Kinetic Theory
Vocabulary/Main Idea:
1. Kinetic theory of matter is the idea that all matter is made up of
tiny, constantly moving particles.
2. Matter is solid when it has a definite volume and shape.
3. Matter is liquid when it has a definite volume and takes the shape
of its container.
4. Matter is a gas when it has no definite shape or volume.
5. The melting point of a substance is the temperature at which that
substance changes from solid to liquid.
6. The freezing point of a substance is the temperature at which that
substance changes from a liquid to a solid.
7. The boiling point of a substance is the temperature at which the
interior of a substance changes from liquid to a gas.
In all substances, molecules and the atoms that make them up are in
constant motion. This idea is called the kinetic theory of matter. The
amount of this motion is related to the state of matter. The state of a
sample of matter depends on its temperature. All substances are in one of
(3) three states of matter – solid, liquid or gas.
A solid is any matter with a definite volume and shape. A block of ice is
water in the solid state. Solids can’t be forced into a space smaller than its
volume. A liquid is any matter that has definite volume but takes the shape
of its container. Tap water is in the liquid state. Liquids also can’t normally
be squeezed into a container smaller than its volume. A gas is any matter
with no definite shape or volume. Water that evaporates from puddles
becomes water vapor, which is invisible water in the gas state. Gases can
expand and contract to fill the space they are in. gases can also be
compressed into a smaller space. Propane is a gas that is pumped into a
metal container. As more and more propane is pumped, the gas is placed
under pressure because more gas is forced into the same space.
For any substance, the motion of its molecules increases as it changes
state from solid, to liquid, to gas. Because of this, a change in state also
indicates a change in molecular motion.
In the different states, the motion of particles is different. Scientists use
the kinetic theory to explain the properties of each state of matter in terms
of the motion of its particles. Kinetic energy is the energy of movement.
Kinetic theory describes the movement of molecules.
According to the kinetic theory, the molecules in a gas have enough
energy to move freely and collide freely with one another. The molecules in
a gas are much farther apart than in liquid or solid. In a liquid, the molecules
have just enough energy to slide past one another. For this reason, liquids
flow and can be poured from one container to another. In a solid, molecules
don’t have enough energy to move past one another. They can only vibrate
in place.
A liquid can also change to a gas without ever reaching the boiling point.
At the surface of a liquid, some molecules have enough energy to break free
and become a gas. This process is evaporation. A shallow pan of water left
out on a counter will evaporate over time, leaving an empty pan.
Scientists have measured the temperature and pressure conditions that
exist when most substances change state. The point at which a solid
changes to liquid is the melting point of that substance. The point at which a
liquid changes to a solid is the freezing point of that substance.
It is important to understand that the melting point and freezing point
for any substance are the same. Whether the substance is being cooled or
warmed determines the change in state that results. For example, when
liquid water is cooled at 0 deg Celsius (32 deg Fahrenheit), it starts changing
to solid ice. Ice warmed at this temperature changes to liquid water.
Not all substances change from solid to liquid to gas. Some substances,
like dry ice (solid carbon dioxide), go directly from the solid state to the gas
state in a process called sublimation. Also, some substances can exist in two
solid states. Graphite, for example, can be soft black solid or a hard, clear,
crystalline solid.
Another change of state happens at the boiling point. The boiling point
of a liquid is the temperature at which the molecules of the substances
change to gas and make bubbles. When liquid water is heated to 100 deg
Celsius (212 deg Fahrenheit), its molecules form bubbles of water vapor. As
water boils, these water vapor bubbles escape and rise into the air.
Condensation is the reverse of boiling. When a gas is cooled, its particles
move more slowly. As the gas further cools, it changes back to a liquid. The
temperature at which a gas changes to a liquid is called condensation point.
The chart shows some substances and the temperatures at which they boil
and condense, and melt and freeze.