How do you know if CHEMISTRY is happening?
What are the signs?
Something has to change, right?
When an ice cube melts, you get liquid water.
Water is water, right, whether it’s frozen solid or a liquid.
Or even if you boil water, and it changes to steam, or water vapor.
It hasn’t changed, chemically - it’s still H2O in all these forms.
Nothing has happened to the water, CHEMICALLY.
What about melting something more complicated, like butter.
That’s still butter, right?
Melted butter?
But what happens if you leave it in the pan and it BROWNS?
Browned butter?
Mmm.
Something has happened - a chemical reaction.
A DELICIOUS chemical reaction.
Can we figure out from these examples what distinguishes a chemical change from a physical
change?
It looked like we were doing the same things to the water and the butter, so how do you
know when a chemical reaction has taken place?
It’s actually easier to say if a chemical reaction HASN’T taken place - when it’s
just a PHYSICAL change.
And that means recognizing what is a PHYSICAL property of matter.
Think of all the ways you can change something physically, without changing its chemical
composition.
Imagine cutting or chopping - if you divide something in half, like a bar of metal - both
pieces are still the same metal.
Let’s do it again.
Now you have four pieces.
They all still have the same chemical composition as the original metal bar.
You can keep chopping, and chopping, and chopping, and you’ll just get smaller nuggets of the
same material.
As an aside, this was the same road the ancient Greek scholar Democritus went down when he
developed the philosophy of atomism.
He was trying to account for observations about how the world worked, but in the absence
of scientific experimentation, back in the day, philosophers tried to reason out what
must be true.
In his thought experiment, Democritus kept dividing a material over and over until you
reached the smallest portion possible, beyond which you would lose that material’s essential
unique qualities.
That smallest portion is called the ATOM, derived from the Greek word ATOMOS, meaning
indivisible.
Democritus reasoned that everything was made of a variety of different kinds of atoms that
could combine and recombine with little hooks and eyes or balls and sockets.
This was the first statement of the atomic theory of the universe.
Of course, it wasn’t until much, much later that we had SCIENTIFIC evidence of atoms.
You can learn more in our video about the History of the Atom.
Now back to all the ways we can PHYSICALLY change a material - let’s make a list of
related words - all sorts of physical disruptions: Bend
Break
Cut
Crumple
Chop
Crack
Grind
Fracture
Mix
Pour
Split
Shatter
Tear
Can you think of any other physical changes?
If you think back to our water example,
we can add a few more terms:
Melt,
Freeze,
Boil,
Evaporate,
Condense...
some of these seem pretty dramatic, like boiling over a flame.
Are we saying applying a flame DOESN’T cause a chemical change?
That’s weird isn’t it?
Here’s how we know a chemical change hasn’t happened.
You can boil water, and it changes into water vapor,
and that’s still water (H2O).
You can capture that water vapor, and condense it back into liquid water -
no change, still H2O.
If it hasn’t changed the chemical composition, it’s just a PHYSICAL change.
So what was different about the butter example?
At first, the butter just MELTED, physically.
You could put that melted butter in a dish and stick it in the refrigerator,
and it would change back to solid butter.
Just like melting an ice cube, taking the liquid water,
and putting it in the freezer to change it back to ice.
So MELTING is just a physical change.
It wasn’t until you let the butter sit longer over the flame,
and it BROWNED, that we say there IS a chemical change.
What IS browning?
When you’re cooking, browning is a special chemical reaction
that happens to some food.
It’s called the “Maillard” reaction
- it involves amino acids chemically reacting with some sugars,
within a certain temperature range - around 140 to 165 degrees Celsius.
This is how bread gets a nice brown crust.
It’s also the reaction that happens when we brown meats,
or toast marshmallows.
Notice we can’t reverse it,
like we could in the case of melting butter or melting ice.
Similarly, when you caramelize sugar -
that’s a different kind of browning reaction -
the temperature has to be just right for sugar molecules to actually decompose.
Decomposition is a kind of chemical reaction.
When you caramelize sucrose, you’re changing it from a disaccharide into
the monosaccharides fructose and glucose,
and they undergo additional reactions,
forming a complex mixture of soft brown materials that we call "caramel,"
as well as some nice-smelling molecules that are volatile
and escape into the air.
You’ve changed the chemical formula of the original sugar.
In general, a physical property is something you can observe
without changing the COMPOSITION of a material.
So we can observe water’s boiling point, and
water hasn’t changed composition.
We can observe a freezing point.
We can measure how bendable or malleable a metal is -
and that doesn’t change the composition of the metal.
But if you want to observe a CHEMICAL property of a substance,
you have to actually allow it to take part in a chemical reaction.
In another video, we’ll discuss different TYPES of CHEMICAL REACTIONS.
I’ve mentioned decomposition already.
Can you think of some others?
So, in other words, a CHEMICAL property has to do with
the way a substance reacts, CHEMICALLY.
How does it change, chemically, especially
when in the presence of certain other materials.
Chemical properties include: is it chemically stable,
its flammability,
the heat of combustion,
what’s its oxidation state,
is it acidic or basic,
is it ionized,
what’s its electronegativity...there are a lot more -
there’s a huge variety of chemical changes that can take place.
When you study chemistry, you spend a lot of time thinking about how these chemical changes work,
what allows them to happen.
To be a good chemist - any kind of scientist, really -
you have to hone your observation skills.
Watch carefully, and look for signs of a chemical reaction.
Was there a release of energy in the form of heat or light?
Was a gas produced - was there bubbling?
Is there a smell?
Did the colour change?
Was there a precipitate formed?
Keep in mind, physical changes may cause a dramatic change in appearance,
but you can USUALLY undo a physical change,
if you had the right tools and enough energy.
The only way to reverse a chemical change is by doing another chemical reaction.
Pop Quiz time!
Is this a physical or a chemical change?
Shredding paper
PHYSICAL
Chopping wood
physical
Burning wood
chemical
Melting icicles
physical
Fireworks going off
chemical
Rotting food
chemical
Rusting metal
chemical
Breaking a glass
physical
Dissolving sugar
physical
Dissolving salt
ahh - this is a special challenge!
CHEMICAL
(NaCl separates into Na+ and Cl- ions).
Dissolving is a hard one, you’d have to test it to know
if something chemically came apart like this.
You’ve taken a big step as a chemist today.
It’s important to understand chemical properties,
because these are essential if you want to classify a material,
or purify it, or predict its behavior.
We need to know how a chemical will behave
if we’re going to use it safely.
Remember you can find bonus materials and practice tests at Socratica.com