Video Transcript
In this video, we will learn what
it means for a sample to be pure and how to use purity to describe a sample that is
impure. We’ll also learn how to calculate
the purity of a substance based on its proportion by mass. And we’ll also look at how
impurities impact melting and boiling points.
Firstly, what does pure mean? In everyday life, you might see it
used to describe someone who is very moral. Or you may see it on packaging in
the supermarket, like 100 percent pure beef mince. In everyday life, pure means all of
this and nothing else. So, pure beef is beef and only
beef. But chemists use the word pure to
mean something more specific. Beef is made up of proteins, fats,
water, and many other substances. It’s still pure beef, but it’s not
a pure substance because it’s made of many different substances. So, to a chemist, a sample is a
pure chemical if it contains only one substance. There’s nothing wrong with calling
beef pure beef, but if you’ve got your chemist hat on, you need to be thinking of
the chemical version of purity.
To a chemist, a sample is either
perfectly pure or not. If a sample is not pure, we say
that it is impure. But that’s very black and
white. The slightest impurity will make a
sample impure. So, we can talk in more detail. We can think of purity as lying on
a spectrum between completely impure and completely pure. If we have a sample that’s 99
percent the chemical we want, that’s usually close enough. A sample might be significantly
less pure than this, so we can imagine a range, which we measure in percentage
points. A pure sample is 100 percent
pure. And we could imagine a sample that
doesn’t contain any of the chemical we want to be zero percent pure, although we
wouldn’t use that language very often.
Once we get about the 95 percent,
you might hear chemicals described as nearly pure because for many applications
that’s good enough. Making a relatively pure chemical
is sometimes not that expensive. But making chemicals really pure
can be much, much more expensive. Sometimes, low-purity products are
okay for certain applications like cleaning. But for some applications, even the
smallest amount of impurity can make a sample completely useless. For example, if we had medicine
that was only 99 percent pure and we didn’t know what the other one percent was, it
might not be safe to take the medicine.
Typically in the lab, we’ll use
chemicals that are in between these two extremes of purity. Of course, we don’t need to only
talk about one chemical when discussing the purity of a sample. We could talk about the purity for
all the chemicals in a sample. So, a mixture of chemical X and
chemical Y will be somewhere in between 100 percent X and 100 percent Y. It could be
50 percent of one or 50 percent of the other or anything in between. But what do these percentages
actually relate to?
When talking about percentage
purity, you could talk about a number of things, like the mass, the volume, or the
amount in moles. But the most common we use is
mass. So, we calculate percentage purity
by taking the mass due to the chemical we’re interested in, dividing by the total
mass of the sample, and multiplying by 100 percent.
Here’s an example where the mass of
the sample is 10 grams. We have eight grams of our target
chemical and two grams of our impurities. We can calculate the percentage
purity by taking the mass of the chemical we’re interested in, dividing by the total
mass of the sample, and multiplying by 100 percent, giving us a purity of our target
chemical of 80 percent in this sample. When we’re dealing with purity in
this way, you may see the symbol w/w, which stands for weight of chemical per weight
of sample.
We’ve looked at percentage purity,
but we haven’t really examined what it means to be an impurity. If pure fun is the hour you spend
playing laser tag, the impurity is the 20 minutes it takes to get there. What we call an impurity depends on
the situation. Imagine you’re in the kitchen and
you’re doing some baking, and you want to use sugar or salt. Now, imagine sprinkling a little
bit of salt onto a pile of sugar and a little bit of sugar onto a pile of salt. If you want pure sugar, but there’s
also a little salt there, the salt is the impurity. But if you want pure salt and
there’s a little sugar, the sugar is the impurity.
Quite often, impurities are quite
similar to the chemical we want. For instance, in powdered calcium
carbonate, there might be a little calcium oxide. Both of these are ionic compounds
that contain calcium and oxygen. And impurities could also have
nothing to do with the chemical. When we have sugar and salt, we
have two very different chemicals. One is an organic compound with
covalent bonds between carbon, hydrogen, and oxygen. And the other is an inorganic
compound with bonds between sodium ions and chloride ions.
