Chapter 13 States of Matter

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Chapter 13
The skunk releases its spray!
Within seconds you smell
that all-too-familiar foul
odor. You will discover some
general characteristics of
gases that help explain how
odors travel through the air,
even on a windless day.
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Kinetic Theory and a Model for Gases
◦ What are the three assumptions of the kinetic
theory as it applies to gases?
• The word kinetic refers to motion.
• The energy an object has because of its motion is called
kinetic energy.
• According to the kinetic theory, all matter consists of tiny
particles that are in constant motion.
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• According to kinetic theory:
a. The particles in a gas are considered
to be small, hard spheres with an
insignificant volume.
b. The motion of the particles in a gas
is rapid, constant, and random.
c. All collisions between particles in a
gas are perfectly elastic.
• Particles in a gas are in rapid, constant motion.
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• Gas particles travel in straight-line paths.
• The gas fills the container.
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Gas Pressure
◦ How does kinetic theory explain gas pressure?
• Gas pressure results from the force exerted by a gas
per unit surface area of an object.
• An empty space with no particles and no pressure is called
a vacuum.
• Atmospheric pressure results from the collisions of atoms
and molecules in air with objects.
◦ Gas pressure is the result of simultaneous collisions
of billions of rapidly moving particles in a gas with
an object.
• A barometer is a device that is used to measure
atmospheric pressure.
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• The SI unit of pressure is the pascal (Pa).
• One standard atmosphere (atm) is the pressure required
to support 760 mm of mercury in a mercury barometer at
25°C.
• Reference table A
Kinetic Energy and Temperature
◦ What is the relationship between the temperature in
kelvins and the average kinetic energy of particles?
Average Kinetic Energy
◦ The particles in any collection of atoms or
molecules at a given temperature have a wide range
of kinetic energies. Most of the particles have
kinetic energies somewhere in the middle of this
range.
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• Absolute zero (0 K, or –273.15°C) is the temperature
at which the motion of particles theoretically ceases.
• Particles would have no kinetic energy at absolute zero.
• Absolute zero has never been produced in the
laboratory.
• Reference table A
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◦ Average Kinetic Energy and Kelvin Temperature
• The Kelvin temperature of a substance is directly
proportional to the average kinetic energy of the
particles of the substance.
• In this vacuum chamber, scientists cooled sodium
vapor to nearly absolute zero.
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Hot lava oozes and flows,
scorching everything in its
path, and occasionally
overrunning nearby houses.
When the lava cools, it
solidifies into rock. The
properties of liquids are
related to intermolecular
interactions. You will learn
about some of the properties
of liquids.
A Model for Liquids
◦ What factors determine the physical properties of a
liquid?
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• Substances that can flow are referred to as fluids. Both
liquids and gases are fluids.
◦ The interplay between the disruptive motions of
particles in a liquid and the attractions among the
particles determines the physical properties of
liquids.
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Evaporation
◦ What is the relationship between evaporation and
kinetic energy?
• The conversion of a liquid to a gas or vapor is called
vaporization.
• When such a conversion occurs at the surface of a liquid
that is not boiling, the process is called evaporation.
• In an open container, molecules that evaporate can
escape from the container.
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• In a closed container, the molecules cannot escape.
They collect as a vapor above the liquid. Some
molecules condense back into a liquid.
◦ During evaporation, only those molecules with a
certain minimum kinetic energy can escape from
the surface of the liquid.
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Vapor Pressure
◦ When can a dynamic equilibrium exist between a
liquid and its vapor?
• Vapor pressure is a measure of the force exerted by a
gas above a liquid.
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◦ In a system at constant vapor pressure, a dynamic
equilibrium exists between the vapor and the liquid.
The system is in equilibrium because the rate of
evaporation of liquid equals the rate of
condensation of vapor.
◦ Vapor Pressure and Temperature Change
• An increase in the temperature of a contained liquid
increases the vapor pressure.
