If we take some aqueous solution and dilute it further and further, will the concentration of the solution ever get to zero? I would say no, simply because total dilution implies that all the molecules of the solute have literally disappeared. But, the fact that I am unable to figure out where the molecules have gone doesn't make my argument compelling at all. I am led to believe that there is a far better answer and/or explanation to my question
UPDATE: While it is true that this question closely resembles the linked one, the answer provided in this question is a lot better as it gives a much deeper insight into the dilution process.
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40$\begingroup$ Homeopathy does it all the time :) $\endgroup$– AlchimistaCommented Aug 7, 2020 at 18:45
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$\begingroup$ Sorry, Alchimista; that's simply not true. Homeopathy not only recognises but to a large extent depends on the idea that no such thing could ever be possible. $\endgroup$– Robbie GoodwinCommented Aug 9, 2020 at 0:45
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9$\begingroup$ @RobbieGoodwin You're taking the the logic of homeopathy at face value and using it to justify homeopathy when you say that, so of course that would be the conclusion. $\endgroup$– DKNguyenCommented Aug 9, 2020 at 1:30
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3$\begingroup$ Of course you might find a bottle among others billions of billions containing 1 molecule of the diluted stuff. It does not classify as a solution of that particular stuff as for you can find whatever exist. I did not want to be too serious because the topic doesn't deserve. But there is a nice comment by @ Karl below. Also as a curiosity. Starting from a spoon of NaCl there is not enough water in the known universe to get a C 60 homeopathic solution in one step!!! $\endgroup$– AlchimistaCommented Aug 9, 2020 at 10:39
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3$\begingroup$ This is one of the questions where "In theory, there is no difference between theory and practice. In practice, there is." $\endgroup$– cbeleitesCommented Aug 10, 2020 at 7:49
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5 Answers
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It depends how you dilute it. If you take an aqueous solution of A and just add pure water (absolutely 100% water), the concentration of A will never quite be null. In this case however, you will reach a point where the concentration of A is so small that it can be considered null for your applications.
If, however, you dilute the solution, take a sample, then dilute that sample (and so on), you could reach a concentration of exactly 0M. Imagine you have diluted the solution enough so that it contains exactly 1 molecule of A. When you take your sample for the next dilution, if this molecule isn't in the sample, the concentration will be exactly null. If it does happen to be in the sample, it could be left behind when you draw the next sample, or the next, and so on.
In practice though, the water you use for the dilution will likely contain impurities. You will maybe not achieve exactly 0M, but the concentration could be so small that it is undetectable and have no measurable consequence.
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1$\begingroup$ "...dilute the solution, take a sample, then dilute that sample (and so on), you could reach a concentration of exactly 0M." That's amazing. That's the type of solution I was looking for! $\endgroup$ Commented Aug 7, 2020 at 19:00
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37$\begingroup$ @KaraKirkland It's worth mentioning, explicitly, that the reason this works is because a solution is not a continuous medium—if it were, you could never get to zero. You can only get to zero because molecules are discrete. $\endgroup$– theoristCommented Aug 8, 2020 at 1:42
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30$\begingroup$ I get that it wouldn't be in the spirit of the answer, but if you are doing serial dilution with the explicit aim of getting to zero concentration, you could just skip the part where you add any of the original solution to the solvent and you'll get there faster 😉 $\endgroup$– PaulCommented Aug 8, 2020 at 3:05
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7$\begingroup$ @Paul But water have memory! Ask any homeopath! $\endgroup$– d-bCommented Aug 8, 2020 at 13:27
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1$\begingroup$ @RobbieGoodwin if I have a box with 99 red balls in it and one yellow ball, the concentration of yellow balls is 1%. If I add a million red balls, the concentration is about 1 ppm. If I dump half of the contents of that box into another box, I will have one box with about 2 ppm yellow balls and another with exactly 0 ppm yellow balls. The same is true of molecules in solutions, they ARE different processes. $\endgroup$– llamaCommented Aug 10, 2020 at 21:11
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For an analytical chemist, the concept of zero concentration does not exist. The concentration cannot be exactly 0! Only a limit of detection can be developed in terms of statistics. This is why a senior respectable user here has written an entire monograph on this topic. Suppose you have a NaCl solution, and so called "pure water"*, matter how much you dilutions you perform, you cannot say with 100% confidence that there is no single ion sodium left in the solution now.
*Pure water cannot exist in any ordinary laboratory.