Impurities can be harmful, so you
could have corrosive sodium hydroxide mixed in with salt. Or like a little salt in some
sugar, they can be harmless, although perhaps a baker would disagree. Ultimately, it comes down to the
situation you’re in and the nature of the impurity.
So far, we’ve looked at describing
purity and impurities. But how do we go about testing
purity? Fine table salt is a white
powder. Fine sugar can look very
similar. Imagine that you have a really fine
mixture of the two. And imagine you don’t know how much
is sugar and how much is salt. For solids that are very, very
different chemically, like table salt and sugar, it’s quite a tough job to do. But something interesting happens
if chemicals are similar enough, like a mixture of table sugar and glucose, which is
the molecule that can be made from sugar.
There’s a simple observation that’s
very reliable with mixtures like these that you can use to tell if something is
close to pure in one direction or the other. The melting point of sugar is 186
degrees Celsius. Generally, with a pure chemical
substance, there will be a very precise melting point. Below that melting point, it will
be solid. And above that melting point, it
will be liquid. But for a sample that’s mostly the
chemical with a little impurity, the melting point will be quite broad and will be
lower than the pure value.
This is what we might see for a
sample that’s 99 percent sugar. Perhaps below 184 degrees Celsius,
it’s a solid. And above 185 degrees Celsius, it
will be a liquid. In between, we’ll see a mixture of
solid and liquid, no matter how long we heat at that temperature. These numbers have been made up for
slightly impure sugar. We can’t be sure exactly what the
melting point of 99 percent sugar would be because it can depend on the
impurity. But for something that’s mostly
pure, we can be sure of a broad melting point that’s lower than the standard
value.
Here, we’ve looked at sugar and
slightly impure sugar. But we could’ve looked at glucose,
and we’d see the same effect, the melting point of an impure mixture being slightly
lower than the melting point of pure glucose. All this means that we have a
simple test. If we have a sample we think it’s
pure, its melting point should be exactly that of the pure chemical. So, if we test the melting point of
the sample we think is pure sugar, if it’s 186 degrees Celsius and it’s a very sharp
melting point, then we’re probably dealing with pure sugar. If, however, the melting point is
below 186 degrees Celsius and it’s quite broad, then we’re probably dealing with
impure sugar. And we’ll need to do other tests to
determine what the impurities are.
But so far, we’ve been looking at
mixtures of solids. What about mixtures of liquids? For pure liquids, the boiling point
will be a very precise, repeatable value, like for water; it’s 100 degrees
Celsius. If we add some sodium chloride,
table salt, we may see the boiling point rise as high as 104 degrees Celsius. But bear in mind, there are some
exceptions where adding something to water will decrease its boiling point rather
than increase it. But generally speaking, when
looking at impure samples, we expect a broad melting point lower than the pure value
and a higher boiling point.
The last thing we’re going to look
at is how you adjust if you know that your sample is impure. In fact, for most of human history,
we’ve rarely had samples we would call pure chemicals today. Water from the ocean has lots of
different impurities. Even rainwater has dust from the
atmosphere and dissolved gases in it. Metals or their ores from the
ground are hard to purify. And the technology to make them
pure substances has only existed for the last few hundred years. Chemistry has been happening quite
happily without pure chemicals. This is because in many cases
impurities don’t interfere with the reaction in question.
But if you want to produce
rainwater or you want to make a really pure metal for a certain application, there
are some things you can do. There are many, many forms of
purification, but all of them help to remove impurities, leaving a more pure sample
behind. Filtration, crystallization, and
distillation are just some examples appropriate to different use cases. The alternative is simply to use
more of the sample.
