A Chemical Reaction vs. A Physical Change

Difference Between A Chemical Reaction vs. A Physical Chang

Examples of physical changes include water that freezes into ice and water that evaporates into water vapor. Since this is a reversible phase change, you get the water back on the ice melts and the steam condenses. A physical change does not provide new substances. A reversible change is taken as the criterion that the change is physical, not chemical. But it can not be right, as there are many chemical reactions that are reversible for example a reaction between an alcohol and an organic acid forming ester plus water. However, mix tests with water and add some strong acid dissociates ester and we get back the alcohol and acid.

Solving ammonium nitrate in water temperature drops into the water. The resolution gives an energy change. Yet we would not call a resolution reaction of a chemical reaction.

We have so far kept us at the macro level. But it is not easier if we go at the micro level and define that in a chemical reaction bonds are broken and the new established. When the ice melts and water evaporates bonds are broken, but we do not call melting and evaporation a chemical reaction, instead the phase changes.

There is little about phase transition in school science books and chemistry books (but perhaps more in geology books?).  There may be a color change and an energy change. It may also form a new substance. When applied to phase transitions there are strong bonds that are broken. An example is the transition from white tin and gray tin. The white tin is a metal and the gray one is an insulator. In the white, each tin atom has six neighbors, while gray tin has the same structure as diamond with each tin atom having four neighbors.

When ammonium nitrate dissolves in water, bonds are broken releasing ions, but we do not call resolution reaction a chemical reaction.

So the conclusion is simple: it is in principle not possible to distinguish between a chemical reaction and physical change. I will go one step further: it is not fruitful to try to distinguish between them. There is no such fundamental difference between a change that many agree is a physical change and a change which many agree is a chemical reaction. So, multiple choice questions which students will choose involve chemical reaction among several events that are a part of future studies.


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