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All of the exams that I've sat so far, and the homework questions I've had, always ask for the reaction between a substance and a dilute acid. Is there a difference in the reaction between a dilute acid and a concentrated acid when each is applied to a substance? If so, what is it, and how dilute is dilute?

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    $\begingroup$ Can you provide an example? I might have an answer, but it depends on the context. $\endgroup$ – Molx Mar 29 '15 at 2:13
  • $\begingroup$ Example: drive.google.com/file/d/0B_LohPswqrHqXzRFOERhR0xPUzg/… $\endgroup$ – drunkBrain Mar 29 '15 at 4:18
  • $\begingroup$ I presume you're a high school student, in which case specifying your educational board would yield more relevant answers. $\endgroup$ – Vatsal Manot Mar 29 '15 at 4:28
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While it would be easier with examples, I leave here some ideas.

This could be because the idea of concentration is an approximation of the concept of thermodynamic activity, which is frequently referred to as effective concentration. Depending on the question being asked, it might be important that the solute behaves as if it was in an infinite concentration, where there's no interaction between solute molecules/ions and the concentration equals the activity.

The mention of dilution might also be important if the reaction involves water as a reagent. Even though water usually does not receive much attention when discussing reactions, since it's the solvent with "constant" concentration, it is important to note that several reactions wouldn't happen in its absence. That would be the case if the acid is available in a purified form, such as a gas (like HCl) or as solid (such as benzoic acid). It's also worth mentioning that the concept of acidity is inherent to solutions, so the two above aren't acids unless diluted.

The above also highlights something that is noteworthy. While diluted is the opposite of concentrated, it may also be said as the opposite of "not-in-solution", meaning not mixtured with the solvent. This could also be a reason to mention dilute acids, to make it clear they are in solution instead of any other form.

Edit:

Given the example, I do believe it's because the reactions happen in the presence of water, but honestly, I'm not sure.

Edit 2:

Another possible reason to mention dilute acids would be in case it's necessary to consider the concentration of a medium-strength acid. In dilute solutions, those can be considered completely dissociated, which makes calculations a lot easier, if that's the case.

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  • $\begingroup$ Acidity isn't inherent to solutions - acid-base reactions can occur also in solid or gaseus phases. $\endgroup$ – Mithoron Apr 9 '15 at 14:22
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It depends to some extent on the acid. The most important example would be sulphuric acid because in concentrated form ~98% there is not enough water present to accept the protons available so it becomes a weak acid if concentrated enough. This is not the case for conc. nitric acid (~65%) or conc. hydrochloric acid (~36%). In addition conc. H2SO4 is a powerful dehydrating agent (eg. with sucrose). It is also a strong oxidising agent eg. gives SO2 rather than hydrogen with metals. It can also be simply that the reaction could be inconveniently vigorous with too high a concentration. I hope this helps.

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  • $\begingroup$ It seems rather common misconception to think that lack of water could weaken acid stronger than water because, it seems, people are stuck in pH reasoning which breaks down in very concentrated acids - see chemistry.stackexchange.com/questions/27905/… $\endgroup$ – Mithoron Apr 9 '15 at 14:17

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