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  • 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 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.

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

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B2 中上級

TED-ED】なぜ油と水は混ざらないのか?- ジョン・ポラード (【TED-Ed】Why don't oil and water mix? - John Pollard)

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    阿多賓 に公開 2021 年 01 月 14 日
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