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# examples of reversible and irreversible changes

From Wikipedia

Time reversibility

Time reversibility is an attribute of some stochastic processes and some deterministic processes.

If a stochastic process is time reversible, then it is not possible to determine, given the states at a number of points in time after running the stochastic process, which state came first and which state arrived later.

If a deterministic process is time reversible, then the time-reversed process satisfies the same dynamical equations as the original process, AKA reversible dynamics; the equations are invariant or symmetric under a change in the sign of time. Classical mechanics and optics are both time-reversible. Modern physics is not quite time-reversible; instead it exhibits a broader symmetry, CPT symmetry.

Time reversibility generally occurs when, within a process, it can be broken up into sub-processes which undo the effects of each other. For example, in phylogenetics, a time-reversible nucleotide substitution model such as the generalised time reversible model has the total overall rate into a certain nucleotide equal to the total rate out of that same nucleotide.

Time Reversal, specifically in the field of acoustics, is a process by which the linearity of sound waves is used to reverse a received signal; this signal is then re-emitted and a temporal compression occurs, resulting in a reverse of the initial excitation waveform being played at the initial source. Mathias Fink is credited with proving Acoustic Time Reversal by experiment.

## Stochastic processes

A formal definition of time-reversibility is stated by Tong in the context of time-series. In general, a Gaussian process is time-reversible. The process defined by a time-series model which represents values as a linear combination of past values and of present and past innovations (see Autoregressive moving average model) is, except for limited special cases, not time-reversible unless the innovations have a normal distribution (in which case the model is a Gaussian process).

A stationary Markov Chain is reversible if the transition matrix {pij} and the stationary distribution {Ï€j} satisfy

\pi_i p_{ij} =\pi_j p_{ji}, \,

for all i and j. Such Markov Chains provide examples of stochastic processes which are time-reversible but non-Gaussian.

Physical change

Physical change is a concept introduced to contrast with the concept of chemical change. A physical change is any change not involving a change in the substance's chemical identity. Matter undergoes chemical change when the composition of the substances changes: one or more substances combine or break up (as in a relationship) to form new substances. Physical changes occur when objects undergo a change that does not change their chemical nature. A physical change involves a change in physical properties. Physical properties can be observed without changing the composition of matter. Examples of physical properties include: texture, shape, size, color, volume, mass, weight, and density.

An example of a physical change occurs when making a baseball bat. Wood is carefully crafted into a shape which will allow a batter to best apply force on the ball. Even though the wood has changed shape and therefore physical properties, the chemical nature of the wood has not been altered. The bat and the original piece of wood are still the same chemical substance.

Changes are sometimes hard to categorize strictly as physical or as chemical. Dissolving a salt in water involves the breaking of chemical bonds, yet is often described as a physical change. Some teachers hold that a chemical change is a rearrangement of atoms, but many physical changes also involve the rearrangement of atoms. Many chemical changes are irreversible, and many physical changes are reversible, but reversibility is not a certain criterion for classification. Although chemical changes are often recognized by an indication such as odor, color change, production of a precipitate, or production of a gas, every one of these indicators can result from physical change.

Question:...in everyday life? Also, an example of an irreversible physical change in everyday life. Sorry, I can't think of good ones that you'd come across everyday. haha, thanks Randi I just thought of that too xD i still need a reversible one.. ^^irreversible one, sorry

Question:I know that because there is no heat transfer the entropy of the system is zero, but what about the surroundings? Can the same be said since the system does not transfer any heat to the surroundings? If the entropy of the surroundings is non zero how would I got about calculating it?

Answers:The entropy change of an adiabatic system undergoing a spontaneous *irreversible* process is NOT zero -- it is positive. Only *reversible* adiabatic processes are isentropic (constant entropy). That is, in the inequality expressing the second law, dS q/T the equals sign only holds for reversible processes. For irreversible processes, dS > q/T. So for an adiabatic process (for which q = 0), we have that dS > 0. The forgoing all refers to the entropy of the system. For the surroundings, nothing is going on (i.e., the "process" that is occurring is restricted to the system). Because there is no transfer of thermal energy to the surroundings, the entropy change of the surroundings is zero. Note, however, that the system can do work on the surroundings, and thus transfer energy (in the form or work) to the surroundings.

Question:Ok here's the thing. I know that after decomposing water through electrolysis you end up with oxygen and hydrogen gas. Is it true that if you put the gases together and supply energy in the form of heat (i.e. throw in a lit match) it'll burn for a while and then recombine into water again? I know that chemical changes are usually irreversible but there are exceptions, is this one of them? Or doesn't it count because electrolysis and combustion are not considered "opposite" actions? Please don't give any copied-and-pasted answers because they're usually too long especially when they're from wikipedia, and too complicated for someone of my level of understanding to take in easily. Try to keep it simple! =) EDIT: Thanks for that explanation. Would choose yours as best answer but I can't do that yet because I have to wait 4 hours.

Answers:Yes. Oxygen and Hydrogen explode into a cloud of steam if ignited. The energy released in the form of heat and light (and sound!) is equal to the energy needed to separate the water into two gasses. The synthesis of water is reversable and spontaneous. The decomposition of water is also reversable, but it is not spontaneous. Spontaneous reactions are driven by the fact the products formed have much less energy than the starting material. Rocks roll downhill because of something known as entropy. This is also the force which drives the reaction between Hydrogen and Oxygen to produce water. Things like water and carbon dioxide are very low in chemical energy. This is why they form when things burn.

Question:

Answers:Cooking something. Like cooking eggs, meat, or baking. Once it is cooked then it is a physical change that can not be reversed.