Why Saltwater is Saltwater

General Chemistry Index

Some context, please?

The substances in a homogeneous mixture are so evenly distributed that it is both hard to distinguish one substance in the mixture from another and also difficult to separate them.

The parts of the mixture are not noticeably different from one another.

Pouring saltwater from one cup to another results in two cups of saltwater. You can't look into a cup of saltwater and say, "Oh! Look! There's some salt and look! There's some water." It is all saltwater. 

The composition is the same throughout.

One drop of saltwater is like any other drop. You'd have to get down to nearly a molecular scale to have a chance at seeing any difference between one sample and the other. Why? 

You CANNOT easily divide out different parts.

You can't just reach in and grab the salt from the saltwater.

So about that… Think about molecules. All the way down to the smallest molecule of a substance that is still that substance.

H2O and NaCl in this case.

Well, suppose we want to make some saltwater, so we start with a glass of water. In that glass are about 1000 bazillion H2O molecules. Now we mix in 1 bazillion molecules of NaCl.

Because saltwater is a homogeneous mixture, a solution, to be exact, the molecules of the NaCl move around and find places to fit in within the H2O molecules. (To complicate things, the Na and Cl come apart, but that's not needed for understanding homogeneous solutions). They scootchy around until they all find a place. 

So, the pretend numbers above tell us that for every 1000 H2O molecules, there is 1 NaCl molecule (2 atoms) lodged in and among the water. The (if you will) spread out until they are evenly mixed within all those H2O molecules.

Grabbing some quantity of the mixture (solution) will result in that same 1000:1 ratio. Dividing that into two samples still results in the 1000:1 ratio. Divide again, still 1000:1. Any non-theoretical sample would have so many molecules that the ratio would stay the same. Thus, the composition is the same throughout.

Taking this to a theoretical limit, if you could somehow grab only 1000 molecules, there would be a tiny chance that none of them would be NaCl. For all practical purposes, this is impossible. Therefore, in a homogeneous mixture (reprise):

The substances in a homogeneous mixture are so evenly distributed that it is both hard to distinguish one substance in the mixture from another and also difficult to separate them.

• The parts of the mixture are not noticeably different from one another. 

• The composition is the same throughout. 

• You CANNOT easily divide out different parts.