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: 
The statement you challenge is a gross oversimplification and generalisation that fails to capture much of the subtlety of chemical reactions.

It seems to be pitched at describing the difference between simple examples such as the melting of water (physical, easily reversible by putting the water in a freezer) and the simple reaction between chlorine and hydrogen to give common salt (chemical, not spontaneously reversible or dinner tables would be far more entertaining).

But the variety of possible chemical reactions is far bigger than that. Many chemical transformations can be reversed easily given the right conditions. Esterification, the reaction of an acid with an alcohol, makes new molecules and water, for example. But to push the reaction to produce the ester, chemists need to remove the water or the reverse reaction just undoes the new bonds. So they have a cool piece of equipment called a [Dean-Stark apparatus][1] that removes water from the reaction to ensure they get the product they want. The whole point of this is a recognition that the underlying reaction is *reversible* and needs to be pushed in one direction.

The key to reversible reactions is that there is some *mechanism* that can undo the new bonds created in a chemical change. In many cases there is no such mechanism which makes many chemical changes impossible to reverse, which is why a simplistic view might distinguish physical from chemical change. But this is unfair on the variety of chemical behaviours.

Another example where you might expect reversibility is the conversion of diamond to graphite. Graphite is more stable under normal conditions but the conversion is not seen. This is because the *mechanism* to convert diamond t graphite would need to break a very large number of chemical bonds in diamond (every carbon connected to other carbons in an infinite tetrahedral grid) to get graphite (every carbon connected to 3 other carbons in an infinite hexagonal array using delocalised multiple bonds). At very high temperatures and pressures (such as carbon dissolved in liquid iron at pressure) many of the bonds in any form of carbon are already broken and the two forms of carbon can interconvert (this is roughly how diamond is formed in nature and laboratories). The reason why reversibility is not normally seen is that, at room temperature, there is no available mechanism to break the bonds.

Sometimes reversibility has tragic real-world consequences. Thalidomide is a chiral molecule one form of which is a mild sedative and is active against some diseases and cancers. The other form is a dangerous teratogen, causing monstrous deformities in the fetus. Unfortunately there is a reaction path in living systems that interconverts the two isomers, hence tragedy for pregnant women.

In short, reversibility is sometimes possible in chemical reactions, *if* there is an easily accessible *mechanism* to undo a chemical reaction under the right conditions. It is wrong to say that *all* chemical reactions are irreversible.

[1]: [https://en.wikipedia.org/wiki/Dean%E2%80%93Stark\_apparatus#:~:text=The%20Dean%E2%80%93Stark%20apparatus%2C%20Dean,other%20liquid](https://en.wikipedia.org/wiki/Dean%E2%80%93Stark_apparatus#:%7E:text=The%20Dean%E2%80%93Stark%20apparatus%2C%20Dean,other%20liquid))%20from%20a%20reactor.
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.