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
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