Applying pressure can result in chemical changes, does this hold true for an avocado?
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$\begingroup$ It's physical change, but it can increase the chemical reaction rate as surface area of Avocado is increased. $\endgroup$– BigSackCommented Oct 12, 2012 at 12:43
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5 Answers
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I don't think so. A chemical change implies changing molecular-level structure usually accompanied by transferring electrons. When you squish an avocado you're only rearranging the fats, carbohydrates, and fiber in it - I don't think a chemical reaction occurs.
If you left it out, and it turned brown (oxidized), that would be a chemical change!
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I second Phillip’s answer, which is right in practice, with a small additional detail: it depends on the pressure applied. If you apply pressure by hand, you are not likely to create or destroy chemicals bonds, and the changes will be physical. Applying pressure can change chemical structure, as in the case of solid phase transitions or pressure-induced reactions, but the pressure has to be much larger than that: typically of the order of gigapascals (1 GPa is rougly 10,000 atmospheres).
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Your title and question are a bit different.
Is crushing an avocado a chemical or physical change?
Physical. Crushing (or any other physical manipulation of) any substance is, by definition, a physical change. However, those physical changes can affect the conditions for a chemical change, potentially causing a noticeable change in the rate of said reaction. You somewhat acknowledge this in your question, though I'm not sure it's quite intentional:
Applying pressure can result in chemical changes, does this hold true for an avocado?
Under reasonable conditions, I'm fairly sure that nothing in an avocado is reactive enough to undergo a chemical change because of any conditions the physical crushing altered. Except that it will increase the surface area and speed up oxidization. Given enough of a crushing pressure (e.g. equivalent to the pressure put on coal to create diamond) I'm sure some chemical reaction or another will occur.
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The actual act of crushing the avocado (applying simple pressure) is a physical change. While the act of applying pressure may allow oxidation to happen faster, the application of pressure is still physical. The type of pressure you would apply to an avocado under normal circumstances would be insufficient to result in chemical changes. Chemical changes result in the breaking or formation of molecular bonds which are simply not possible by normal kitchen appliances. Pressure and heat are related which could cause the enzymes and proteins within the avocado to denature, but the act of applying the pressure is not directly responsible for any chemical change in this case, and the application would only be a physical change. If there were chemical changes, these changes would be caused by factors other than the pressure itself.
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My 5 cents. In the beginning it's only physical. However if you crush it a lot of things change. The most obvious is that oxygen can penetrate it much easier than before. There are a lot of substances that can be readily oxidated (f.e. unsaturated compounds). You gain bigger surface so the cells on surface will dry. The nice equilibrium in which this fruit exist is changed. I can't explain it on the level of biochemistry but as every living thing avocado must have some mechanisms that protect it from decomposing. Some of them can't work properly if there is not enough moisture and to much O2.
