Name: 沈殿反応。クラッシュコース化学 #9 (Precipitation Reactions: Crash Course Chemistry #9)
Uploaded: 2021-01-14T06:40:10.000Z
Duration: 11 min 31 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

You're at dinner with your best friend: fine conversation, fine wine, some barbecued beef cheeks.

You look outside to admire the full moon, but when you glance back you realize that

Fortunately, the cutlery is made of silver, and you know how to use it!

Or perhaps, you're in the bath one day, and as you reach for the soap you notice a wart on your big toe.

Well squeeze a little silver nitrate on that big boy and you'll be ready for sandals season in no time.

Shiny, electrically conductive, and oh-so-useful, silver has been valued since ancient times

and has a reputation for purity and warding off evil, whether in the form of werewolves or warts.

Silver was also a big driver of settlement in the western United States, including Montana, where I live.

And, of course, all that silver got here because of chemistry.

Specifically, it's here because of countless chemical reactions that took place over the

when chemicals in a solution react to form a solid.

Precipitation reactions are what create geological deposits in the earth as well as rings around your bathtub.

They're what we use to make our waste water drinkable

and they've been used by folks for thousands of years to get rich.

Because precipitation reactions happen to be one of the best ways to produce chemicals of the highest purity.

So they're not only the key to how silver was deposited in these mountains hundreds of millions of years ago.

They're also the key to getting that silver back out. I can do it, right here on this desk.

Precipitation. It's stuff falling out of other stuff: water falling out of the sky, solids falling out of solution.

And for us here it all comes down to a little thing called solubility.

Water, as we've discussed here before, is pretty dang good at dissolving stuff, ionic compounds in particular.

A positively charged ion and a negatively charged ion held together by their charges

might form a crystal when they're dry, but add a bit of water

and those little polar molecules slide their way between the ions dissolving massive amounts of ionic compounds.

But some ionic compounds can overcome even the dissolving power of water

and when they form through reactions in solution, they fall out as a solid precipitate.

Yes, precipitate is both a noun and verb. Get used to it.

When we talk about an ionic compound that's fallen out of solution I say precipitate [pre-sip-uh-tit]

to distinguish it from from precipitate [pre-sip-eh-tate] which is more the verb-y sound.

And this is purely my preference because that's how my teacher said it when I was being taught.

So the rich silver veins in Montana formed

when water stuffed with ionic compounds ran through cracks in paleozoic limestone.

Where conditions were right, silver ions in the water reacted with ionic compounds,

or salts, in the limestone to make insoluble silver compounds that fell out of solution.

And it looks, a little bit, like this. It actually looks exactly like this.

It's pretty cool because, uh, you can't feel this but, it is extremely heavy, because silver is a pretty heavy element.

And it wasn't just the silver salts in solution, all kinds of stuff: gold and potassium and copper salts,

and, most notably, sodium salts are dissolved as water rushes across the landscape.

If these dissolved compounds stay in solution until they get to the ocean, they pretty much stay there forever.

The water evaporates, leaving the salts behind in the ocean where over the eons it has built up,

leaving the ocean super salty. As we know it today.

And while sodium chloride, what we call salt when not doing chemistry, is the most common salt,

there are also tons of other things dissolved in the ocean, including quite a lot of gold.

In fact, at today's market value the ocean contains about one hundred million trillion dollars of gold.

And that was not a stutter, a hundred million trillion.

That's a hundred trillion with six more zeros after it.

So you can see why it might be nice to master some precipitation reactions.

There have been chemists that have driven themselves crazy trying to figure out

how to economically extract gold from seawater, but thus far none have done it.

This solution here of silver nitrate is similar to that ion rich water

that steeped through the Montana limestone millions of years ago.

And we can use it right here at this desk to recreate

the ancient reactions that deposited silver in veins across our landscape.

But instead of the types of salts found in limestone,

we can use a very similar and substantially more familiar ion compound: table salt, good old NaCl.

Add some drops of sodium chloride, also known as your salt water,

to the silver nitrate solution and there you see your precipitate. Ooooo...gross.

Now the question that we immediately want to ask is, "What is this white stuff down here?"

The key to understanding what just happened here is that both of the compounds are ionic.

You remember there are two kinds of ions, right?

Cations are positively charged and anions are negatively charged.

Just like little bar magnets, they attract.

So cations only react with anions to form new compounds.

And don't just think that there's one anion and one cation.

The sodium ion in sodium chloride will have chloride ions on all four sides,

which in turn are surrounded by four sodiums, and this pattern repeats many, many, many times

until we end up with the salt crystals that we dissolved in the water.

But how do we know which ions are cations and which are anions?

Well sodium is positively charged so it's a cation

and we know that it's positively charged because sodium is a metal from the left side of the periodic table

and those are always cations when they're alone.

Silver is also a metal and is also a cation.

We know that chlorine is a gas from the right side of the periodic table, so that is an anion.

Nitrates, sulfates and phosphates are really common and they're always anions.

Whenever you see an N, S, or P followed by a bunch of oxygens, you know you're looking at an anion.

With that in mind, look at the possible products of this reaction.

What we're looking for is a product that doesn't dissolve in water, so we know it's not sodium chloride.

That was one of our reactants and it dissolves readily in water, hence the oceans.

And it isn't silver nitrate, our other reactant, or sodium nitrate,

because as a rule, nitrates dissolve really easily in water, so we know that's dissolved.

So we're left with silver chloride. Just process of elimination.

This makes sense because silver also makes insoluble compounds with bromine and iodine,

which are in the same column of the periodic table as chlorine.

Elements in the same column often behave in similar ways.

And you'll notice of course that we don't end up with, like, a huge, nice chunk of pure silver here;

Kinda like table salt, silver chloride is a crystalline solid.

Unlike salt though it's not very soluble in water.

Now getting the silver out of this compound will involve another kind of reaction, a redox reaction,

In the meantime, we still have to learn the language of describing this sort of reaction.

Because of the neat and somewhat unique interactions that are involved in precipitation reactions

-- dissolved substances producing solids, ions dissociating and rebonding --

there are special ways to write and balance them as equations.

One way is to include notations in parenthesis that tell you what state the chemicals are in:

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沈殿反応。クラッシュコース化学 #9 (Precipitation Reactions: Crash Course Chemistry #9)

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