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Reading 14.1
What Happens to Molecules When a
Substance Melts?
JUMPSTART
Ice cream melts. Butter melts. You have probably observed both of
these. But you probably have not seen a glacier melting. Glaciers
are like big rivers of ice. They are found on every continent except
Australia. The glacier in the picture is the white part that curves
around the land.
Maybe you have heard that the world’s glaciers are getting smaller. It’s probably easier
to think about a glacier getting smaller if you think about watching an ice cube melt.
As you read about the molecules in an ice cube, think about how what you’re reading
might apply to glaciers too.
© IQWST
WHAT HAPPENS WHEN I PUT ICE IN A GLASS OF SODAPOP?
Many people like their drinks cold. One way to make a drink cold is to put ice
cubes in it. When you put ice in a warm drink, the ice melts and the drink gets
colder.
The diagram below can help you think about what happens to the molecules
when ice sits in a glass of warm liquid. At first, the ice molecules are stuck in a
specific arrangement. They are in the solid state, so they move by jiggling. The
warm liquid molecules surround the ice cube and move around it. The moving
liquid molecules collide with and bounce off of the jiggling ice molecules.
The jiggling ice molecules get more energy from the liquid molecules. Some
of the ice molecules gain enough energy to start moving past each other. The solid
water begins to change into liquid water when the ice molecules start to move around.
Another way to say that a substance changes from the solid state to the liquid state is
to say it melts. The ice continues to melt as more ice molecules get enough energy to
move around. This continues until all of the solid water changes to liquid water.
Water molecules are locked
into a hexagonal pattern
Ice
Liquid
Water molecules have too much energy to
remain in place, but are slow enough
to stay clumped together
6th Grade Chemistry
Vapor
Water molecules
move fast enough to break
free of each other
Reading 14.1
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Reading 14.1, continued
WHY DOES A DRINK GET COLD WHEN THE ICE CUBES MELT?
In class, you saw that when you heated candle wax, the wax changed from the solid
state to the liquid state. Wax is a solid at room temperature. It needs to be heated to
become a liquid. When wax is heated, the wax molecules get enough energy so that
they can move around. When that happens, the wax melts into a liquid.
Ice cubes also need to be heated to melt. An ice cube is heated by the warm drink. The
warm drink particles move around and collide with the ice cube molecules. Those
collisions can make the ice cube molecules jiggle faster. But the collisions can also
make the warm drink particles move slower. As the drink particles continue to collide
into the ice cube molecules, several things happen:
1.
2.
3.
4.
Collisions make the ice cube molecules move around.
The ice cube melts.
The warm drink molecules start to slow down.
The drink becomes cold.
So, when you put ice in a warm glass of sodapop, as the ice melts it also cools your
beverage!
HOW IS MELTING A PROPERTY OF A SUBSTANCE?
Have you ever melted butter in a frying pan or spread butter on hot corn on the
cob? Butter starts to melt when it reaches a temperature of 32.3° C or 90.1° F. This
temperature is called the melting point of butter. Melting point is the temperature
at which a solid substance starts to become a liquid. Once a solid reaches its
melting point, it stays at the same temperature until it is completely melted. Solid
butter stays at 90.1° F until it is completely melted.
WHAT HAPPENS TO MOLECULES AS SOMETHING FREEZES?
If you live in the northern part of the United States, you have felt how cold winters can
be. Often people say, “It’s freezing outside,” when it really isn’t freezing. When the air is
cold enough, rain can turn to snow. Rain turns to snow at about 32° F. Snow is a solid
form of water. So, 32° F is the temperature at which liquid water “freezes.” When liquid
water freezes, it changes into a solid. The liquid water molecules do not move around
fast enough to continue moving. Instead, the molecules slow down to the point that
they stay in a fixed spot and move by jiggling. They freeze.
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The melting point of a substance is a property of that substance. Remember that a
property of a substance is characteristic of that substance. That means that the melting
point is the same no matter how much of the substance you measure out. A spoon full
of butter melts faster than a whole stick of butter. But both of them start to melt at the
same melting point.
GO
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Date
Reading 12.1
How Do Water and Odors Go into the Air?
JUMPSTART
Here are three phenomena with something in common:
1. Water boils on the stove, and steam fills the kitchen.
2. Wet clothes hang on a clothesline and become dry.
3. After it rains, puddles form on the ground, but they get smaller over time.
Can you tell what all three of these have in common? They all occur because water
molecules can move from the liquid to the gas phase. In this reading, you’ll learn more
about water changing phases. You’ll also learn that understanding what water does can
be related to pancakes and waffles!
WATER CHANGES PHASES IN EVERYDAY LIFE!
When you take a bath or shower and dry yourself with a towel, the towel gets wet. If
you hang the towel up, and then feel it at the end of the day, the towel will not be as
wet. If it hangs up over night, it will be dry by the next day. How does a towel dry?
