Video Transcript
In this video, we will learn how to
describe and explain changes of state and identify these changes through diagrams
and heating curves.
The atoms and molecules that make
up matter may have different speeds. Scientists have shown that atoms
and molecules are generally in constant motion. In a solid, the atoms move very
slowly. They vibrate and have relatively
fixed positions. If a solid is heated, it gains
thermal energy. This energy causes the molecules in
the solid to move faster. If we heat a solid enough, we may
cause it to change from the solid state to the liquid state. If we heat a liquid, its molecules
will move faster. And if we heat it enough, it may
change from the liquid state to the gas state. So, adding thermal energy changes
the speed of molecules.
Adding thermal energy also changes
the strength of the intermolecular attractions between molecules. In the word “intermolecular,” the
prefix inter- means “between,” and “molecular” refers to molecules. So, intermolecular attractions are
attractive forces between molecules.
Attractive forces are forces that
pull molecules closer together. In a solid, the attractive forces
between molecules are very strong. This keeps the molecules stuck in
position and unable to move apart very much. In a liquid, the attractive forces
between molecules are weaker than those in a solid. This means that the liquid
molecules can move more freely, allowing a liquid to flow and take the shape of its
container. Finally, in a gas, the attractive
forces are the weakest. Gas molecules can move very freely,
which is why they can fill the volume of any container they are in. In summary, we can see that as
thermal energy is added to matter, the attractive forces between molecules get
weaker.
Another property that changes when
thermal energy is added is the intermolecular distance. Intermolecular distance is the
distance between molecules. This distance changes when the
state changes. In a solid, the molecules are
relatively close together. In a liquid, molecules tend to be
farther apart. In other words, the distance
between molecules is longer. And in a gas, the distance between
molecules is even longer.
Now that we know some of the
properties of molecules that change when matter is heated, let’s discuss the changes
of physical state that might result from heating. When we heat matter, we are adding
thermal energy to the matter. Another way to think about this is
that the matter is absorbing energy. Let’s say we heat a solid, and then
it changes from a solid to a liquid state as it gains enough thermal energy. This change of state is called
melting. Melting happens when a solid is
heated to its melting point, which is the specific temperature at which a solid
begins to melt.
So, what causes melting? Well, we already know that heating
a solid makes its molecules move faster. As the molecules move faster, the
attractive forces between the molecules get weaker. This causes the molecules to get
farther apart. In other words, the molecules move
away from their positions. When all of the molecules in the
solid have undergone these changes, the solid will have changed into a liquid. We can see in our cartoon of a
liquid here that the molecules in the liquid are less ordered than the molecules in
the solid. This makes sense because the liquid
molecules are more free to move about.
Now, if we heated a liquid, a
different change of state tends to happen. If a liquid absorbs enough thermal
energy, its state will ordinarily change from liquid to gas. This change of state is called
vaporization. Vaporization happens when a liquid
is heated to its boiling point, which is the specific temperature at which a liquid
begins to boil. Heating the liquid makes its
molecules move even faster. As the molecules move faster, the
attractive forces between the molecules get weaker, and the distance between the
molecules gets much larger. When all of the molecules in the
liquid have undergone these changes, the liquid will have changed to a gas.
In certain situations, a solid may
go through a change of state called sublimation. During sublimation, a solid changes
directly from a solid to a gas state. Melting is not part of the process,
so there is no intermediate liquid stage.
Now that we know the changes of
state that can happen when matter is heated, let’s talk about cooling matter. When matter cools, it loses thermal
energy. We can say that the matter releases
energy to the surrounding environment. If we cooled a gas enough, its
state would change from a gas to a liquid state. The name of this change of state is
condensation. How does condensation happen? Well, as the gas molecules lose
thermal energy, they begin to move more slowly. The attractions between the slower
molecules get stronger. These stronger attractions pull the
molecules closer together, and the gas turns into a liquid.
Now, if we cooled a liquid, a
different change of state tends to happen. If a liquid loses enough thermal
energy, it ordinarily changes from the liquid to solid state. The name of this change of state is
freezing. During freezing, the liquid
molecules slow down even more, and the attractions between the molecules get very
strong. These strong forces pull the
molecules very close together, and the solid forms. We can see in our cartoon here that
the molecules in a solid are much more ordered than in a liquid or a gas.
Now, let’s turn our discussion to
density and mass. During a change of state, the
density of a substance tends to change. Density is defined as the amount of
mass per unit volume. We can think of density as a
measure of compactness or how compact a substance is. When matter absorbs thermal energy
and changes state, it tends to become less dense. In general, when a substance goes
through melting, vaporization, or sublimation, its density decreases. The blue boxes drawn here are all
the same size. We notice that in the solid the
blue box holds the most molecules, the liquid holds less, and the gas holds the
least. This is because, in general, solids
are the most dense, liquids are less dense, and gases are the least dense state of
matter.
As we might think, when matter
releases thermal energy and changes state, it tends to become more dense. For example, when a gas condenses
to form a liquid, it becomes more dense. When a liquid freezes, it also
becomes more dense. Deposition is the name of the
change when a gas turns directly from a gas to a solid state without the
intermediate liquid stage. Even though density changes when
matter changes its state, mass does not. Mass remains constant. Mass is defined as the quantity of
matter in an object.
