Question: I just read that physical changes are temporary and reversible while chemical changes are permanent and irreversible. If that is the case, why are some chemical reactions reversible. Does it mean that chemical changes are reversible?

Answer: 
I am actually pretty old to this subject and still I cannot really answer, because **the boundary between physical changes and chemical reactions is indistinct.** Essentially anything that involves rearrangement at an atomic or molecular level, even something as seemingly straightforward as melting ice or dissolving sugar into water, has characteristics of a chemical reaction.

Let us compare the melting of ice with something that is obviously a chemical reaction. For the latter, we choose the reaction of elemental carbon at high temperature with carbon dioxide gas, forming carbon monoxide:

$\text{C(s) + CO}\_2\text{(g)}\overset {\Delta}{\to} 2\text{CO(g)}$

First off, both processes are advanced by heating the material. True, carbon requires a higher temperature to react at an appreciable rate with carbon dioxide than ice requires to melt, but this amounts only to a difference in degree rather than a difference in kind of process. Moreover, and more to the point, both the "physical change" of melting ice and the "chemical reaction" of carbon with carbon dioxide can be reversed upon cooling; in the latter case it can lead to a real industrial problem of furnace parts being covered with soot in the *cooler* parts of the furnace.

One might suppose that we could differentiate the above processes hecause the reaction of carbon with carbon dioxide surely involves the breaking and making of bonds. True, the bonds in carbon monoxide are arranged quite differently from those in the carbon and carbon dioxide from which it can be made. But the same holds true with melting ice, which involves the breaking of hydrogen bonds. The [molecular orbital description of hydrogen bonds[1]](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4764947/#!po=1.11111) identifies this hydrogen bonding as a form of (delocalized) covalent bonding -- weaker bonds than those rearranged when carbon and carbon dioxide are heated in a furnace, but again there is only a difference in degree and not in the fundamental type of process.

So there is no clear boundary between physical and chemical changes. Physical changes can be reversible, and so can chemical reactions. Chemical reactions may be essentially irreversible, and (as I look at the scar permanently left on my arm after a long-ago surgery) so can physical changes.

**Reference**

1. Bo Wang, Wanrun Jiang, Xin Dai, Yang Gao, Zhigang Wang, and Rui-Qin Zhangb (2016). "Molecular orbital analysis of the hydrogen bonded water dimer". *Sci Rep.* **6**: 22099. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4764947/#!po=1.11111>
A chemical reaction is reversible when and if it reaches chemical equilibrium; [A steady state can possibly be a close approximation.] This is almost restricted to simple, elementary reactions that are controlled by "Microscopic Reversibility"; this means that the forward path and reverse path have the exact same mechanism. The conditions for equilibrium are reasonably simple: 1. The reactants and products must be present and properly mixed. 2. There must be a possible mechanism for the reaction [catalysts are permitted]. 3. There must be sufficient energy present for the reaction to happen at a reasonable rate. At equilibrium the rates of the forward and reverse reactions are the same. If these conditions are present the equilibrium position will be reached from either direction. A corollary to this is that if the equilibrium is gently perturbed by changing a factor in the equation: concentration, T, P, the equilibrium will adjust to new equilibrium conditions or even to a nonequilibrium state if a component is completely removed [Le Chatelier's Principle]. At equilibrium Delta G = 0 and the concentrations [activities] satisfy the equilibrium constant conditions. This means that equilibrium is dependent on both the energy change and entropy change between reactants and products. These ideas are explained rather obtusely in all Physical and some Organic Chemistry Texts. Go to the library, read several and get confused then think hard about it.

Most common reactions seem to be irreversible because the above conditions are not met: Products are not confined, energy is removed, and they are a mix of several if not thousands of elementary reactions. Also some cyclic reactions such as the processes of respiration and photosynthesis are not reversible processes because the mechanisms of the various reactions are entirely different and the energetics are profoundly different. It does seem to be equilibrium though. The difference is that the complicated process is not energy driven but entropy driven by the vast entropy differences [increases] in the energy forms as it passes thru reaction steps. The individual steps in the mechanisms are mostly steady state reactions approaching equilibrium.