homogeneous mixture - Are they truly uniform in composition and proportion?

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**?




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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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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