Chapter 8
States of Matter
ALABAMA 8TH GiDE SCIENCE STANDARDS COVERED IN THIS CHAPTER INCLUDE:
7
Describe the states of mailer based on kinetic energy of particles in matter.
Have you ever looked at a pot of boiling water? The water is a liquid, but the bubbles
are a gas. And how about this: have you ever seen a half-frozen ice cube with water
trapped in ice pockets? Here we have a solid (ice) that is surrounding the liquid water.
Water can be found in solid, liquid and gas forms. How does that happen?
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Solid, liquid and gas are the names of different physical forms of
matter, or phases These are called states of matter
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Solids are made of closely-packed atoms or molecules, and maintain a rigid form
without a container. Solids are incompressible, meaning that they cannot be
“squeezed” down to a smaller size.
Liquids are made of closely associated atoms or molecules, and will flow to
conform to the shape of a container. Liquids are mostly incompressible.
Gases are made of mostly unassociated atoms or molecules, and will expand to
fill a container. Gases are compressible, meaning that their volume can be
reduced by applying force.
What causes matter to be in a certain phase?
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Substances are in certain phases depending on the amount of motion between its
particles. So, each state of matter can be described according to its molecular motion.
Energy in motion is called kinetic energy. The kinetic energy of an object is the energy
it possesses because of its motion. Typically, the higher an object’s temperature is, the
greater its energy and the motion between its particles. The temperature of an object
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States of Matter
describes its thermal energy. Thermal energy is a type ofkinetic energy. Adding or subtracting
heat (heating or cooling) changes matter from one state to another. In general, the gaseous state
of a substance has the most thermal energy, and its solid state has the least thermal energy.
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Figure 8.1 Molecules in States of Matter
SOLIDS
Solids are formed when the attractive forces between
atoms are greater than the energy causing them to move
apart. This mean they have low kinetic energy. The atoms
are locked in position near each other. Sometimes they
are in very fixed, structured positions. This is referred to
as a crystaffine solid. Examples are salt, diamond, quartz
and ice.
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Figure 8.2 Crystalline Solids
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Other times, the solid has no structure to it; these are
amorphous solids. Examples are wax and many foods, like
cotton candy.
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Chapter 8
in the solid phase, the atoms cannot move past one
another. You might think there is no motion at all, but that is
not so! Each atom still has energy associated with it, so it
must move. In fact, atoms in the solid phase vibrate. You can
think of them as small children that have been told to sit in a
chair; since they cannot get up, they squirm around in place.
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As the temperature of a solid increases, the amount of
vibration increases (and amount ofkinetic energy). The solid
will keep its shape until the amount ofenergy added is greater
figure 8.4 Particle Motion in Solids than the attractive forces between the atoms. Now the atoms
start to move, forming a liquid.
Particles can vibrate, but remain
in fixed positions with strong
association between them.
LIQuIDs
In liquids, atoms can move past one another and bump into
each other. This is why liquids can “flow” to take the shape
oftheir container. However, they still remain relatively close
to each other, like solids. As the temperature (and motion of
the atoms) of a liquid is increased, the amount of movement
of individual atoms increases. They bump into each other
with increasing thermal energy. Eventually, some particles
have enough energy to “escape.” These atoms have entered
the gas phase.
Figure 8.5 Particle Motion in Liquids
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Translation is the primary mode 1
of movement for these fastmoving particles. There is little
attraction between particles.
Particles can vibrate, rotate and
translate. Particles are not fixed,
but do associate with one another.
Atoms and molecules in the gas phase have little interaction
with each other, beyond occasionally bumping into one
that is, there are few
another. Gases have a low density
particles occupying a given space. Increasing the temperature
of a gas increases the movement of the gas particles. If they
are placed in a container at fixed volume, this increased
movement will cause them to hit the walls of that container
with greater frequency and greater force. This creates
increased pressure.
