Chemists are concerned about MATTER.
Very concerned.
It occupies all their waking hours.
What is “matter?”
It’s anything that has mass and volume.
So if you put a piece of matter on a scale, you can measure its mass in grams, or tenths
of grams, or millionths of grams.
You can measure its volume - how much space it takes up - in terms of cubic meters, or
cubic millimeters, or something even smaller.
In chemistry, we’re usually talking about very SMALL bits of matter - things like atoms
and molecules.
Here’s how we know we’re in CHEMISTRY world and not the real world - in chemistry
class, we’re always talking about PURE samples of matter.
What does potassium do.
How does magnesium behave when you put it in water.
What’s the pH of hydrochloric acid.
In the real world, we generally don’t stumble upon perfectly pure samples of anything.
We have to actually do a lot of work to purify materials before we can study their chemical
behavior in the lab.
The more pure a sample of matter is, the better we can predict how it will behave, based on
chemical principles.
This is one reason why chemists CLASSIFY matter, based on how pure of a sample it is.
Here’s the basic flowchart chemists refer to when they’re classifying matter: {show
flowchart}
All of matter can be classified into either pure substances, or mixtures.
A pure substance has a definite composition that doesn’t change.
That’s why Chemists want PURE SUBSTANCES to work with.
Imagine trying to run an experiment on the chemical behavior of something, and all your
samples had different compositions.
You wouldn’t be able to conclude anything from your experiments!
When we say “pure substances,” those might be either elements, or compounds.
An ELEMENT is composed of just ONE kind of atom.
Like a gold bar - it’s pure gold, all the way through, made up only of gold atoms.
Elements CANNOT be separated into simpler substances.
A COMPOUND, on the other hand, is made up of more than one element, but it always has
the same chemical formula.
H2O, for instance, water, is a compound made of two kinds of atoms, hydrogen and oxygen.
This compound always, always has two atoms of hydrogen bound to one atom of oxygen.
If you somehow force those atoms apart from each other, for instance, by electrolysis
- running electricity through it - it’s no longer that compound.
This notion that compounds always have the exact same formula is known as the Law of
Constant Composition, or the Law of Definite Proportions.
It was proposed around 1800 by French chemist Joseph Louis Proust.
There are a few fundamental ideas like this in chemistry that we almost take for granted
- of course water is always 2 hydrogen atoms for every 1 oxygen atom!
But it still has to be stated, and occasionally even defended.
For instance, you may have come across people who think a vitamin from a vegetable is different
from a vitamin synthesized in the lab.
Nope.
A pure sample of a compound is chemically identical to any other pure sample of a compound,
no matter where you got it from.
There are DIFFERENT compounds you can make of the same two kinds of atoms - for instance,
hydrogen peroxide, H2O2 is made of the same kinds of atoms as water.
Hydrogen peroxide always has this exact composition - two atoms of hydrogen bound to two atoms
of oxygen.
Water and hydrogen peroxide are two DISTINCT compounds, even though they are made of the
same kinds of atoms - the hydrogen and oxygen atoms are combined together differently in
the two compounds, and form different 3-dimensional shapes.
You wouldn’t confuse these two compounds, because their chemical composition gives them
very different chemical behavior.
Even if they look similar standing next to each other, clear liquids in a beaker, you
could tell the difference between them if you test their physical and chemical properties.
We have a video about physical and chemical properties if you want to learn more about
this idea.
Now let’s turn to MIXTURES, which is more like real life.
We run into mixtures in nature all the time.
A mixture is a PHYSICAL blend of two or more substances.
But unlike a compound, where the substances are there in fixed ratios, always identical
- a mixture can vary in its composition.
For instance, we call the blend of gases that we breathe “AIR.”
Air doesn’t have an identical composition everywhere at all times.
In general, it has a lot of nitrogen, some oxygen, some carbon dioxide, and a few other
gases.
But air in a polluted city is going to have more methane, more carbon monoxide...Air is
a mixture, not a compound - there’s no Law of Definite Proportions here.
One important thing to keep in mind about mixtures is that all the component parts keep
their original chemical behavior.
So all those gases mixed together in air - they don’t have a different chemical behavior
because they are physically mixed together.
Nitrogen still behaves like nitrogen.
Carbon dioxide still behaves like carbon dioxide.
It’s not until atoms react with each other in a CHEMICAL reaction that they exhibit different
chemical behavior as part of a new compound.
The compound HCN, hydrogen cyanide, is a completely different chemical species than a mix of those
three kinds of atoms.
