Why does salt dissolve in water but oil doesn't?
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 those possible arrangments
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.