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Quarter 2 Competency 4 | What Makes a Pure Substance PURE? | MATATAG Science Grade 8
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What makes a pure substance pure? Is it
because it looks clean? Because it
tastes plain? Or is there something
deeper? Something invisible that defines
it? In this video, we're diving into
pure substances and what they really
Imagine [Music]
you're known for your voice, your style,
even your smile. These little things,
they make you uniquely you. You don't
randomly wake up with a new identity,
right? Well, in science, some materials
have an identity, too. We call them pure
substances made of just one kind of
building block. No surprises, no mixups.
And that's what makes them stand out in
chemistry. But what exactly is a pure substance?
substance?
It's a material made of only one kind of
particle, either single atoms or
particles made of atoms bonded together.
To understand this better, let's explore
two examples. a gold bar and a beaker of
water. If we can zoom in to observe the
gold bar, we'd see only gold atoms
packed closely, all exactly the same.
Now, let's examine the water. Every drop
contains identical particles, each made
of two hydrogen atoms joined to one
oxygen atom.
Gold and water may seem different, but
each one is stays the same all
throughout. The key is this. The
composition never changes.
That's what makes them pure.
When scientists say that a substance has
a fixed chemical composition, they mean
something very specific.
It means that every single sample of
that substance, no matter where you find
it, always contains the same kind of
atoms in the same ratio.
Take water for example. Pure water
always has two hydrogen atoms bonded to
one oxygen atom. That's what H2O means.
Not 2 and 1/2, not 3 and 1, always 2 to
1. No more, no less. That's what we mean
by fixed chemical composition.
So remember, pure substances have the
same kind of particles in the same ratio
every time.
So if both gold and water are pure substances,
substances,
what makes them different?
Well, pure substances come in two types,
elements and compounds.
Gold is an element. It's made of just
one kind of atom. Water, on the other
hand, is a compound made of different
atoms chemically bonded in a fixed ratio.
ratio.
Let's dive in on the first type, elements.
elements.
Elements are made of only one kind of
atom. All the particles are exactly the
same. Whether it's gold, oxygen, or
argon, an element never mixes in atoms
of a different kind, each atom type has
a unique number of protons. That's its
atomic identity.
And because every atom in an element is
the same, the substance stays consistent
all throughout.
So what makes a gold atom gold or an
oxygen atom oxygen? The answer is
protons. The number of protons in the
nucleus decides the element. Gold always
has 79 protons. Oxygen always eight.
Argon 18. That never changes. It's like
a fingerprint. If the proton count
changes, it's not the same element
anymore. So in a pure substance like
gold, every atom has the same number of
protons, 79. When all the atoms are the
same, you get a fixed composition. And
that's what makes it a pure substance.
Now let's talk compounds. Compounds are
pure substances too, but with a twist.
They are made of more than one kind of
atom. And those atoms are always
combined in a specific unchanging ratio.
Take salt also called sodium chloride.
Every unit is made of one sodium atom
and one chlorine atom. This atoms form a
chemical bond locking together in that
exact ratio. That ratio doesn't change.
No matter where you get it, table salt
is always an ACC.
Never Na2 CL or N A C2.
Now, not all compounds are as simple as
salt. Let's look at two more examples.
Carbon dioxide and ammonia. Carbon
dioxide or CO2 is always made of one
carbon atom and two oxygen atoms. Ammonia
Ammonia
or NH3 is always made of one nitrogen
atom and three hydrogen atoms. Even
though they're made of different atoms,
the rule stays the same. They're always
bonded in a fixed ratio. And that's what
makes them pure substances just like
water, salt, and gold.
Now, not everything around us has this
fixed recipe. Let's clear things up by
comparing pure substances to mixtures.
A pure substance has particles that are
all the same, whether it's a single
element or a compound. A mixture, on the
other hand, combines different particles
and the ratio can change.
Think of salt water. When salt dissolves
in water, it doesn't disappear. It
breaks apart into ions.
Inside the mix, you'll find water
molecules, sodium ions, and chloride
ions all swimming around together.
They're not held together by chemical
bonds, it's just a physical mix. And
because there's more than one kind of
particle, salt water is a mixture, not a
pure substance.
All right, now that you've seen what
makes something a pure substance, let's
put your eyes and your brain to the
test. Can you tell which ones are truly
pure and which are just clever mixtures
in disguise? Let's test your science
instincts. Are these materials pure
substances or not?
Take a look at each one. Just drink air
in a balloon, ethanol, medical oxygen,
and vinegar. Think carefully, then get
ready for the answers. This lemon drink
might look uniform, but it's actually a
mixture. Sugar, water, flavoring, maybe
even coloring, all blended, not
chemically bonded.
Air seems invisible and even, but it's a
mix of gases.
Nitrogen, oxygen, carbon dioxide, and
more. So, nope, not a pure substance.
Ethanol, on the other hand, is a
compound. It's made of carbon, hydrogen,
and oxygen atoms. And every molecule has
the same formula C2 H5O.
That fixed structure makes ethanol a
pure substance.
Medical oxygen is a pure substance too.
And it's an element. It contains only
oxygen molecules, nothing else. That's
why it's safe for patients who need it.
Now, vinegar, it often tricks people.
Some think it's a compound, but it's
actually a mixture. Why? Because it's
just water and acetic acid mixed
together, not chemically bonded.
Each part keeps its identity. So,
vinegar is a mixture.
So, remember pure substances have only
one kind of particle.
Mixtures have different particles
physically combined, not chemically bonded.
bonded.
All right, quick recap. Pure substances,
they're made of just one kind of
particle, either an element or a
compound. Their makeup always the same.
No surprises.
Elements, they've got only one kind of
atom like gold or oxygen. Compounds made
of different atoms chemically bonded in
a fixed ratio like water or salt.
Now mixtures, that's a totally different
story. They're just physically combined
and the ratio can change. Think salt
water, lemon drink, or even the air you
breathe. So next time you're holding
something and thinking, is this pure?
Just remember what you've learned today.
Science is all about asking the right questions.
questions.
This is Learning with G. Stay tuned for
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