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If 5ml alcohol is dissolved in 1 L of water ( the purpose is to highlight the low quantity of one of the liquids) , how is it possible for 5 ml to evenly dissolve in 1 L of water ? There is not enough of alcohol to sit between water molecules , isn't it ? So how is the mixture's proportion/composition is uniform ?

Or is there anything that I miss to understand in the term - composition?

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  • $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, you can ‎visit the help center or take a ‎‎tour of the website.‎ || You just missed azeotropes! $\endgroup$ – M.A.R. Jun 9 '15 at 16:50
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Your question is one of my favorite question. The concept of microscopic homogeneity and macroscopic homogeneity can explain this. Like if you add sugar and sand together and mix them well, you can see a homogeneous mixture but if you take a random sample you will find more sugar than sand or vice versa and even if your sample size is so small that it contains only one particle than it might be either sand or sugar. So the consistency of your system depends on the sample size. It has statistical significance also. In statistics if you want to make an average of large pile of data (Whatever data you have), you have to make sure that the sample you are analyzing is a true representative of the whole pile. In statistical mechanics microscopic description is used to find out the macroscopic behavior and so this simple question has lot of potential. Now this very concept is also used to find the freezing point depression or colloidal behavior.

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  • $\begingroup$ Thank you.Quoting you "So the consistency of your system depends on the sample size. ". Thank you for this , this is exactly I was thinking as well. However , there is a difference in the way a homogeneous mixture behaves (macro scope) right ? I am more curious to know what enables asugar or salt to dissolve in such a way that it is not heterogeneous. $\endgroup$ – Curious Jun 10 '15 at 11:03
  • $\begingroup$ Because salt dissolves as separate ion and in case of sugar it dissolves because of weak van-der-waals type forces between polar solvent and apolar solute. $\endgroup$ – Osman Mamun Jun 10 '15 at 14:59
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Maybe it is just a question of what you mean by "evenly dissolved". True enough, in your example, the proportion of alcohol to water is not "even" - rather the solution is indeed mostly water containing only a little bit of alcohol. But for a true solution such as this, the alcohol is dispersed evenly (uniformly) throughout the bulk of the solution, so that if you repeatedly withdrew a sample and analyzed it, you would get the same result no matter which part of the liquid you sampled.

That would not be the case for oil and water - they would exist in separate phases and which phase you sampled would give a different analysis result.

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  • $\begingroup$ Thank you for the response. Quoting you - "so that if you repeatedly withdrew a sample and analyzed it, you would get the same result no matter which part of the liquid you sampled." - The question is , how is that possible ? Lets increase the volume of alcohol to 250 ml. There would not be sufficient alcohol to spread over 1 L. Could you provide a specific measure that you could analyze - such as concentration ? Wont it depend on the sample as mamun described below? $\endgroup$ – Curious Jun 10 '15 at 11:01
  • $\begingroup$ If not , what property of bonding between alcohol and water molecules( not the chemical bond since its mixture but the weak bonding that exists between molecules) results in same result in an analysis of concentration ? $\endgroup$ – Curious Jun 10 '15 at 11:01
  • $\begingroup$ (1) I agree that if you combine 1 liter of water and 250 ml of alcohol, the resulting solution will probably have a volume greater than 1L (note that it is not always true that when you dissolve A in B you get an increase in volume). The alcohol present will distribute itself evenly throughout the mixture. $\endgroup$ – iad22agp Jun 10 '15 at 14:15
  • $\begingroup$ (2) To answer your question about bonding, the alcohol and water molecules are mutually attracted due primarily to "hydrogen bonding" - where the hydrogens on water or on the OH group of alcohol are attracted to the oxygen on another water or alcohol molecule. Oil and water, on the other hand, do not hydrogen-bond together, so they separate into two phases. $\endgroup$ – iad22agp Jun 10 '15 at 14:19
  • $\begingroup$ (3) Yes, sample size is important, as Mamun points out in his answer. If you took a series of extremely small samples of a water-alcohol mixture, you could get inconsistent results - some would have more alcohol and some less. Or, if you had an extremely dilute solution of alcohol in water, even if you took normal sized samples, you could see a variation in the amount of alcohol present in the samples. $\endgroup$ – iad22agp Jun 10 '15 at 14:22

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