Well, in a word, chemistry,

but that's not very satisfying, is it?

Well, the reason salt dissolves and oil does not

comes down to the two big reasons

why anything happens at all:

energetics

and entropy.

Energetics deals primarily

with the attractive forces between things.

When we look at oil or salt in water,

we focus on the forces between particles

on a very, very, very small scale,

the molecular level.

To give you a sense of this scale,

in one glass of water,

there are more molecules

than known stars in the universe.

Now, all of these molecules are in constant motion,

moving, vibrating, and rotating.

What prevents almost all of those molecules

from just flying out of the glass

are the attractive interactions between molecules.

The strength of the interactions

between water, itself, and other substances

is what we mean when we say energetics.

You can think of the water molecules engaging

in a constant dance,

sort of like a square dance

where they constantly and randomly exchange partners.

Put simply, the ability for substances

to interact with water,

balanced with how they disrupt

how water interacts with itself,

plays an important role in explaining

why certain things mix well into water

and others don't.

Entropy basically describes

the way things and energy can be arranged

based on random motion.

For example, think of the air in a room.

Imagine all the different possible arrangements

in space for the trillions of particles

that make up the air.

Some of those arrangments

might have all the oxygen molecules over here

and all the nitrogen molecules over there,

separated.

But far more of the possible arrangements

have those molecules mixed up with one another.

So, entropy favors mixing.

Energetics deals with attractive forces.

And so, if attractive forces are present,

the probability of some arrangements

can be enhanced,

the ones where things are attracted to each other.

So, it is always the balance of these two things

that determines what happens.

On the molecular level,

water is comprised of water molecules,

made up of two hydrogen atoms and an oxygen atom.

As liquid water, these molecules are engaged

in a constant and random square dance

that is called the hydrogen bonding network.

Entropy favors keeping

the square dance going at all times.

There are always more ways

that all the water molecules can arrange

in a square dance,

as compared to if the water molecules

did a line dance.

So, the square dance constantly goes on.

So, what happens when you put salt in the water?

Well, on the molecular level,

salt is actually made up of two different ions,

chlorine and sodium,

that are organized like a brick wall.

They show up to the dance

as a big group in formation

and sit on the side at first,

shy and a bit reluctant to break apart

into individual ions to join the dance.

But secretly, those shy dancers

just want someone to ask them to join.

So, when a water randomly bumps into one of them

and pulls them into the dance away from their group,

they go.

And once they go into the dance,

they don't come back out.

And in fact, the addition of the salt ions

adds more possible dance positions

in the square dance,

so it is favored for them to stay dancing with water.

Now, let's take oil.

With oil, the molecules are sort of interested

in dancing with water,

so entropy favors them joining the dance.

The problem is that oil molecules

are wearing gigantic ballgowns,

and they're way bigger than water molecules.

So, when an oil molecule gets pulled in,

their size is really disruptive to the dance

and the random exchange of partners

that the waters engage in,

a very important part of the dance.

In addition, they are not great dancers.

The water molecules try to engage

the oil molecules in the dance,

but they just keep bumping into their dresses

and taking up all the room on the dance floor.

There are way more ways the waters can dance

when the oil gets off the floor,

so the waters squeeze out the oil,

pushing it back to the bench with the others.

Pretty soon, when a large number of oils

have been squeezed over to the side,

they band together to commiserate

about how unfair the waters are being

and stick together as a group.

So, it is this combination

of the interactions between molecules

and the configurations available to them

when they're moving randomly

that dictates whether they mix.

In other words, water and oil don't mix

because they just don't make great dance partners.