13.1 Liquids/Evaporation
Liquids can not be pushed closer together; they have
no space between molecules. Strong attractives
between molecules hold them together. Liquids have
high densities and combining liquids increases the volume.
Models for liquids and solids relate to the forces of
attraction between molecules. The open spaces above any
liquid contains some molecules in the gaseous or vapor
state due to evaporation. Some of the liquid molecules
have greater kinetic energy and overcome their attractive
forces. When this occurs the volume decreases and this
change that takes place is called evaporation.
Certain solids can go directly to the gaseous state without
entering into the liquid state and the term is sublimination.
13.2 Vapor Pressure/Surface Tension
Vapor pressure is the partial pressure exerted by gaseous
molecules in the open space above any liquid. In a closed
system the vapor molecules strike the side of the container
and return to the liquid state by the process of condensation.
This condensation-vaporarization process will continue
until an equilibrium is reached. This evaporation process
is dependant on the attractive forces between the molecules,
thus some substances may be higher or lower than others.
Volatile substances are those which have a high vapor
pressure; that is they evaporate readily. Molecules of liquid
act differently at the surface than they do in the interior.
Surface molecules experience an inward force, whereas
those in the interior are attracted equally in all directions.
In order to expand the surface area of a liquid a certain
amount of energy is required to overcome the inward force
and is given by the surface tension. High surface tension
substances are those substances with large attractive forces.
Forces that bind like molecules to one another are called
cohesive forces. Forces that bind a substance to a surface are
called adhesive forces. It is these forces which cause the
surface, or meniscus of a liquid in a tube to take a certain
shape. For example, a test tube of water will display a
u-shape meniscus whereas a tube of mercury will have a
meniscus of a downward curve at the edges. The adhesive
forces between the mercury atoms and the glass are less
that the cohesive forces between the mercury atoms
themselves.
13.3 Change of State
The ease at which substances change from one state to
another relates to many important properties. Outlined
below is an illistration of transformation of the three states.
Phase changes can be endothermic and require energy to
go from one state to another.
Exp:
sublimination (solid to gas)
melting (fusion for solids)
vaporization (liquid to gas)
As noted when a solid goes through a melting process it is
called fusion. "The energy required to change 1 gram of a
solid at its melting point into a liquid is called heat of fusion.
The heat neede tfor the vaporization of a liquid is called
molar heat of vaporization.
13.4 Physical Properties of Water/Structure
Trends amoung physical and chemical properties as
related to the periodic table were noted earlier. However,
water is in a catagory all to itself. While other H2 family
members (H2S, H2Se, and H2Te) have boiling points that
decrease as the molecule becomes smaller, H2O boils at
100°C. The structure of the water molecule influences the
properties of water. Aside from Fluorine, oxygen is the most
electronegative element. It's strong attractive forces creates
two very polar bonds. A molecule of water has a bent structure
of about 105 degree angle between the two bonds.
One of the most important contributions to the strong
intermolecular attractions in water is the hydrogen bond
that it forms.
13.5 Chemical Properties of Water
Metal hydroxides (NaOH, KOH, and Ca(OH )2 are
produced by reacting them with water. Caution should be
taken when reacting Na because it frequently ignites.
Exp:
Some elements (Al, Zn, Fe) react with steam at high
temperatures to form H2 and a metallic oxide.
Exp:
Basic anhydrides can be formed by reacting a metal oxide
with water to form hydroxides.
Exp:
Acid anhydrides can be formed by reacting nonmetal oxide
with water to form acids.
Exp:
Hydrates are solids that when crystallized from water
solutions have water molecules as part of their crystal
structure.
Exp:
