Let’s think about what makes you, well, you.
Without getting too deep into the philosophical nature of identity, we could define ourselves
in a couple different ways:
By our innate qualities, physical appearances, and the individual cells in our bodies, or
by our actions and our reactions to the things around us.
Just like people, chemical substances can be identified by their individual composition
and by the way they react to their surroundings.
They have distinct and important relationships with each other. And, like all of us, when
put in certain situations, they are capable of radical change.
Today, we’ll discuss the physical and chemical changes substances can undergo, as well as
how to recognize which of those changes has taken place.
Welcome to Study Hall: Chemistry, presented by Arizona State University and Crash Course.
I’m your host, Will Comar. Let’s get started!
We can’t dive right into the world of chemistry without defining a few things.
First, a substance is matter that has a distinct composition and unique properties. It’s
like a person, with a certain look and personality.
When we say “substance,” we might mean an element, a substance comprised of only
one type of atom that can’t be broken down any further.
Or we might be talking about a compound, a substance comprised of
one or more elements bonded together.
And when we think about change from a chemistry perspective, there are two main kinds a substance
can experience: physical and chemical.
Let’s take water as an example.
It's a compound made of two hydrogen atoms and one oxygen atom bonded together
to make H2O molecules!
We’re pretty familiar with the solid, liquid, and gaseous states water can occupy because
we interact with them all the time.
For example, when it’s cold out, you might boil water for tea or find the duck pond in
the park frozen into ice.
When water freezes, its molecules move closer together and are less free to move around,
so they form a solid.
If we were to melt the ice, its molecules would have more energy to move apart and the
water would take the shape of a liquid.
If we kept going and boiled the water, we’d see the H2O molecules getting more excited
and breaking free to become steam, further spreading apart from each other in its gaseous state.
There’s a lot more involved in this process and we can't get into all of it right now,
but it's still important to mention!
The molecules that make up the water are still H2O. They’re just changing their proximity
to each other.
This is a physical change.
To put this in perspective, if you were to cut your hair, that would be a physical change.
It changes the state of your hair from longer to shorter, but doesn’t really change anything
about who you are as a person. You’re still you, just with a cool haircut.
Now, the world isn’t made entirely of H2O, and change isn’t just physical.
We’ve got lots of other substances with their own molecular structures, all interacting
with each other to make other combinations.
Those interactions have to be described somehow.
This is where chemical change comes in. We can recognize a chemical change has occurred
when one or more substances undergoes a reaction involving all of the following:
first, the rearrangement of their atoms, second, a change in their properties or composition,
and third, the formation of at least one new substance.
Let’s visualize this with an example.
Rust forms from a chemical process where iron oxidizes in the presence of water. You might
see it on historic vehicles or license plates, or old mysterious keys. It’s pretty common.
The process begins when iron from the car, oxygen in the air, and water from humidity
all come into contact.
When the humidity reaches a high enough level, it tips the scales and initiates a chemical change.
Then, first, the atoms in iron, water, and oxygen rearrange. Second, the chemical composition changes
from three separate substances into a combination of them.
Third, that combo is a new substance: iron [three] hydroxide, or rust.
But, hang on. In cases like rust or souring milk, it may be easy to see that something
has occurred here, but not all chemical reactions are obvious.
Luckily, chemistry comes with a side of detective work.
We can look for evidence of a chemical change, because there are a couple clues these processes
leave for us to find.
Those clues could be a change in color or temperature, the formation of a gas, the formation
of a solid in solution, called a precipitate, or the emission of heat, light, or odor.
Each of these occurrences is proof to us that there has been a rearrangement of atoms, a
change in composition, and the production of a new substance.
Let’s use our detective skills to solve a couple cases using these clues as evidence.
Case #1: Say we add a small amount of liquid sodium chloride to liquid silver nitrate,
and as a result, a visible, separate solid forms in the combined solution.
This precipitate informs us that a chemical reaction has taken place, and we can eventually
conclude the solid is silver chloride, suspended in sodium nitrate.
Case #2: If we were to mix sodium bicarbonate, also known as baking soda, with vinegar, carbon
dioxide would form as a result.
By attaching an inflatable balloon to the top of the container where the two substances
are combined, we can watch it expand as carbon dioxide is produced.
This should provide us with enough evidence to crack the case: the formation of a gas
reveals a chemical reaction has occurred.
Case #3: If we go to a New Year’s celebration where fireworks are set off across the skyline,
those fireworks are actually the result of a chemical change.
Exploding fireworks produce both a temperature increase and an emission of light, proving
beyond doubt that a chemical change has happened.
Case #4: In some places, companies add a sulfur compound to their natural gas as a safety precaution.
If we were to experience a gas leak, it’d be impossible to see the gas entering the room,
but we’d be able to smell the rotten-egg odor of the sulfur interacting with the air
and know a dangerous chemical change was underway.
Luckily, the smell would help us solve the case and resolve the leak safely.
