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  • 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

  • your friend has turned into a werewolf.

  • 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

  • eons called precipitation reactions:

  • 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.

  • And all I need to get started is this.

  • [Theme Music]

  • 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.

  • Now what about the nitrate? Also 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;

  • now it's bonded to chlorine.

  • 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,

  • which we'll talk more about next week.

  • 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:

  • Aq meaning aqueous, or in solution,

  • and s for solid, meaning that it's your precipitate, this is called the molecular equation.

  • Another way which tends to give a clearer picture of what actually happens during the

  • reaction is to write everything out as ions.

  • Here you list the compounds that dissolve completely in solution as ions;

  • makes sense because as soon as the salts are dissolved every ion is on it's own,

  • and it doesn't really matter where it originally came from.

  • So the left side shows silver, and nitrate, and sodium, and chloride ions all in one solution.

  • And the right side shows sodium and nitrate still as separate ions in solution

  • with the silver chloride precipitated out as solid.

  • Now if we don't care about the complete equation,

  • and only want to see the active participants, we can write it in yet a third way.

  • We just leave out the so-called spectator ions: nitrate and sodium, which don't participate directly in the reaction,

  • and end up with a net ionic equation showing just silver and chloride ions reacting to form silver chloride.

  • This is nice, and short, and to the point, which is what chemists love

  • because remember some of them have terrible writing skills and have to dictate their stuff to their mom.

  • Now as an aside remember when I first brought up the weirdness of using Ag to denote the word silver?

  • Well all that stems from the fact that the Latin world for silver is argentum,

  • and the ancients were, as most people are today,

  • obsessed with what silver represented, not just wealth, but also health.

  • Ancient Indo-Europeans associated silver with purity and goodness.

  • Hippocrates, the ancient Greek doctor, wrote about silver's anti-disease properties.

  • And there's good science behind silver's medical uses.

  • A lot of the metals are toxic to things like fungi and microbes.

  • But unlike say, lead, silver isn't that toxic to humans.

  • Silver nitrate and a compound called silver sulfadiazine,

  • were used to disinfect wounds in World War I, before antibiotics were discovered.

  • Silver sulfadiazine is still used to dress burns.

  • And researchers are now looking at the antimicrobial uses for silver nanoparticles.

  • Some people even take colloidal silver, basically silver particles in a liquid suspension,

  • as a kind of general health booster, but there's not actually any evidence that it boosts health.

  • It can turn you blue though. Now you want that silver even more, don't you.

  • Now, as we always do, in order to make a reaction practical,

  • you have to go through the final step of converting the formula equation into a molar mass equation.

  • If we wanted to get the silver out of solution, how much salt would we need?

  • Specifically, let's say that we want to get one troy ounce of silver.

  • The troy ounce is part of the troy weight system which is used to weigh precious metals.

  • It's derived from the way the Romans measured bronze and silver bars they used for currency 2,000 years ago.

  • And we are still stuck with it.

  • But let's be at least a little bit modern about this, a troy ounce equals 31.1 grams.

  • So we want 31.1 grams of silver, the molar mass of silver is 107.868 grams per mole.

  • Do the calculation and we find that 31.1 grams equals 0.288 moles of silver.

  • From the molecular equation we can see that in order for it to balance

  • we'll need one mole of sodium chloride for every mole of silver.

  • So to get 0.288 moles of silver, how much sodium chloride do we need?

  • We've made it easy for you; it's 0.288 moles.

  • So then we convert 0.288 moles of sodium chloride into units of mass and grams.

  • Sodium chloride's molar mass is 58.45 grams per mole.

  • Multiply 58.45 grams per mole by 0.288 and we find:

  • we need 16.8 grams of sodium chloride to precipitate out 1 troy ounce of silver out of silver nitrate solution.

  • And look at that you get a nice pile of silver chloride in solution. Yes, it's not pure silver not yet.

  • Just like real miners who dig ores out of the ground that contain just a few percent silver,

  • we need to do some refining.

  • In our case, there's another type of reaction necessary called a redox reaction.

  • Redox is short for reduction-oxidation and that is what we'll be talking about next time.

  • Thank you for watching this episode of Crash Course Chemistry.

  • This week we discussed what defines precipitation reactions

  • and how you can determine what precipitants they form.

  • We also learned how to write precipitate reaction equations

  • and finally we calculated a molar mass equation to figure out how much of a reactant we need