Let’s say we have some ice in a
beaker, and its mass is 70.0 grams. We then allow the ice to melt
completely into liquid water. If we used a balance to find the
mass of the water, we would see that it did not change. The mass of the water is the same
as the ice before melting. It is still 70.0 grams. This is because we have the same
number of molecules of water before and after melting. In fact, as long as we have a
closed system, the mass of a substance will not change when its state changes. If we use a closed container in our
experiment, then no molecules can escape, and the mass remains the same.
Now, we’re ready to discuss the
types of graphs that scientists use to keep track of changes in the temperature of
matter over time. The graph we will look at first is
called a heating curve. This type of graph also shows the
state of a substance as it gains thermal energy. The 𝑦-axis of the graph represents
temperature, and the 𝑥-axis represents increasing energy. But scientists also label the
𝑥-axis with time. The graph shown here is the heating
curve for water. Let’s start at the bottom-left part
of the graph.
This part of the graph represents
solid ice. As we heat the ice, its temperature
increases, which is why the graph has a positive slope here. The melting point of ice is zero
degrees Celsius. When the temperature of the ice is
zero degrees Celsius, it begins to melt. The ice does not change instantly
from a solid to a liquid as soon as it reaches its melting point. We must continue to heat the ice
for a period of time. During this time, both ice and
water are present. While the ice is melting, the
temperature remains constant, which is why the graph is flat here. The heat that is absorbed or
released by a substance as its state changes at a constant temperature is called
latent heat. So, the heat that is absorbed or
gained by a substance as it melts at a constant temperature is called the latent
heat of melting.
Now that the ice has melted
completely, the next section of the graph represents liquid water. If we heat the liquid water, its
temperature will rise. This is why the graph has a
positive slope here. If we heat water for long enough,
it will reach its boiling point, which is 100 degrees Celsius. At 100 degrees Celsius, water
begins to vaporize. But this change does not happen all
at once. We will need to continue to add
heat to convert all the liquid water into a gas. Both water and water vapor will be
present as we are heating. While vaporization is happening,
the temperature remains constant. The heat absorbed by water as it
vaporizes at a constant temperature is the latent heat of vaporization.
The last section of the graph
represents heating the gas form of water, which is known as water vapor or
steam. As we heat the steam, its
temperature increases. So, this part of the graph again
has a positive upward slope.
Now that we’ve discussed a heating
curve, let’s take a look at a cooling curve. A graph that shows the temperature
and state of a substance as it loses thermal energy is called a cooling curve. Let’s start at the upper-left part
of the graph. The first section of the graph
represents cooling a gas. As we cool the gas, its temperature
decreases. When the temperature of the gas
reaches the boiling point, then the gas begins to change to a liquid. This change of state is called
condensation and is represented by the first flat part of the graph.
The next part of the graph
represents cooling the liquid. While the liquid is cooled, its
temperature decreases. When the temperature of the liquid
reaches the freezing point, then the liquid begins to change to a solid. This change of state is called
freezing and is represented by the second flat part of the graph.
The last part of the graph
represents cooling the solid. While the solid is cooled, its
temperature decreases.
During condensation and freezing,
heat is being released, but the temperature of the matter is not changing. Heat that is released when the
state of a substance changes at constant temperature is called latent heat.
Before we summarize what we’ve
learned, let’s take a look at a question.
Which of the following is not the
name for a change of state? (A) Melting, (B) freezing, (C)
dissolution, (D) vaporization, (E) condensation.
Matter can have a change of state
if it gains or loses enough thermal energy. Let’s begin by looking at the
answer choices. Melting and vaporization are both
changes of state that can happen when matter gains thermal energy. On the other hand, freezing and
condensation are changes of state that can happen when matter loses thermal
energy. Melting is the change of matter
from the solid to liquid state.
Since we are looking for the word
that is not the name for a change of state, we can eliminate answer choice (A). Freezing is the change of matter
from the liquid to solid state. So, it cannot be the correct
answer. Vaporization is the change of
matter from the liquid to gas state. And condensation is the change of
matter from the gas to liquid state. Since these two options are the
names of changes of state, we can eliminate them.
This leaves only dissolution, which
must be the correct answer. Dissolution is the process where a
solute dissolves in a solvent. Dissolution is not the name of a
change of state. Therefore, the correct answer is
answer choice (C), dissolution.
Now, let’s summarize our learning
by going over some key points. Substances can change state when
they absorb or release thermal energy. The properties of molecules change
as they gain or lose thermal energy. Melting, vaporization, and
sublimation are changes of state that can happen when matter absorbs thermal
energy. Melting is the change from the
solid to liquid state, and vaporization is the change from the liquid to gas
state. Sublimation is the change from
solid directly to gas with no intermediate liquid phase.
Condensation and freezing are
changes of state that can happen when matter loses thermal energy. Condensation is the change from the
gas to liquid state, and freezing is the change from the liquid to solid state. Density changes during a change of
state, but mass does not. Latent heat is the energy absorbed
or released by matter as its state changes at a constant temperature.