Let’s say, for instance, you’ll
need exactly one gram of sodium bicarbonate for a reaction. But you only have 95 percent pure
bicarbonate of soda and the rest is perhaps something unimportant, like sodium
chloride. All we need to do is make an
adjustment. We know it’s 95 percent pure, so
all we need to do is add a little bit more so that the total mass of bicarbonate is
one gram. In this case, we’d need to add
about 1.05 grams of our 95 percent pure sodium bicarbonate to have one gram of
sodium bicarbonate in there. This is just the principle; we
won’t be going into the mathematics of this in this video. Instead, it’s time for some
practice.
The image below shows a labeled
bottle of orange juice. Why might the company claim the
orange juice is 100 percent pure?
Here’s our bottle, here’s the
label, and here we can see stamped on the bottle the claim the orange juice is
100 percent pure. In everyday language, when we
see the word pure, it generally means that we’re dealing with that thing and
nothing else. So, this is orange juice and
nothing but orange juice. This means there’s no apple
juice, no blackberry juice, and no bananas. So, all of the juice in the
bottle came from oranges. Something else we might expect
when we see the word pure on food or drink is that nothing’s been added. This means sugars and
artificial sweeteners and so forth haven’t been introduced. The exception we might make for
orange juice is that water’s been added. Therefore, a company might
claim the orange juice is 100 percent pure because it contains no added or
artificial products.
Why might a chemist say that
the orange juice is not pure?
To a chemist, the word pure has
a slightly more specific meaning. We use pure when referring to a
sample that contains only one chemical substance. The substances inside orange
juice include water, different types of acid, sugars, and, depending on the
type, there may be more or less fiber from the pulp. But this is only a short list
of the many, many substances you might find in orange juice. All these substances are
different chemicals, so orange juice is not a pure chemical. So, a chemist might say that
the orange juice is not pure because it does not contain just one substance.
Next, let’s have a go at
calculating percentage purity.
An impure sample of magnesium
chloride has a mass of 50 grams. After perfect purification, 45
grams of magnesium chloride is recovered. What is the percentage purity
of the original sample?
Magnesium chloride is a salt
with formula MgCl2. And we’re told we have a sample
containing magnesium chloride with a mass of 50 grams. But it’s impure. Some of the sample is magnesium
chloride, but some of the sample is not. Next, we’re told this sample’s
undergone perfect purification. When we perform a purification,
our aim is to remove impurities. In a perfect purification,
we’re removing all the impurities and not losing any of our target chemical. In this case, what we’re
getting out is 45 grams of 100 percent pure magnesium chloride.
The question is asking us, what
is the percentage purity of the starting sample? So, just to recap, we have our
starting sample, which weighed 50 grams. It was then purified, removing
the impurities, leaving 45 grams of magnesium chloride. So, we must’ve removed five
grams of impurities because we can’t gain or lose mass in cases like this. We know there were five grams
of impurities because 50 grams minus 45 grams is five grams.
Now, the question isn’t after
the mass of impurities. It’s after the percentage
purity of the original sample. And we calculate the percentage
purity by taking the mass of the chemical in the sample, divide by the total
mass of the sample, and multiply the result by 100 percent. The mass of chemical is the
mass of magnesium chloride recovered. And the mass of the original
sample was 50 grams. We can then multiply that by
100 percent. This gets us 0.9 times 100
percent, which is 90 percent. So, the mass of the original
sample that was due to magnesium chloride is 90 percent.
Next, let’s look back at the key
points. In chemistry, when we use pure to
describe a sample, we mean it’s made of only one chemical substance. And we might call that sample 100
percent pure. We often describe purity using
percentage purity, which gives us the percentage of the mass of the sample that’s
due to the particular chemical. We calculate this by taking the
mass of chemical in the sample, divide it by the total mass of the sample, and
multiply the result by 100 percent.
An impurity is simply any other
substance that isn’t the desired one. If we introduce impurities into a
pure sample, we’ll lower the melting point. Doing this will also broaden the
distance between the temperature at which we have 100 percent liquid and the
temperature at which we have 100 percent solid. And generally speaking, if we
introduce an impurity to water, we’ll raise the boiling point of the water. And this occurs with other liquids
too.
