Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
125
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 12
Practice Problems
9. Classify each crystalline solid as molecular, ionic, covalent
network, or metallic.
a. NaCl
c. Fe
b. SiO 2
d. H 2 O
12.4 Phase Changes
Most substances can exist in three states—solid, liquid, and gas—
depending on the temperature and pressure. States of substances are
called phases when they coexist as physically distinct parts of a mixture, such as ice water. When energy is added to or taken away from
a system, one phase can change into another.
Phase changes that require energy You know the three phases
of water: ice, liquid water, and water vapor. When you add ice to
water, heat flows from the water to the ice and disrupts the hydrogen
bonds that hold the water molecules in the ice together. The ice
melts and becomes liquid. The amount of energy required to melt
one mole of a solid depends on the strength of the forces keeping
the particles together. The melting point of a crystalline solid is the
temperature at which the forces holding the crystal lattice together
are broken and the solid becomes a liquid. Because amorphous
solids tend to act like liquids when they are in the solid state, it’s
hard to specify their melting points.
When liquid water is heated, some molecules escape from the
liquid and enter the gas phase. If a substance is usually a liquid at
room temperature (as water is), the gas phase is called a vapor.
Vaporization is the process by which a liquid changes into a gas or
vapor. When vaporization occurs only at the surface of a liquid, the
process is called evaporation.
Vapor pressure is the pressure exerted by a vapor over a liquid.
As temperature increases, water molecules gain kinetic energy and
vapor pressure increases. When the vapor pressure of a liquid equals
atmospheric pressure, the liquid has reached its boiling point, which
is 100°C for water at sea level. At this point, molecules throughout
the liquid have the energy to enter the gas or vapor phase.
▲
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
125
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 12
Practice Problems
9. Classify each crystalline solid as molecular, ionic, covalent
network, or metallic.
a. NaCl
c. Fe
b. SiO 2
d. H 2 O
12.4 Phase Changes
Most substances can exist in three states—solid, liquid, and gas—
depending on the temperature and pressure. States of substances are
called phases when they coexist as physically distinct parts of a mixture, such as ice water. When energy is added to or taken away from
a system, one phase can change into another.
Phase changes that require energy You know the three phases
of water: ice, liquid water, and water vapor. When you add ice to
water, heat flows from the water to the ice and disrupts the hydrogen
bonds that hold the water molecules in the ice together. The ice
melts and becomes liquid. The amount of energy required to melt
one mole of a solid depends on the strength of the forces keeping
the particles together. The melting point of a crystalline solid is the
temperature at which the forces holding the crystal lattice together
are broken and the solid becomes a liquid. Because amorphous
solids tend to act like liquids when they are in the solid state, it’s
hard to specify their melting points.
When liquid water is heated, some molecules escape from the
liquid and enter the gas phase. If a substance is usually a liquid at
room temperature (as water is), the gas phase is called a vapor.
Vaporization is the process by which a liquid changes into a gas or
vapor. When vaporization occurs only at the surface of a liquid, the
process is called evaporation.
Vapor pressure is the pressure exerted by a vapor over a liquid.
As temperature increases, water molecules gain kinetic energy and
vapor pressure increases. When the vapor pressure of a liquid equals
atmospheric pressure, the liquid has reached its boiling point, which
is 100°C for water at sea level. At this point, molecules throughout
the liquid have the energy to enter the gas or vapor phase.
▲