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# Solutions

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```									Solutions

Homogeneous Mixtures
Key Definitions
   Solute
   Dissolved particles in a
solution
   The lesser component
   Solvent
   The dissolving medium in a
solution
   The greater component
   Example: Saline Solution
for contact lenses
   Solute = sodium chloride
   Solvent = water
More definitions
   Miscible
   Liquids that will
dissolve in each
other
   Ex. Oil and gasoline
for 2 stroke engines
   Immiscible
   2 liquids that are
insoluble in each
other
   Ex. Oil and water
How much solute is dissolved?
   Saturated solution
   Contains the maximum
amount of dissolved
solute
   If you add more solute, it
will fall to the bottom
   Unsaturated solution
   Contains less than the
maximum amount of
dissolved solute
   If you add more solute, it
will dissolve
Relative Humidity
   A measure of how much water vapor is
in the air, compared to maximum
amount of water the air can hold at that
temperature
   100% humidity: rain is possible!

   People feel most comfortable indoors when
the relative humidity is between 40-60%.
Supersaturated Solutions
   Sometimes solutions can be prepared
that have MORE dissolved solute
than a saturated solution
   Unstable!
   Often used in candy making processes

   Video clip:
Solubility Diagrams
   Solubility
information is
often
presented
graphically
Interpreting Solubility
Diagrams
   Most substance become more soluble
as temperature increases.
   Gases typically become LESS soluble as
temperature increases.
   Note the units on the y axis: often
given as grams of solute/100 g solvent.
To speed up the solution process:
   Increase the surface area of the solute
   Which would dissolve faster: a sugar cube
or powdered sugar?
   Stir the solution
   Increase the temperature of the
solution
Rule of thumb for solubility
   “Like dissolves like”
   Solutes tend to dissolve in solvents with
similar properties.
   Nonpolar substances typically dissolve in
nonpolar solvents.
   Ionic solids tend to dissolve in polar
solvents.
Solution Concentrations
   Ratio of solute to solvent
   Concentrated: high ratio of solute to solvent
   Dilute: low solute/solvent ratio
   Several different ways to calculate
   Most common:
   Molarity symbol: M
   Percent solutions: m/m, m/v, v/v
   Parts per million
   Molality
Molarity
   Definition:

   Since mol = g/gfm,
Percent Solutions
   Easy and reliable to calculate
   Mass/mass, mass/volume, volume/volume

   Frequently used in biology & medicine
Molality
   Used to calculate changes in physical
properties of solutions

   Note subtle differences from molarity
formula!
   “molal” solutions
Vapor Pressure
   Particles at the surface of a liquid may
evaporate or vaporize, if they have
enough kinetic energy.
   If this happens in a sealed container,
the particles of escaped liquid (now in
the gas phase) will collide with the
container walls and exert a “vapor
pressure” above the liquid
Vapor Pressure Diagrams

   Vapor pressure
data is often
presented
graphically
Boiling Point
   Boiling
   when bubbles of gas form anywhere in the liquid
and then rise to the surface
   Boiling point
   the temperature at which the vapor pressure of
the liquid is just equal to the external pressure
   Normal boiling point
   the boiling point of a liquid at a pressure of 101.3
kPa (1 atm)
Colligative Properties
   Addition of a solute changes the
properties of the system:
   The solution will have different properties
than the pure solvent
   Adding solute changes the freezing point,
boiling point, vapor pressure
   These changes depend only on the number
of particles added (NOT the identity of the
particles)
Phase Diagrams
Phase Diagrams
   Closed system
   Triple Point
   Where solid, liquid, and gas can coexist
   Critical Point
   Above this point, gas and liquid are
indistinguishable
   “supercritical fluids”
Colligative Properties
   Freezing Point Depression
   The solution will freeze at a lower
temperature than the pure solvent
   Why we put salt on sidewalks during winter
   Antifreeze in cars
   Boiling Point Elevation
   The solution will boil at a higher
temperature than the pure solvent
   Cool simulation
van’t Hoff Factor (i)
   Some substances dissociate into ions,
resulting in more solute particles
dissolving
   van’t Hoff Factor = the theoretical
(maximum) number of particles formed
when a substance dissociates
   The real degree of ionization is typically
slightly less than predicted by the van’t
Hoff factor
Predicting van’t Hoff factors
   For all covalently bonded substances,
   i=1
   For ionic substances
   I = number of ions present in formula
   Ex.
   NaCl       i=2
   Al(NO3)3   i=4
   MgCl2      i=3
Calculating Colligative
Properties
   Tbp = kbpim where kbp is a constant for
each solvent
   Calculate the new boiling point by adding Tbp to
the boiling point of the pure solvent
   Tfp = kfpim where kfp is a constant for
each solvent
   Calculate the new boiling point by subtracting Tbp
from the freezing point of the pure solvent

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