That's a great question, and the answer can be pretty complicated. Usually when we call a change "irreversible" or non-reversible, a new material is formed and it would be impossible (or at least really really really hard) to get the original materials back. A good example is when you burn firewood. Even if you could capture all of the heat and smoke the fire created, you wouldn't be able to make the ashes back into a log. In reversible changes, you can always get back to what you started with.

A great example is to think about all the different ways water can change. Liquid water can change into a gas or a solid - things which seem to be completely new and different than what it originally was -but it can always go back to being the same kind of liquid that it started out as. The main thing to think about when you're trying to figure out if a change is reversible or non-reversible is "can I get back the things I started with?" I hope that helps answer your question!

A reversible change means that you can go back to the original state of the system by doing the opposite action. A non-reversible change means that there is no action that you could do in order to go back the starting point of the system.

An example of a reversible state is mixing and de-mixing of components that do not interact. If the components interact, the product will not always be able to go back to the initial components.