CHANGES IN MATTER
 Molecules in Solids, Liquids, and Gases
 Heat and Temperature
 Changes in Phase or States (Physical
Changes)
 Classifying Matter
Teachers – Before viewing the slide show, right click on the document
below. Go to “Document Object” and select “Open”. Print out
worksheet and have students complete while viewing the PowerPoint.
WORKSHEET TO ACCOMPANY
“PCM 1, 4, 8 PHYSICAL CHANGES”
1. What is a “phase” as it relates to matter?
2. List two things that can cause a change in phase, or a “phase transition”
3. What about the molecules determines the physical state or phase of a substance?
4. What is the only kind of motion possible for a molecule in a solid?
5. Sketch and describe the arrangements of atoms and molecules in a solid, and in a
liquid, and in a gas.
6. What is the name of the process that is the opposite of condensation? What phases
are involved?
MATTER EXISTS IN PHASES
Matter exists in various
forms, or phases.
If the temperature and/or
pressure of a sample of
matter is adjusted, the
matter may undergo a phase
transition.
During a phase transition,
matter shifts between its
three states: solid, liquid,
and gas.
PHYSICAL STATES
 The state of a material is determined by the attraction between its
atoms or molecules and by the temperature of the material.
 In the solid state, the attraction between the atoms or molecules is
so strong that it holds them rigidly in place. The energy of
vibration of the molecules of a material increases with a rise in
temperature.
 As the temperature rises, the molecules eventually acquire enough
energy to break away from their fixed positions, and the solid
either melts or transforms directly into gas (a process called
sublimation).
 The material melts if the molecular attraction remains great
enough to hold the molecules together, and the material sublimes
to a gas (in which the molecules are free to move randomly) if the
attraction is too small.
MOLECULAR
ARRANGEMENTS
SHAPE and VOLUME
PHASE TRANSITIONS
SOLIDS
 Solids are matter that
maintain their own shape
instead of conforming to the
shape of their containers.
 Under normal conditions,
steel, copper, and diamonds
are solids.
 Individual grains of table salt
are also solids; each keeps
its own shape and size.
MMM, Sugar!
This electron microscope image of
raw cane sugar reveals the shape of
sugar crystals.
The crystals form after purified cane
juice has been heated and some of
the water in the juice has
evaporated, leaving behind a cane
syrup.
Seed crystals added to the syrup
make the sugar molecules dissolved
in the syrup separate from the liquid
to form larger, solid crystals around
the seed crystals.
HEAT
Heat is a form of energy that is
transferred from one body to another
because of a difference in temperature.
Temperature is the property that gives
physical meaning to the concept of heat.
The temperature of a substance is a
measure of the average speed of all atoms
and molecules in the substance.
WHAT IS HEAT ENERGY?
All matter is made of atoms and molecules in
constant motion. When heat energy is
absorbed by matter, the random internal
energy and the motion of these atoms and
molecules are increased.
The increase is of two kinds--an increase in
straight-line motion and in rotational motion
of the atom about its own axis.
This increase makes itself felt in the form of
heat, and when it occurs the temperature of
the matter rises.
TEMPERATURE
 It can be observed that if a hot poker is plunged into cold water,
the poker becomes cooler and the water becomes warmer. This
means that the hot body gives up some of its heat to the cold
body. The exchange of heat will continue until the water and
the poker have the same temperature.
 Thus the temperature of a substance will determine whether
heat flows from it or to it when the substance is in contact with
another body at a different temperature.
 In 1802, the chemist Joseph L. Gay-Lussac found that all gases,
when heated through one degree, expand by 1/273 of the
volume that they occupy at the freezing point of water. It was
reasoned that if the gas were cooled, its volume would decrease
by the same amount as the temperature decreased.
 Further study has supported the idea of an absolute zero. It is
now defined as the temperature at which all molecular and
atomic motion stops completely.
 The temperature of a substance is a measure of the average
speed of all atoms and molecules in the substance. Absolute
zero is also the temperature below which it is impossible to go.
THERMAL EXPANSION
 The expansion of solids as
they grow warmer has
practical consequences.
 Engineers must make
sure there are gaps in the
metal of bridges so that
there is room for the
bridge to expand in warm
weather. Otherwise, the
structure would buckle or
crack.
Why do solids usually
expand when heated?
THERMAL EXPANSION
In solids, each molecule is held in
place by the attractive forces of its
neighbors, so it moves very little. But
adding heat to a solid gives its
molecules more energy of
movement, so that they rotate and
vibrate more strongly. The space
each molecule takes up therefore
increases, because it is moving about
in a greater area. This causes the
solid that comprises these molecules
to expand.
MELTING
 When the melting point for a substance has
been reached, its molecules can gain too
much energy to stay in one place.They break
away from their fixed positions and move
randomly.
 While the solid is melting, its temperature
holds steady because all the heat applied to it
goes to overcome the forces that hold the
molecules in one place.
 Once the solid has melted completely, the
heat applied to the substance again serves to
speed up the movement of its molecules
rather than to overcome the forces between
them.
 The temperature of the substance therefore
resumes its rise.
LIQUIDS
EVAPORATION and BOILING
 As heat is applied to a liquid, some
molecules gain sufficient thermal
energy to overcome the
intermolecular attraction--surface
tension--exerted by molecules at
the surface of the liquid.
 These high-energy molecules break
free from the liquid and move away.
Such molecules are now in the
gaseous state.
 As more heat is applied, more
molecules gain enough energy to
move away until at a temperature
called the boiling point of the liquid
all the molecules can gain enough
energy to escape from the liquid
state.
Gas Under Pressure
 The average distance between
molecules in the gaseous state is
extremely large compared to the size
of the molecules, so the
intermolecular forces in a gas are
quite weak. This explains why a gas
fills the entire volume of its
container.
 Since intermolecular forces are so
small, a gas molecule moves until it
strikes either another gas molecule
or the container wall. The net effect
of the many molecules striking the
container walls is observed as
pressure.
Gases act like this because they are mostly empty space between molecules!
HOW GASES BEHAVE
CLASSIFYING MATTER
SOLUTIONS
 A solution is a homogeneous mixture of two or more substances.
Solutions form because even electrically neutral molecules have
weak attractions for one another.
 Much of this attraction comes from the polarity, or slight
unevenness of the electrical charge distribution within the
molecules—a local region of slight negative charge in one molecule
attracting a region of slight positive charge in another.
 These weak opposite charges hold molecules together in a liquid
and also account for the ability of a liquid to dissolve other
substances.
 When a substance (called the solute) dissolves in a liquid (called
the solvent), the molecules of the solvent must force their way
between molecules of the solute. This occurs, for example, when
water dissolves crystals of sugar.
SATURATED SOLUTION
 While some liquids, such as water and alcohol, can
dissolve in each other in any proportion, other
compounds cannot.
 For example, salt added to water will dissolve until a
threshold is reached, after which new salt added will
no longer dissolve. This solution of salt water is then
called saturated.
 A compound’s solubility in a given solvent is
measured as the maximum amount of the compound
that a solution can dissolve.