Re: Miscible versus Soluble with water. What's the difference?

In my opinion, "miscible" versus "soluble" is not a very useful 
distinction, primarily because "soluble" is not a well-defined concept.  
*Everything* is soluble in water to some extent -- ranging from 
immeasurably small (like mercury) to moderate amounts (sodium chloride) to 
as much as you can put in (ethanol).  So saying something is "soluble" in 
water is in some sense incomplete, because you need to ask "how much".  
100% soluble would be the same as miscible.

A better classification would be "miscible", "partially miscible", 
and "immiscible".  Even that leaves some imprecision, as nothing is 
totally immiscible and "partially" covers a lot.  Something is "miscible" 
with water if you can mix together all possible proportions and it all 
remains a single liquid phase.  Ethanol is an example.  If something 
is "partially miscible" with water, a significant amount of it dissolves 
(what qualifies as "significant" might depend on the application), but 
some proportion exists where no more will go in and a second liquid phase 
(consisting of the other chemical with some water dissolved in it) will 
form.  Hydrocarbons would typically be in the "partially miscible" 
category (though their solubility in water is small enough that for some 
purposes you might think of them as "immiscible").  If memory serves, t-
butyl alcohol is fully miscible with water at some temperatures and 
partially miscible at others, to give another example.  "Immiscible" would 
mean that the two liquids stay completely separate.  This is actually 
thermodynamically impossible, but for some things (mercury, maybe heavy 
hydrocarbons if you didn't care about 1 ppm in your water) you can think 
of them as immiscible.

With regard to the rest of your question, any time things are totally 
miscible there will just be one liquid phase, so there will be no floating 
or sinking and shaking things up won't matter because it will be mixed 
together already.  For things that are partially miscible or immiscible, 
there will be two liquids (at least if you choose the proper proportions), 
and of course the heavier one will sink to the bottom of a container.  If 
you shake up the container, the phases will get mixed together on some 
scale, but the heavy phase will eventually settle out.  You might be able 
to demonstrate this with a mixture of water and some cooking oil, or if 
you could find something darker colored like a motor oil or some tarry 
hydrocarbon it might show up better.