Suppose I have the power of Jean's (X-Men). If I separate the all the atoms in the water in a water bottle, would the volume remain the same?
Imagine you have one litre of water, with the molecular weight of water being 18g/mol, this gives about 56 moles of substance. If you were to seperate all the atoms, you would be left with 56 moles of oxygen and 112 moles of oxygen (both would be gaseous, assuming normal* conditions). Manipulation of the ideal gas law gives the result that one mole of gas occupies 22.4 litres at standard pressure and temperature - or a total of nearly 3800 litres of our completely disassociated water. That pressure would certainly be high enough to make (I would imagine) any bottle explode, and then you've got the added fun of the hydrogen and oxygen explosively recombining to remake their preferred watery combination.
*As normal as you can get in this situation, at least.
Since the question is rather in-precise, so shall the answer be.
When you talk about separating all the atoms you have to specify in which manner you to this. When you disassociate water you will need to add some energy to break the hydrogen bonds. Afterwards you will have a pile of hydrogen and a pile of oxygen. There would likely (even though the temperature would be slightly lower) both be gasses and you thus be of a immensely larger volume.
I'll leave it so someone else to give a more thorough answer with all the energies involved as well as a proper application of the ideal gas law that gives the final volumes (provided the two gases don't mix again).