Let’s pretend for a second that you’ve been invited to a costume party to celebrate Halloween.
Except, when you arrive dressed as C-3PO, you realize it isn’t a costume party at all.
Everyone else is wearing formal clothes, and you are the only one in a shiny droid getup,
sticking out like a lightsaber in the dark.
Chemical substances can also experience this awkward situation.
Not every substance dissolves well into another, and some are just bound to end up separated.
At the not-so-costumed party, everyone else’s attire is soluble; it blends in well with the crowd.
We could call your stellar costume insoluble — it doesn’t jive with the aesthetic everyone else went for.
How well a substance can dissolve in solution is called solubility, and this is what we’ll
be discussing in today’s episode.
Hi, I’m Will Comar, and welcome back to Study Hall Chemistry, presented by Arizona
State University and Crash Course.
Let’s dive in!
In order to become solubility masters, we need to first understand some key terms.
A solvent is a substance, usually a liquid, that can dissolve another substance.
A solute is the substance that is being dissolved.
It doesn’t have to always be a solid — it can also be a liquid or gas.
Together, a solute and solvent form a solution.
To put this into context, if we made lemonade from a powdered drink mix and some water,
the water would be the solvent,
the powder would be the solute, and the lemonade would be the combined solution.
Solubility is driven by lots of different factors, like the chemical and physical properties
of the substances, or their temperatures and pressures.
For instance, if I want sweet tea and stir in some sugar when the tea is cold, the sugar
won’t dissolve well, and it’ll sit in tiny granules at the bottom of my pitcher.
No thank you.
When substances don’t dissolve once added to a solvent, like your costume at the party,
they are insoluble.
If they combine with the solvent to form an insoluble compound, we call this precipitation.
The solid that forms and separates out from the rest of the solution is called a precipitate.
Substances that do dissolve well, like the more formal dressers, are said to be soluble.
For example, if we mix sodium chloride, or table salt, with water, they form a homogenous
solution without separation.
We can think of a homogenous solution like everyone wearing the same suit at the party.
But sometimes substances aren’t one hundred percent soluble or one hundred percent insoluble.
They may be slightly soluble, where only a small amount, between 0.1 and 1 grams of the
solute, can be dissolved in 100 milliliters of solvent.
An example of this is calcium oxide, which won’t fully dissolve in water.
The amounts of solute and solvent are also very important in determining solubility.
We call the ratio of solute to solvent in a solution its concentration.
We’ll talk a bit more about concentration in future episodes, but for now we just need
to know why it affects solubility.
Think about it this way.
If we add a tiny amount of salt to another big glass of water, we’ll have an unsaturated solution.
There’s less than the maximum amount of solute the solvent is capable of dissolving.
If we add more salt to it, it’ll dissolve in the water until it reaches saturation,
the point where the maximum amount of salt has dissolved.
This is also known as equilibrium, where everything that can dissolve does.
If the amount of salt goes past saturation, this is called a supersaturated solution,
where any added solute will remain in its solid form or precipitate out.
Think of this like musical chairs — every atom of solute finds a partner in the solvent
up until there are no more left,
and the rest of the solute has to stand awkwardly off to the side.
With a high enough amount of salt in the water, we’d see salt granules at the bottom missing
out on all the fun.
We can see supersaturation in volcanic lakes too.
Carbon dioxide gas is emitted through volcanic activity and supersaturates in the water.
Since there’s too much carbon dioxide absorbed, some is released as gas back into the atmosphere.
If too much gas is released all at once, though, it can cut off oxygen from organisms in the area,
so vents are set up in lakes where this is common to release the gas in safe levels.
But there are even more factors to consider when there are ionic substances involved.
To begin, let’s talk about dissociation, which is what happens once the solute and
solvent are combined.
The components split up into their positively charged cations and negatively charged anions
when in solution.
We’ve described ions in previous episodes if you want to go back and review.
And we talk more about them in the next episode.
Not all substances are ionic, so some won’t dissociate in solution, but the ones that
do are called electrolytes.
You may recognize the term from products like Gatorade, which promise to replenish important
electrolytes lost during exercise.
That’s because electrolytes dissolved in solution give it the capability to conduct electricity.
It’s probably not on the same scale as the Emperor’s Force lightning,
but our neurons and muscle tissues need to conduct electricity in order to
keep us alive and functioning.
Some electrolytes we’ll see a lot in general chemistry are sodium chloride, nitric acid,
and calcium chloride.
But not all electrolytes are created equal.
There are strong electrolytes, which dissociate completely into their ions, and weak electrolytes,
which only dissociate partially.
Solubility and dissociation are also influenced by how reactive the substances are when combined.
The more reactive an element is, the easier it will be for it to dissociate, and the more
likely it is to make a strong electrolyte.
The less reactive it is, the harder it will be to dissociate, and the more likely it is
to make a weak electrolyte.
So we need to know where elements fall in the activity series, which is a list of elements
from most reactive to least reactive.