As a towel hangs, single water molecules on the towel get enough energy to leave the
towel and go into the air. The word for this process is evaporation.
© IQWST
Another example of water evaporating happens in a puddle in your
yard or on the road after it rains. If you measured the amount of water
in a puddle, and every hour you went back and measured the amount
of water again, you would notice the puddle getting smaller. You might
have thought that the puddle gets smaller because the water soaks into
the ground. Some of the water could do that, but a lot of it evaporates.
Water molecules on the surface of the puddle leave the puddle and
move into the air.
WHAT ELSE EVAPORATES?
In class, you saw another substance (bromine) evaporate. Unlike invisible
gaseous water, bromine is a reddish substance. You saw bromine in the gaseous
state and the liquid state. First, you watched bromine gas condense into a liquid.
Second, you saw liquid bromine evaporate back into a gas. You also drew models to
show how substances like bromine can condense and evaporate.
Describe how the model you used in class explained how bromine can go from
the liquid state to the gas state:
GO
6th Grade Chemistry
Reading 12 .1
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Reading 12.1,continued
MORE ABOUT PHASE CHANGES: HOW HOT CAN I HEAT WATER?
In class you heated water and measured the temperature. At first, the more you heated
the water, the higher the temperature the water got. But then, the temperature of the
water stopped increasing. It stayed at 100° Celsius, even though you continued to heat
the water. How can a substance stay at a certain temperature even though it is being
heated? Shouldn’t the water keep getting hotter and hotter?
You know that if the temperature of a substance increases, then the speed of the
particles also increases. When a substance is heated, its particles move faster. The
opposite also happens. If the temperature does not increase, then that means the
speed of the particles does not increase. You saw that the temperature of water did not
increase once it reached 100° Celsius. That means the water molecules continued to
move around, but they didn’t move faster. Something else must have been happening to
the molecules of water as they continued to gain energy.
This “something else” is that all of the energy was being used to make the water particles
separate, and turn into water vapor. Liquid water particles became gaseous water
particles. This is called boiling. Boiling happens at a specific temperature. The boiling
point of water is 100° Celsius (or 212° Fahrenheit). That temperature is the point at
which all of the water molecules begin to separate and change to a gaseous state.
© IQWST
OUTSIDE OF SCIENCE CLASS, WHEN ARE BOILING AND EVAPORATION IMPORTANT?
Do you ever eat pancakes, waffles, or French toast with syrup? Maple syrup is
made using the processes you have been studying in class, and that you have
just read about. Maple syrup comes from the sap inside of maple trees.
On the following page is some information from the Internet that tells
about maple syrup. As you read, underline the parts that describe how
boiling and evaporation are used to make syrup. Before you read, you should
know that the article refers to boiling sap in an “evaporator.” So, it sounds like
boiling and evaporating are the same thing. You know that boiling and evaporating are
two different processes. When you read, keep in mind what you’ve already learned.
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Reading 12.1, continued
(From http://www.bennersfarm.com/HA%20folder/Festival%20lore/ha_maplelore.htm)
Making Maple Syrup
THE SAP
Maple sap is a barely sweet, thin, watery
liquid, much like weak sugar water. It looks
nothing like the thick amber syrup it becomes.
Sunlight, carbon dioxide from the air, and
chlorophyll in the leaves work together to
make the sugar that nourishes the tree. The
sap is stored in the bark and wood all winter,
and begins to flow throughout the tree as the
days begin to warm in late winter and early
spring.
The sugary sap helps the tree grow and live.
carry the sap from the tree.
Sap is collected in a gathering tank that
is often pulled into the sugarbush on a
horsedrawn sled. The sap is poured into the
gathering tank through a cone-shaped metal
filter that removes pieces of bark and leaves.
This is the first of three filterings.
When the gathering tank is full, the sled
is driven to a location uphill from the
sugarhouse and the sap poured into a storage
tank or an evaporator. Sap must be boiled
within a week, or it may spoil.
HOW IT’S DONE NOW:
An upwardly slanted hole no more than half
an inch wide or one and one half inches deep,
is drilled into the trunk of a healthy, mature
sugar maple. More than one hole can be
drilled in a single tree, but not too many; the
tree also needs the sap. Long ago when trees
were gashed with an ax, the sap would pour
out, but the wounds didn’t heal well and the
trees soon died.
A tap, or spile, is hammered into the hole and
a bucket hung on the spile to collect the sap
as it drips from the tree. Buckets are covered
with “hats” to keep out rain and snow. In some
large operations, plastic tubes are used to
The sap is boiled in an evaporator, a series
of partitioned boiling pans about 4' by 3' by
1' deep. The boiling sap is moved through
the partitions as it reaches graduated stages
of evaporation and thickens into syrup. The
darker the syrup, the sweeter it gets. It takes 40
gallons of sap to make one gallon of syrup.
GO
© IQWST
The pans are divided into sections to separate the
more concentrated sap from the more dilute. The
sections are not closed. Sap can move freely as the
water evaporates.
