14.1 General Properties
Solutions are homogenous mixtures. A homogenous
mixture is an equal distribution of solution components.
A gaseous solution surrounds us everyday and is
known as air. Even the water we drink is a solution,
although many times water is said to be purified. From
these statements you can see that a solution can be in
a form other than liquid.
Certain properties hold true for all solutions.
1) Solutions are homogenous mixtures.
2) Particles in solution are very small.
3) Solutions are usually transparent but can be colored.
4) Separation of the solute from the solvent can be
done by physical means(evaporation).
Determining whether a substance is soluble or not is tough.
However, some general rules are given as an outline.
Many reference books are available for solubility because
factors such as temperature, ion interaction, size of ions,
and others complicate predictions.
14.2 Solubility/Related Factors
A substance that is dissolved in another substance is
called a solute. The substance the solute is being
dissolved in is the solvent. Describing the amount of these
substances is the basis for solubility. Solubility ranges
from very soluble, moderately soluble, slightly soluble to
insoluble.
The general terms that describe whether substances are
soluble or not are: miscible and immiscible. Liquids that
dissolve in each other in all proportions are miscible(exp:
salt in water). Liquids that do not mix, such as oil and
water, are immiscible.
A substances ability to dissolve or not is dependant on the
nature of both the solvent and solute. Temperature and for
gases(pressure) also play important roles. "Substances
with similar intermolecular forces tend to be soluble in one
another". Polar and ionic substances are soluble in other
polar substances. Nonpolar substances solubilize in
nonpolar solvents. A major influence on chemical equilibrium
is temperature. With some exceptions the solubility of
solids increases with temperature. Alternatively, gases in
liquids show a decrease in solubility with an increase in
temperature. An example would be CO2 escaping from
a soda can or bottle. Another factor that influences
solubility is pressure. The only notable effect is that of
gases in liquid, such as, when your soda goes flat. If the
gas over a solution is doubled the amount of gas that is
dissolved in the solution is doubled.
14.3 Rate of Dissolving Solids
Four factors govern the rate at which a substance
dissolves:
1) Size of the particle- the smaller the surface area the
easier the dissolution.
2) Temperature- usually a increase in temperature will
increase solubility. Kinetic energy increases with increased
temperature.
3) Solution concentration- As the solute is added to
solvent and the saturated state is reached , the rate of
dissolution decreases.
4) Mixing or agitating- Mixing the solute causes an
increase in the kinetic forces as well as a increased
interaction between solute-solvent.
The amount of solute that can be dissolved in solvent can
be saturated, unsaturated, or supersaturated. A saturated
solution is one that at a given temperature, is at its limit
as to the amount of solute that will dissolve. Unsaturated
solutions are capable of dissolving more solute without
altering the conditions. Under controlled conditions a
supersaturated solution can be prepared by warming the
solvent and cooling slowly.
14.4 Expressing Concentrations
A qualitative way of expressing concentration is termed
dilute and concentrated. A relatively small quantity of solute
per unit of solution is dilute. Concentrated is a relatively
large amount of solute per unit of solution.
Quantitative expressions are used in chemistry to attain
specific information.
For dilute concentration, concentrations are expressed in
parts per million(ppm).
Exp:
12.5g of glucose (C6H12O6) are dissolved in 0.100kg
of H2O. Calculate the percentage concentration of the
solute.
Exp:
4.6µg of zinc2+ was found in a sample of 2.3g sample
of ground water. What is the concentration of Zn2+ in
ppm?
Exp:
How many grams of silver nitrate(AgNO3) must be
dissolved to prepare 4.00x 102ml 0.140M AgNO3?
Exp:
What is the molarity of a solution made by dissolving
3.00g potassium chlorate(KClO3) in water to make 160mL
solution?
First: Think about what we are given
3.00g/160ml
we don't want mL so we must convert to liters
Second: we need to apply a unit of grams/mole
Third: we need to use the moles we calculated to one liter
to get molarity of the solution.
The answer is 0.153M
"Solution properties that are determined by the number
of solute particles dissolved in a fixed quantity of solvent
are called colligative properties".
It is possible to change some physical properties of solvents
by introducing and altering the quantity of solutes in a
solution. An example of this is adding antifreeze to water
in a radiator to lower the freezing point. The lowering of
the freezing point, raising of boiling point, and reduction of
vapor pressures are all colligative properties. Both the change
in freezing point and the boiling point can be related to the
reduction of vapor pressure. When the vapor pressure
decreases the boiling point increases because a liquid boils
when its vapor pressure equals atmospheric pressure. When
a nonvolatile solute is introduced to a pure solvent the solution
volatility decreases from its original amount by a certain
percentage. In other words the solvent no longer reaches
atmospheric pressure as fast as it did before the nonvolatile
solute was introduced. Freezing of water occurs when water
and ice have the Same vapor pressure(O°C). However, in a
solution (water and nonvolatile solute) an equilibrium of
solution-ice vapor pressure occurs at a point below O°C.
Colligative properties can be used to determine molar
mass. The number of moles of solute per kilogram of solvent
are proportional to boiling point elevation and freezing point
depression. The concentration expression is molality(m)
which is the number of moles of solute per kilogram of solvent.
The change in freezing point depression and the boiling point
elevation have equations and constants associated with them.
