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Knowing some about thermodynamics and reactions, I do understand how it can be shown that a change is reversible. But irreversible? Why can't it be that a change that was deemed irreversible thousands of years ago via new changes perhaps developed by physicists or changes, processes and reactions from some other part of the world or space, a change that was deemed irreversible in the future can be shown to be reversible?

I think that for instance diseases that were deemed irreversible as science progressed, we could make changes that were priorly said to be irreversible, in fact reversible.

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naI think what you are essentially asking is that, "Can we violate or circumvent the second law of thermodyanmics?"

The answer is no, based on all the physics we know so far and observations we have made so far. When you freeze the melted ice back to ice you are creating irreversible changes elsewhere in the universe (e.g., outside your refrigerator).

That said, there is a deeper conversation on whether the second law fundamental or only a measure of our ignorance about the complex system. A conversation that is not philosophically well resolved. You can search for work by Ilya Prigogine, or about the arrow of time. But setting philosophical questions aside, in the real-known world we cannot violate or circumvent the second law of thermodynamics.

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There's some confusion here. Thermodynamics involves changes in going from an initial state to a final state. The path taken between those two states can be either reversible or irreversible.

The process that moves the system along the path between the two states is reversible if and only if two conditions are fulfilled: (1) that a microscopic change in the external conditions would cause the direction of the process to reverse. The other, (2) is that the system always be in a state of total equilibrium. Point (2) implies that an equilibrium process be infinitely slow. (There are a few exceptions to this but we won't go there now.) All other processes are irreversible.

Thus it is clear that essentially all real processes are irreversible.

Nevertheless, most all thermodynamic variables are independent of path. That's a consequence of the first law. So it is always possible to envision a reversible path and to calculate thermodynamic properties for that change. The same thermodynamics properties apply to the real process.

So changes thought to be irreversible 100 years ago are still irreversible. But it is possible that we have found new reversible paths between states. That may make calculations easier, but it won't change the numerical answers.

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Thanks for the answer. But are you telling me that the real process of for instance melting ice is not reversible? Would not freezing the melted ice reverse the change? Or reactions with water at constant tempareature to make the example easier. –  909 Niklas Feb 3 '13 at 1:08

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