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Reading 12.1, continued
Sap turns to syrup at 218° F. (At this point
things can happen fast, and the sap can
turn to sugar, burn, and even explode as the
temperature rises.)
There are signs that the sap is about to turn to
syrup; it becomes darker, an amber color, and
the bubbles become very fine, then suddenly
grow huge and explosive looking. The final
test is called sheeting or aproning, when the
liquid slowly gathers along the edge of the
scoop and does not dribble off in separate
drops. Sap does not sheet, only syrup does.
At the sheeting stage, the syrup is drawn
off through a spigot at the end of the last
partition in the evaporator.
The finished syrup is now filtered through
felt funnels lined with heavy paper, the final
step in cleansing the syrup of any foreign
matter . . . .
. . . the finished syrup is poured into
containers to be sold and the trees prepare
to greet the summer warmth and begin the
process over again.
It’s all over in a few weeks.
BOIL OR FREEZE?
An alternate method used by Native
Americans was to allow the collected sap to
freeze overnight in shallow vessels. In the
morning they would discard the ice and and
repeat the process until a thick syrup was left
in the containers. Some feel that this method
makes the sweetest, clearest syrup.
WHAT HAPPENS TO THE MOLECULES WHEN YOU HEAT MAPLE SAP?
The reason people can make syrup is because the sap has both sugar molecules and
water molecules. Think more about what happens to the molecules, and answer these
two questions.
What happens to the water molecules in the sap when the sap is heated?
A) the water molecules evaporate.
© IQWST
B) the water molecules stay in the co ntainer.
C) the water molecules melt.
D) the water molecules change into syrup molecules.
What happens to the sugar molecules in the sap when sap is heated?
A) the sugar molecules evaporate.
B) the sugar molecules stay in the container.
C) the sugar molecules melt.
D) the sugar molecules change into syrup molecules.
6th Grade Chemistry
STOP
Reading 12.1
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Reading 14.2, continued
Activity/Reading 8.3, Are There Other Phase Changes?
HOW MANY WAYS CAN A SUBSTANCE CHANGE PHASES?
So far you learned a substance can change phases:
1. From a solid to a liquid—and—from a liquid to a solid.
Solid
Liquid
2. From a liquid to a gas—and—from a gas to a liquid
Liquid
Gas
Now you know a substance may be able to change into the gas state without first
melting. You can show this by grouping the states of matter these together, and then
using another arrow to show that solids can go directly to the gaseous state:
Solid
Liquid
Gas
IS THERE A PROCESS OPPOSITE OF SUBLIMATION?
You now know that some substances skip the liquid phase, and change from a solid
right to a gas. Some substances can also change from the gaseous state to the solid
state. In fact, you observed menthol change from the gas state to the solid state in
Lesson 3. You saw the menthol crystals form on the top of a flask, where there was no
liquid menthol!
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6th Grade Chemistry
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© IQWST
Draw an arrow above, that connects “gas” to “solid.” That arrow can help you
remember that some substances can also change from a gas to a solid, without ever
being a liquid! In fact, snow is an example of this phenomenon! The section on the
following page describes this process.
Name
Date
Below, read about how snowflakes form to find out more about changing phases between solids
Reading 14.2, continued
and gases. Note that this article uses the terms condenses instead of deposition to describe the
process of going from gas to solid.
http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm
The Life of a Snowflake
The story of a snowflake begins with water
vapor in the air. Evaporation from oceans,
lakes, and rivers puts water vapor into the air,
as does transpiration from plants. Even you,
every time you exhale, put water vapor into
the air.
When you take a parcel of air and cool it
down, at some point the water vapor it
holds will begin to condense out. When this
happens near the ground, the water may
condense as dew on the grass. High above
the ground, water vapor condenses onto dust
particles in the air. It condenses into countless
Water
minute droplets, where each droplet contains
droplet
at least one dust particle. A cloud is nothing
more than a huge collection of these water
droplets suspended in the air.
In the winter, snow-forming clouds are still
mostly made of liquid water droplets, even
when the temperature is below freezing.
The water is said to be supercooled, meaning
simply that it is cooled below the freezing
point. As the clouds gets colder, however,
the droplets do start to freeze. This begins
happening around –10° C (14° F), but it’s a
gradual process and the droplets don’t all
freeze at once.
If a particular droplet freezes, it becomes
a small particle of ice surrounded by the
remaining liquid water droplets in the cloud.
The ice grows as water vapor condenses
onto its surface, forming a snowflake in
the process. As the ice grows larger, the
remaining water droplets slowly evaporate
and put more water vapor into the air.
Note what happens to the water—it
evaporates from the water droplets and goes
Snowflake
into the air, and it comes out of the air as it
condenses on the growing snow crystals. As
the snow falls there is a net flow of water from
the liquid state (cloud droplets) to the solid
state (snowflakes). This rather complicated
chain of events is how a cloud freezes.
QWST
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