Question

What is irreversible change?

Answer 1

Irreversible change are those changes which are permanent changes and cannot be brought back into its original form. For example:-)when you burn a piece a paper it convert to ash and you cannot convert that ash into piece of paper again. This type of changes are knows as irreversible change.

Answer 2

Technically, its when the entropy increases, we call it irreversible. Like many other answers pointed out, its when a change is not reversible. But, the catch here is, going back to original possible is not possible spontaneously. If you are ready to do some work, you can always make it go back.

There is one more catch here, going back to the original position, is highly improbable, but not impossible, without any work. Consider this, lets say you break a glass of milk. Now, will the glass mend itself and milk go back into it spontaneously? most probably not. But, if you wait long enough, just by randomness, all the molecules of the milk and glass may probably make it back and find themselves in the same position. It may take more than the age of universe for that to happen, but not completely impossible.

hope it helps you friend.

@ sk