Hopefully by now the difference between a physical and chemical change is clear.
If not, think of it this way: if you were to dye your hair, that would be a chemical change.
You’re altering the chemical structure of the hair cortex with ammonia and peroxide
to permanently recolor it.
Turns out there’s a lot of science going on at the hair salon.
While there are plenty of ways to catch a chemical change in real life, we won’t be
able to do a full experiment each time we need to figure out what certain substances
will do when combined.
We need some way of denoting chemical changes, or putting them on paper.
Though there are variations, we represent chemical changes, which we can call chemical
reactions, with a similar structure.
Think about this like a sentence or a mathematical equation. Let’s denote the chemical reaction
for rust as an example.
The reaction here starts with reactants on the left side of the arrow. These are the
substances the reaction originates with.
To get from the left side to the right side of the equation, the substances must undergo
a chemical reaction.
This is typically represented by an arrow, although sometimes the arrow is accompanied
by symbols to indicate what kind of reaction is taking place, or the conditions required.
You might run into arrows with the kind of catalysis, or initiating factor, needed for
a reaction, or a delta symbol indicating the reactants are heated.
In some cases, the arrow might point in both directions, signifying the reaction is reversible.
On the right side of the equation are the products, the substances formed by the reaction.
Now, there are a lot of numbers here. First, each substance has its own set of coefficients,
which are used to balance equations.
The key to interpreting a coefficient is to treat it the same as you would in a mathematical
equation: it applies for all the terms to its right.
So if we look at 4 Fe, the coefficient is four.
That means we have 4 iron atoms in that expression.
The 4 only applies to Fe, the 3 to O2, the 6 to H2O, and so on.
We’ll discuss coefficients in more detail in future episodes, so all we need to know
right now is that chemical equations need to be balanced on either side, like a math problem.
Next, there are subscripts in some chemical formulas. Unlike coefficients, subscripts
only apply to the element they directly follow.
For instance, look at the iron 3 hydroxide on the products side. The hydroxide molecule
(the OH) has a subscript of three.
This means there are three oxygen atoms and three hydrogen atoms each.
This, when multiplied by the overarching coefficient of four, becomes twelve oxygen atoms and twelve
hydrogen atoms total.
Subscripts can tell us more than just quantities.
In the case of oxygen on the reactants side, the subscript 2 below the O also denotes that
oxygen is diatomic.
So one oxygen molecule has two oxygen atoms.
In their pure elemental form, diatomic elements always exist as a molecule with two atoms
of that element bonded together. There are seven of these in the periodic table:
Hydrogen, oxygen, nitrogen, bromine, fluorine, chlorine, and iodine. This’ll come in handy
later in the series.
If all of this information sounds like a lot to take in, that’s because it is!
Chemistry can often be a challenging and non-intuitive subject.
So, throughout the course of this series, we’ll take things one step at a time to
shake off those mental blocks.
We’re all capable of excelling at chemistry. We just need the right combination of preparedness and confidence.
To contextualize what we’ve covered, let’s go to the TestTube to take a look at a real-world
example of a set of life-giving chemical reactions.
Say we plant an apple tree in a garden.
We don’t need to feed the apple tree the same way we’d need to provide hay for a
pet chinchilla, or make pancakes for our best friend. We just give it water.
And yet, the tree still produces fruit. Because chemistry!
More specifically, because of two chemical reactions that serve as a fundamental energy
cycle for most living things on our planet.
The apple tree uses photosynthesis to convert water, carbon dioxide from the air, and light
energy from the sun into two things:
glucose, which is a sugar plants use for energy, and oxygen, which is emitted into the surrounding air.
On paper, we see that carbon dioxide and water are the reactants, with glucose and oxygen
as the products.
The reaction arrow specifies sunlight as the energy driving the reaction.
This little line of numbers and letters reveals how our tree gives us apples to snack on and
oxygen gas to make air breathable for us, our buddy, and our adorable chinchilla.
Eating an apple completes the other part of this chemical reaction process, called cellular respiration.
Your body converts the sugars from the apple, along with oxygen you inhale, into energy and carbon dioxide.
You’ll notice the equation for cellular respiration is familiar. It’s the photosynthesis
chemical reaction we just discussed, but reversed.
We can write our equation out with a double-sided arrow to show this back and forth.
Eating an apple allows us to produce reactants for future photosynthesis, so we and our tree
coexist in a chemical reaction cycle that is mutually beneficial.
Chemical reactions summarize the changes happening around us every day.
We can write them out to represent the important and life-giving processes taking place, and
we can use them to gain a better understanding of the amazing things physical
and chemical changes can accomplish.
As we continue through this series, we’ll get to see more of what a world empowered
by chemistry knowledge looks like, and why this is so important not just for our education,
but to give context to our own human experiences.
Next time, we’ll delve deeper into the types of chemical reactions, as well as where we
might encounter them in daily life.
There might also be a dance party. See you next time!
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 next time!