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7$\begingroup$ There's plenty of room at the bottom, as they say, and I suppose this includes room for impurities. The lowest (indirectly) measured concentration I know in a condensed phase solution comes from a dark matter detection study, where the authors propose a 14 yM (that's yoctomolar) concentration of tritium in liquid xenon may have produced enough noise to mask signals from dark matter decay. Somehow it still remains after very stringent purification, and even though tritium itself is a trace radioisotope with modest half-life. $\endgroup$ Commented Aug 7, 2020 at 23:31
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2$\begingroup$ Very nice example. No reagent is 100% pure. $\endgroup$– ACRCommented Aug 7, 2020 at 23:32
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1$\begingroup$ Searching, there are 10 posts in Astronomy SE and 8 in Physics SE that contain "echelle" and many are interesting. It looks like the ones in Physics SE indicate there is more of a chance of a "DIY-based answer" there but there are some observational astronomers active in Astronomy SE who might really enjoy answering this as long as you mention it might use it to analyze sunlight or something astronomical. $\endgroup$– uhohCommented Aug 8, 2020 at 3:16
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1$\begingroup$ Let us continue this discussion in chat. $\endgroup$– uhohCommented Aug 8, 2020 at 3:28
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1$\begingroup$ You need to go for larger molecules, which are also not ubiquitous in the environment. $\endgroup$– KarlCommented Aug 8, 2020 at 6:42
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Unequivocally yes! If your solute is something of which you can identify a single molecule in a macroscopic solvent sample (let´s say a fluorescent dye), then you can dilute down to zero, and be sure about it.
(For the nitpickers: This can be done in a finite number of steps, if you split the solution in two after each step, and keep diluting the part which shows a lower concentration. Divide et impera.)
If you´re unable to measure the concentration down to actual zero, then the statement "this has been diluted to zero" is not falsifiable, therefore unscientific (see Karl Popper), and as such not worth pondering. ;)
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1$\begingroup$ I'm also a fan of Popper! (and Kuhn, and Lakatos). Anyways: If you knew the initial concentration, and the number of splits, then even if you couldn't measure the final concentration, you could make a scientific statement about its probability of being zero :). $\endgroup$– theoristCommented Aug 8, 2020 at 1:39
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7$\begingroup$ I would take a minor issue with your last paragraph: just because you (the hypothetical scientist doing this) are unable to measure the concentration down to zero doesn't mean that the hypothesis that it's zero is unscientific. Maybe someone else can measure it, or people in the future will be able to measure it, or it has some physical consequence that you're not aware of, or so on. $\endgroup$– David ZCommented Aug 8, 2020 at 2:36
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3$\begingroup$ @Karl Because if someone in the future, with better technology, could falsify it, then the statement is falsifiable. "There are zero molecules of X in this solution" is a testable prediction when you have a device precise enough to detect whether there is a single molecule of something in a solution. $\endgroup$ Commented Aug 8, 2020 at 18:36
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5$\begingroup$ @Karl The statement is not "becoming correct", it is becoming measurable. Whether it's correct or not doesn't change. You can make correct statements that aren't measurable at the time (for example, Einstein in 1916: "Gravitational waves exist", or Fermi in 1934: "Neutrinos exist"). The fact that they weren't measurable at the time of proposal doesn't mean they weren't correct. $\endgroup$ Commented Aug 8, 2020 at 21:10
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2$\begingroup$ @Karl I answered your question: a statement is "scientific" if it is falsifiable. Falsifiable means that someone could conceivably measure it, not that someone can do that right now. And falsifiability doesn't really have anything to do with correctness, so talking about a statement "becoming correct" isn't really relevant here. $\endgroup$ Commented Aug 8, 2020 at 22:17
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Something like a zero concentration is achieved in the vapor phase in some materials. In this answer the triple-point vapor pressure of gallium is identified as so low that a measurement thereof defaults to zero with a high probability.
answered Aug 7, 2020 at 20:50
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Hypothetically, I think limitation for zero-dilution would be when you take a sample of diluted sample the possibility of having a solute molecule in there should be zero. To achieve this goal, the concentration of your solution should be less than $10^{-24}$ in power $\left(\frac{1}{6.022 \times 10^{23}}=1.66 \times 10^{-24}\right)$. To do so, if you have $\pu{1 M}$ of solution, you should add another $\pu{1.66 \times 10^{24} L}$ of water to get that dilution. Considering the whole water body of the Earth is about $\pu{1.3859 \times 10^{21} L}$ ($\pu{2.5511 \times 10^6 mile^3}$ according to USGS) you may run out of water before you get right concentration.:-)
That's why Raphaël has said its depends on how you want to do it. So, you may use the procedure called serieal dilution.
answered Aug 7, 2020 at 20:14
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4$\begingroup$ If you sample substantially less than a litre (say, 1 µL), then the concentration can be proportionately higher and you can still statistically expect to find zero molecules of the solute in some samplings. $\endgroup$ Commented Aug 7, 2020 at 22:29