hey it's professor Dave, let's talk about
colligative properties a solution will behave differently from a pure solvent
because there are particles of solute that interfere with physical processes
like phase changes. a solution will display certain colligative properties
that depend only on the concentration of solute and not on the chemical identity
of the solute particles. adding solute to a solvent will change things in the
following ways: vapor pressure lowering boiling point elevation, and freezing
point depression. before we analyze these processes we need a new way to express
solute concentration when discussing colligative properties. instead of using
molarity we will often use molality which is moles solute per kilogram of
solvent rather than per liter so the molality of a solution of 10 grams of
iodine in 30 grams of dichloromethane would be as follows we would call this a 1.31 molal
solution like we might say molar when discussing molarity, and while we use an
uppercase M to denote molarity we use a lower case m in italics to denote
molality. when we look at colligative properties the key theme is that the
presence of solute particles interferes with the activity of solvent particles
at the interface between two phases. a liquid will always have a particular
vapor pressure since particles at the surface of the liquid can spontaneously
enter the gas phase. the number of gaseous particles determines the vapor
pressure but solute particles sitting at the interface occupy some of the surface
area which hinders some solvent molecules from evaporating, therefore the
presence of the solute causes the vapor pressure of the liquid to decrease. the
new vapor pressure is equal to the vapor pressure of the pure solvent times the mole fraction of the solvent which
refers to the percentage of particles in a solution that are solvent molecules. a
similar phenomenon is that of boiling point elevation. when a solution is
raised to the temperature at which the solvent should boil, again there are
solute particles blocking the solvent molecules from going into the gas phase.
this means more heat energy will be needed to compensate for this activity
which results in an elevated boiling point. the more solute there is the more
dramatic the effect, so the change in boiling point is given by the molality
of the solution times a constant that is specific to the
solvent. freezing point depression is similar but deals with the interface
between the liquid and solid phases. for a liquid to freeze it has to form a
lattice, but the presence of solute particles interferes with the ability of the
solvent particles to efficiently form the lattice so to compensate the system
will have to reach an even lower temperature to freeze. the equation here
is similar, the change in freezing point is given by the molality of the solution times another constant. these Kb and Kf
constants can be found in tables in your textbook or online. remember to always
add the change in boiling point to the original boiling point since solute
always raises the boiling point and change in freezing point will always be
subtracted from the original freezing point because solute always lowers the
freezing point. this is why we add salt to icy streets, because it causes the
water to freeze at a lower temperature so water will remain liquid instead of
turning to ice until lower temperatures are reached thus reducing the quantity
of ice that will form. let's check comprehension thanks for watching, subscribe to my
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