus recognized as an exothermic process, or alternatively, vaporization of water is an endothermic process. On the other hand when water is heated to give steam, or steam condensed to give water, no bonds are broken, but a change in state occurs. A new product is formed, and this equation is unambiguously a chemical change. Here both #H-H# and #Cl-Cl# bonds are broken, and #H-Cl# bonds are formed. For (i) it is easy to see which bonds are broken and which bonds are formed: So at the risk of starting a long, protracted argument, I'll give you my definitions, and try to supply examples of each.Ĭhemical change is characterized by the formation of new substances and the making and breaking of strong chemical bonds. There seems to be a lot of confusion in the classification of physical versus chemical change, and you will see a lot of differing answers on Socratic when a student asks is this particular reaction a physical or chemical reaction.

Chemical changes