Another real life example of a mixture where the composition varies is LEMONADE.
There isn’t a set formula for making lemonade, although everyone thinks their recipe is the
best.
One classic recipe follows the ratio 1: 2: 4 - 1 cup sugar, 2 cups lemon juice, 4 cups
water.
That’s WAY too sweet for me!
My recipe is 2 Tablespoons of sugar, 2 cups of lemon juice, 4 cups of water.
My lemonade is tart!
But we still call it lemonade.
It’s a mixture.
Chemists also distinguish between a Homogeneous mixture and a Heterogeneous mixture.
Can you figure out the difference between these based on a little etymology?
“Homos” is a Greek prefix meaning the same, and “heteros” is a Greek prefix meaning
“other,” or “different.”
A Homogeneous mixture has the same composition throughout - it’s a uniform mixture all
the way through.
Think of when you mix paint, and if you mix it well enough, you get the same colour all
the way through.
That’s a homogeneous mixture.
A heterogeneous mixture, on the other hand, does NOT have a uniform composition all the
way through.
You can identify different “phases” in the mixture.
Think of a bowl of cereal with milk.
Each time you stick in your spoon, you’ll pull out a different, unpredictable ratio
of components.
Sometimes you’ll get a marshmallow.
Tang vs fresh squeezed orange juice.
One is a homogeneous mixture, and the other is a heterogeneous mixture.
Tang was one of those “better living through chemistry” products from the 1950s.
It’s a powdered orange drink you mix into water.
It’s not very good, but the first astronauts drank it, so I liked to mix it up as a kid
and pretend I was going into space.
You add a tablespoon to a glass of water, mix it well, and you have a glass of uniform,
bright orange Tang.
No difference between the first sip and the last.
Tang is a homogeneous mixture.
Freshly squeezed orange juice, on the other hand, has distinct phases in it.
There are pulpy bits floating around, and each time you take a sip you’ll sometimes
get these little orange bits, and sometimes you won’t.
It’s a mixture of various things including water, and fructose, and little pieces of
orange - but it’s not uniform all the way through.
Fresh squeezed orange juice is a heterogeneous mixture.
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Time for a Pop Quiz - how would you classify the following?
Helium - Pure substance - an element, He.
Milk - Mixture - a homogeneous mixture.
Here’s a caveat - I’m talking about milk from the grocery store - It even says “homogenized”
on the label.
That’s a mechanical process where they shake the milk really hard or, sometimes they spray
the milk through tiny holes, as a way to break up the fat droplets so they’re very small,
and they’ll stay suspended evenly throughout the milk.
If you don’t homogenize milk, like when you get it straight from a cow, that’s actually
a heterogeneous mixture.
If you let it sit for a bit, you’ll see cream rise to the top.
That makes it very clear there are phases in that mixture.
Moving on.
Glucose - a pure substance, a compound.
It has a specific formula C6H12O6.
Gravity - that’s not matter, that’s a force.
Carbon Dioxide - that’s another pure substance, a compound, with the formula CO2.
A sunbeam - that’s not matter.
Photons don’t have any mass.
Trailmix - that’s a mixture (it’s in the name, trail MIX).
Heterogeneous.
Sometimes you just get a bunch of raisins, ugh.
Salt, or table salt - that’s a pure substance, a compound. NaCl.
Beach sand - mixture, heterogeneous.
It’s made up of silica compounds, salt (NaCl), and little bits of crushed shells and
who knows what else.
And beach sand varies, depending on which beach you’re visiting.
Let’s do one more exercise, to get a feel for how these concepts can be used by chemists.
What if you got a bucket of that beach sand - heterogeneous mixture - and you want to
separate out the component parts.
Let’s say, specifically, you wanted to purify out the salt.
How would you do it?
You’d use your knowledge of the properties of salt and sand, to separate out the components
of this mixture.
We know that salt dissolves in water, but sand doesn’t, so let’s add water to the
mixture.
Now you’ve dissolved the salt, and you can see sand settling out along the bottom of
your bucket.
But to catch all the sand, you might want to pour the water into a strainer or filter
it through some cloth or a coffee filter, something where the holes are smaller than
the grains of sand and little pieces of shells.
Just the salty water can run through the sieve.
How do we get the salt away from the water?
You could boil the water (or just leave it out in the sun for a long time) to evaporate
out the water, leaving the salt behind.
Sea salt!
People pay a lot for that in the shops!
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