What type of chemical changes can be reversed?

Chemical changes that consist of a single step can be reversed.

Some chemical changes can be reversed. Most can’t.

Most chemical reactions, like the digestion of food, consist of a sequence of steps. First, something happens, then something else happens, etc., until we end up with the final product. The chemicals are not likely to come together in the reverse sequence to re-form the original reactants.

An example of a reversible reaction is the reaction of hydrogen with iodine to form hydrogen iodide.

##"H"_2"(g)" + "I"_2"(g)" ⇌ "2HI"##

The ##"H"_2## and ##"I"_2## molecules may collide with enough energy to form a transition state. This complex can rearrange its bonds to form two ##"HI"## molecules.

Two ##"HI"## molecules could collide to form the same complex, which could then rearrange to form ##"H"_2## and ##"I"_2## molecules.

The reaction involves a single transition state. It is a single step or elementary reaction.

It is possible to form water by burning hydrogen in oxygen. You can pass an electric current through the water and split it into hydrogen and oxygen. Most chemists would not call this a reversible reaction.