Figure 8.6 Particle Motion in Gases
From this discussion, you have seen that increasing
the thermal energy of a given material increases the
motion of the atoms or molecules of the material.
This is summed up in Figure 8.7.
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Figure 8.7 Energy Trends
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States of Matter
THE DENSITY DIFFERENCE
When there is a high amount of kinetic
energy, there is a high amount of motion
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of something that decreases in that
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situation7 The answer is density
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Density is how closely packed together
atoms are in a given volume of space. If we want to look at it
Figure 8.8 Density Differences
mathematically, the formula for density is d mlv, where d
stands for density, m stands for mass and v stands for volume. To put it another way, density is
the amount ofmafter you have to fill in a specific space. When there is more matter in a given
space, it is more dense. Let’s go back to our high energy situation. When there is a high amount
of kinetic energy it means the molecules are moving very fast. .so fast that they cannot stay
together. They spread out and as a result fewer of them remain in a given space.
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Solids comprise the densest phase of matter, followed by liquids, and in a distant third are
gasses.
PHASE TRANSITIoNS
The transformation ofmatter from one state to another
is called a phase transition. Phase transitions occur at
very precise points, when the energy of motion
(measured as temperature) in the atoms is too much or
too little for the atom to remain in their current state.
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Phase transitions are explained in Table 8.1 and
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Figure 8.9 Phase Transitions Illustrated
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Table 8.1 Phase Transitions Described
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Phase Transition
Description
Melting
Evaporation
Sublimation
Condensation
Freezing
Deposition
Solid to Liquid
Liquid to Gas
Solid to Gas (skipping Liquid)
Gas to Liquid
Liquid to Solid
Gas to Solid (skipping Liquid)
At the beginning ofthis chapter, we looked at the transition from the liquid to the gas phase and
from the solid to liquid phase. Now, let’s look back at our old friend WATER, and examine its
phase changes more closely.
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Liquid water can exist in a range oftemperatures. Cold drinking water may be around 4 °C. Hot
shower water has more energy, and it may be around 40 °C. However, at 100 °C, water will
begin to undergo a phase transition from liquid to gas. At this point, (at least for a little while)
the energy added to the liquid will not go into increasing the temperature. Instead, it willbe used
to send molecules ofwater into the gas state. So, no matter how high the flame is on the stove,
a pot ofboiling water will remain at 100 °C until all ofthe water has undergone transition to the
gas phase! Said another way, turning up the stovetop heat will accelerate the liquid-to-gas
transition, but it will not change the temperature of the water. Figure 8.10 illustrates how this
works.
Steam ncreases in temperature. —‘/
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Figure 8.10 Phase Transitions of Water
This same process can be seen in reverse if we simply look at figure 8. 10, starting on the right
side and moving left.
Molecular Motion Activity
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1 Place two clear glass beakers on a heat-resistant surface.
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2. Fill the first beaker with water at room temperature.
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3 Fill the second beaker with hot water (be careful when handling
hot water!).
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the difference in how the coloring spreads throughout the water.
Observations:
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States of Matter
CHAPTER
8 REVIEW
The figure below shows the three phases of matter of a material, X, in a closed container. Use
it to answer questions 1 3.
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Solid
Liquid
Gas
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1.
Which phase contains the least amount ofkinetic energy?
A
B
C
B
2.
the solid phase container
the liquid phase container
the gas phase container
The temperature must be known to answer this question.
Which container has the lowest density?
A
B
C
B
4.
the solid phase
the liquid phase
the gas phase
The temperature must be known to answer the question.
Tina uses a vice to squeeze each container. Which turns out to be the most
compressible?
A
B
C
D
3.
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the solid phase container
the liquid phase container
the gas phase container
they all have the same density
What would cause water to change from a solid (ice) to a liquid?
A
B
C
D
decrease its temperature
increase its temperature
decrease its energy
increase the attractive force between atoms
5. Which state of matter does not have a definite shape or volume?
A
asolid
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B
a liquid
B
none ofthem do
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