• The particles in the warmed liquid have increased kinetic
energy. As a result, more of the particles will have the
minimum kinetic energy necessary to escape the surface
of the liquid.
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◦ Vapor Pressure Measurements
• The vapor pressure of a liquid can be determined with
a device called a manometer.
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Boiling Point
◦ Under what conditions does boiling occur?
◦ When a liquid is heated to a temperature at
which particles throughout the liquid have
enough kinetic energy to vaporize, the liquid
begins to boil.
◦ The temperature at which the vapor pressure
of the liquid is just equal to the external
pressure on the liquid is the boiling point (bp).
◦ Boiling Point and Pressure Changes
• Because a liquid boils when its vapor pressure is
equal to the external pressure, liquids don’t always
boil at the same temperature.
• At a lower external pressure, the boiling point
decreases.
• At a higher external pressure, the boiling point
increases.
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• Altitude and Boiling Point
Ref. table H
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◦ Normal Boiling Point
• Because a liquid can have various boiling points
depending on pressure, the normal boiling point is
defined as the boiling point of a liquid at a pressure of
101.3 kPa.
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In 1985, scientists
discovered a new form of
carbon. They called this
form of carbon
buckminsterfullerene, or
buckyball for short. You will
learn how the arrangement
of particles in solids
determines some general
properties of solids.
A Model for Solids
◦ How are the structure and properties of solids
related?
◦ The general properties of solids reflect the orderly
arrangement of their particles and the fixed
locations of their particles.
◦ The melting point (mp) is the temperature at which
a solid changes into a liquid.
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Crystal Structure and Unit Cells
◦ What determines the shape of a crystal?
◦ In a crystal, the particles are arranged in an orderly,
repeating, three-dimensional pattern called a
crystal lattice.
The shape of a crystal
reflects the arrangement
of the particles within the
solid.
◦ Crystal Systems
• A crystal has sides, or faces. Crystals are classified into
seven crystal systems.
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• These minerals show four out of the seven crystal
systems.
• The smallest group of particles within a crystal that
retains the geometric shape of the crystal is known as a
unit cell.
• A crystal lattice is a repeating array of any one of fourteen
kinds of unit cells.
• There are from one to four types of unit cells that can be
associated with each crystal system.
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• Three kinds of unit cells can make up a cubic crystal
system.
◦ Allotropes
• Allotropes are two or more different molecular forms
of the same element in the same physical state.
• Allotropes have different properties because their
structures are different.
• Only a few elements have allotropes.
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• Carbon Allotropes
◦ Non-Crystalline Solids
• An amorphous solid lacks an ordered internal
structure.
• Rubber, plastic, asphalt, and glass are amorphous solids.
• A glass is a transparent fusion product of inorganic
substances that have cooled to a rigid state without
crystallizing.
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Familiar weather events can
remind you that water exists on
Earth as a liquid, a solid, and a
vapor. As water cycles through
the atmosphere, the oceans,
and Earth’s crust, it undergoes
repeated changes of state. You
will learn what conditions can
control the state of a
substance.
Sublimation
◦ When can sublimation occur?
◦ The change of a substance from a solid to a vapor
without passing through the liquid state is called
sublimation.
◦ Sublimation occurs in solids with vapor pressures
that exceed atmospheric pressure at or near room
temperature.
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• When solid iodine is heated,
the crystals sublime, going
directly from the solid to
the gaseous state. When the
vapor cools, it goes directly
from the gaseous to the
solid state. This is called
deposition.
Phase Diagrams
◦ How are the conditions at which phases are in
equilibrium represented on a phase diagram?
◦ A phase diagram is a graph that gives the
conditions of temperature and pressure at which a
substance exists as solid, liquid, and gas (vapor).
◦ The conditions of pressure and temperature at
which two phases exist in equilibrium are indicated
on a phase diagram by a line separating the phases.
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• The triple point describes the only set of conditions at
which all three phases can exist in equilibrium with
one another.
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