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Why does salt dissolve in water but oil doesn't?

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Well, in a word, chemistry,

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but that's not very satisfying, is it?

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Well, the reason salt dissolves and oil does not

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comes down to the two big reasons

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why anything happens at all:

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energetics

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and entropy.

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Energetics deals primarily

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with the attractive forces between things.

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When we look at oil or salt in water,

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we focus on the forces between particles

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on a very, very, very small scale,

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the molecular level.

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To give you a sense of this scale,

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in one glass of water,

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there are more molecules

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than known stars in the universe.

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Now, all of these molecules are in constant motion,

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moving, vibrating, and rotating.

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What prevents almost all of those molecules

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from just flying out of the glass

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are the attractive interactions between molecules.

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The strength of the interactions

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between water, itself, and other substances

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is what we mean when we say energetics.

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You can think of the water molecules engaging

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in a constant dance,

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sort of like a square dance

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where they constantly and randomly exchange partners.

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Put simply, the ability for substances

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to interact with water,

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balanced with how they disrupt

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how water interacts with itself,

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plays an important role in explaining

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why certain things mix well into water

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and others don't.

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Entropy basically describes

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the way things and energy can be arranged

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based on random motion.

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For example, think of the air in a room.

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Imagine all the different possible arrangements

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in space for the trillions of particles

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that make up the air.

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Some of those arrangments

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might have all the oxygen molecules over here

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and all the nitrogen molecules over there,

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separated.

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But far more of those possible arrangments

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have those molecules mixed up with one another.

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So, entropy favors mixing.

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Energetics deals with attractive forces.

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And so, if attractive forces are present,

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the probability of some arrangements

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can be enhanced,

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the ones where things are attracted to each other.

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So, it is always the balance of these two things

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that determines what happens.

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On the molecular level,

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water is comprised of water molecules,

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made up of two hydrogen atoms and an oxygen atom.

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As liquid water, these molecules are engaged

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in a constant and random square dance

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that is called the hydrogen-bonding network.

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Entropy favors keeping

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the square dance going at all times.

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There are always more ways

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that all the water molecules can arrange

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in a square dance,

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as compared to if the water molecules

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did a line dance.

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So, the square dance constantly goes on.

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

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Well, on the molecular level,

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salt is actually made up of two different ions,

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chlorine and sodium,

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that are organized like a brick wall.

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They show up to the dance

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as a big group in formation

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and sit on the side at first,

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shy and a bit reluctant to break apart

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into individual ions to join the dance.

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But secretly, those shy dancers

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just want someone to ask them to join.

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So, when a water randomly bumps into one of them

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and pulls them into the dance away from their group,

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they go.

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And once they go into the dance,

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they don't come back out.

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And in fact, the addition of the salt ions

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adds more possible dance positions

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in the square dance,

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

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Now, let's take oil.

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With oil, the molecules are sort of interested

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in dancing with water,

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so entropy favors them joining the dance.

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The problem is that oil molecules

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are wearing gigantic ballgowns,

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and they're way bigger than water molecules.

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So, when an oil molecule gets pulled in,

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their size is really disruptive to the dance

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and the random exchange of partners

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that the waters engage in,

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a very important part of the dance.

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In addition, they are not great dancers.

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The water molecules try to engage

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the oil molecules in the dance,

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but they just keep bumping into their dresses

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and taking up all the room on the dance floor.

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There are way more ways the waters can dance

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when the oil gets off the floor,

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so the waters squeeze out the oil,

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pushing it back to the bench with the others.

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Pretty soon, when a large number of oils

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have been squeezed over to the side,

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they band together to commiserate

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about how unfair the waters are being

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and stick together as a group.

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So, it is this combination

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of the interactions between molecules

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and the configurations available to them

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when they're moving randomly

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that dictates whether they mix.

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In other words, water and oil don't mix

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because they just don't make great dance partners.