Thanks to ACS Central Science for supporting PBS Digital Studios.
This is gold.
This is water.
Wow, that’s boring.
But this is called “royal water”.
THIS IS CHEMISTRY Today we’re talking acids, but before we
get rolling, a strong note of caution:
all of the demos in this video were done by a highly trained chemist, in a chemistry lab
fully equipped to deal with the substances we’re showing.
NO ONE out there should even attempt these demonstrations without the right facilities,
training, or equipment.
Now that we’re clear on that:
Acids are clearly reactive.
Who hasn’t been awed in chemistry class mixing some acidic vinegar together with basic
old baking soda?
Some more than others.
Here’s baking soda reacting with strong acids, even after they’ve reacted with some
of the stuff we’ll show in this video.
That clump of baking soda is bubble surfing because the reaction is so vigorous!
Vinegar doesn’t do that.
So how do acids work, and why do chemists value them so much?
The classic definition of acids is that they throw hydrogen ions, aka protons, off.
Protons are reactive little things because nature in general hates to concentrate charge
into single points.
When a loose proton comes upon certain molecules, an electron melee ensues and we’re left
with new products.
For example, an acid plus a metal will form hydrogen gas and a “salt” of the metal
and the rest of the acid molecule.
And if you’re thinking table salt, that’s ONE kind.
But there are lots of others.
In chemistry, a salt just means a metal plus a nonmetal.
Fun fact!
Chalk is a salt.
Now you know.
So those bubbles when we stick this here metal thing into hydrochloric acid are pure hydrogen.
That grey stuff is forming an aluminum chloride salt.
Because this is Reactions, we need to mention this model of how acids work is not the only
one.
A more general rule says that acids LOVE grabbing electron pairs from other molecules.
That rule lets chemists talk about acids that don’t have hydrogen atoms.
That’s for another video.
But regardless of the theory you use, acids can be really useful in the chemistry lab.
Centuries ago the alchemists figured out acids like hydrochloric or nitric could do a number
on metals.
Even metals that don’t rust or melt in your average oven.
Have another look at that hydrochloric acid having a go at the aluminum.
And here is nitric acid going to town on some copper.
That’s right, copper - the stuff that aside from a little surface verdigris, makes for
pretty lasting landmarks.
Even silver isn’t safe from nitric acid, though we didn’t have any of the shiny stuff
kicking around to show you.
So this ability of acids to readily chew up metals could make for some clever detective
work that allows us to figure out whether this gold is real or not...
These hunks of billionaire tissue paper look the same to the eye..
This one dissolves in 2 of these vials.
But this one only dissolves in that last vial on the right.
Unfortunately, we wanted to show you pure sulfuric acid, but the stuff we used was…
well it was crap.
It was still pretty strong, strong enough to mess with this paper towel.
But this demo with the sugar was supposed to be a lot more interesting.
[sad trombone sound].
Left to right here we’ve got sulfuric acid, then hydrochloric acid, followed by nitric
acid, then this regal water we’ll get to in a sec.
This is imitation silver, which happens to be aluminum.
Regardless, that’s still a metal that you probably know from kitchen experience doesn’t
just dissolve willy nilly in water nor do anything remotely exciting with vinegar or
lemon juice.
But here’s what it does when subjected to strong acids.
Now here’s some copper foil.
Look at those green hues in the nitric.
And the dark blue in the vial on the right.
We expected a deep blue color in the sulfuric - this was our first hint that acid had something
wrong with it.
Now let’s look at these two sparkly golden foils.
Here’s foil A in the acids, and below is foil B.
Chemistry tells us they are not the same thing: A is reacting in the two right-most vials.
B on the other hand is only doing anything in this vial of goldish-red.
That is a liquid of near magical properties passed down from way back when chemists were
still alchemists.
That is a vial of “royal water” that you might have heard of its Harry Potter moniker,
aqua regia.
This stuff is made from 3 parts hydrochloric and 1 part nitric acid.
Unlike the aluminum, the copper, or metal A, this metal B does not do anything except
in the aqua regia.
That means metal B is gold.
Let that sink in for a minute … we have dissolved real gold.
Yep, aqua regia dissolves the noble metal of gold.
Hence its royal name.
So this begs the question what the heck is metal A?
It reacts the same as the copper.
Looking at its label, we’re told it too is a mix of copper and zinc, making it … plain
old boring brass.
Not only did we dissolve gold, but we were able to spot an impostor, all by throwing
acids at them, making us...chemistry detectives.
Coming this fall to CBS!
Ok, maybe not.
BIG THANKS to Dr. Matt Hartings, a professor at American University here in Washington,
DC.
Again, Matt is a trained professional allowing us access to a facility where these demos
can be done safely.
Thanks Matt!
Thanks to ACS Central Science for making this video possible.
Acids can do all sorts of chemistry from catalyzing reactions to helping us investigate the origins
of life.
You can read about them and more in the American Chemical Society’s premier open access journal
ACS Central Science.
It’s free to both authors and readers.
Click the link in the description to find out more.
Let us know in the comments if you’d like our take on superacids … but we gotta say
we’re just not gonna do demonstrations for those.
Stay safe everyone, thumbs up, and thanks for watching.