Like we mentioned last episode, elements are more reactive the closer they are to the left
side and the lower down they are on the periodic table.
So elements at the bottom left of the periodic table are our most reactive, including lithium,
potassium, calcium, sodium, and magnesium.
Toward the other, less reactive end would be metals like silver and mercury.
Things might get more complicated when we start looking at compounds, but don’t worry.
As any scientist would love to tell you, we have a chart for this!
It’s called a solubility chart, and it’ll save us the headache of memorizing the solubilities
for each individual compound.
Solubility charts list the solubilities of common substances, and there are lots of versions
set up in varying formats.
For example, if we wanted to know the solubility of potassium chloride, let’s use these special
Ion Vision glasses to split up the compound.
With these on, we can see the ions that make up our substances, and in this case, we see
a potassium cation and chloride anion.
If we look on the solubility chart for the potassium ion, we can tell it’s soluble,
and so is the chloride ion.
Neither of these ions falls into an exception category either, so we’ll conclude that
the overall compound potassium chloride is soluble.
We could try a harder one.
If we wanted to find out the solubility of calcium hydroxide, we could check the table
for the hydroxide anion, OH minus.
It looks like hydroxide is insoluble, but wait...it says except with the calcium ion,
with a star next to it that says slightly soluble.
So, if hydroxide is paired with calcium, that compound is slightly soluble.
Let’s try another table.
Pretend we have a reaction between the base sodium hydroxide and magnesium chloride.
We’ll first predict the products, sodium chloride and magnesium hydroxide, using our
previous knowledge of double displacement reactions.
Next, we balance it, and then we need to determine its state of matter by using the solubility rules.
Let’s watch this play out on TestTube.
We should first use our Ion Vision to split out our reactants, sodium hydroxide and magnesium
chloride, into cations and anions.
Let’s distribute the coefficients around to keep the reaction balanced.
If we look at this solubility chart for our ions, we can find out which of these are soluble
and assign our states of matter.
Sodium and hydroxide paired together is soluble, and so is magnesium when paired with chloride,
so those are both aqueous.
Now let’s try our products, sodium chloride and magnesium hydroxide.
In Ion Vision, sodium chloride dissociates.
Chloride and sodium are both soluble for this reaction, so we’ll put (aq) for sodium chloride.
But on the table, when hydroxide is paired with magnesium, the compound is insoluble,
so we can expect magnesium hydroxide to form a precipitate.
This also means it won’t dissociate into ions, so we’ll keep it in its solid compound
form even in Ion Vision.
Earlier, we saw how concentration and temperature could make a soluble substance less soluble.
But some compounds, like magnesium hydroxide, just are insoluble, no matter how much or
how little we have in a solution or what the conditions are.
That’s why solubility tables are so helpful.
We won’t have to memorize the solubility of tons of substances or do anything drastic
to discover when we’ll have a precipitate.
Of course, there are some circumstances where all the substances in a solution are soluble.
When that happens, it just means no precipitation reaction has occurred.
So every ion’s found its place in the crowd! Thanks, Test Tube!
Solubility has a lot of important and real world applications.
From an environmental chemistry perspective, certain pollutants can become soluble, like
when nanoparticles of silver are used in washing machines and refrigerators.
They’re marketed as a disinfectant, but the downside is the nanosilver finds its way
into drinking water and dissolves,
which might cause health problems for humans.
Since the particles are so small, they’re difficult to remove during water treatment,
and may not always precipitate out — even though most silver compounds are insoluble.
As another example, the medical industry relies on something called The Biopharmaceutics Classification System, or BCS,
to classify different drugs based on their ability to be absorbed into the human body.
When prescribing medicines, doctors have to take into account important factors,
like how quickly the medicine dissolves in the body, how soluble it is, and whether it
should be taken as a pill or an injection.
This affects the size of the dose administered and how soon the drug will start working.
For us non-medical doctors, the solubility of a medication could affect when we need to take it.
We might want to give our allergy pill a couple hours to dissolve before we meet our friend’s
new puppy, for instance,
or remember to always pair a more insoluble med with food so it’ll absorb better.
Solubility is one of those topics in chemistry that affects a wide range of human experiences.
It gives us the ability to evaluate our drinking water for dangerous pollutants, devise medicines
that dissolve well in our bodies to make us feel better in no time,
and make delicious sweet tea for friends.
We’ll continue using the concepts from this episode, including our solubility rules and
helpful tables, through many other chemistry discussions to come.
Solubility is a bit like the Force, always there when we need it to guide us through reactions.
Next time, we’ll talk about ionic equations, and what we can learn about them when
we use our Ion Vision.
Thanks for watching Study Hall: Chemistry, which is produced by Arizona State University
and the Crash Course team at Complexly.
If you liked this video and want to keep learning with us here in Study Hall, be sure to subscribe.
You can learn more about ASU and the videos produced by Crash Course in the links in the description.
See you all next time!