148 DIY Science: Illustrated Guide to Home Chemistry Experiments
Colligative properties change with changing concentration of
a solution. For most procedures, chemists use molarity (moles
per liter of solution or mol/L) to specify concentration, but
when working with colligative properties, molality (moles per
kilogram of solvent or mol/kg) is a more useful metric, because
molality can be determined with extreme accuracy using only an
analytical balance and because molality does not change with
increasing or decreasing volume as the solution is heated or
cooled.
In this laboratory, we’ll examine all three colligative properties
of solutions.
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The van’t Hoff factor depends on the nature of the
solute, and takes into account the fact that different
solutes behave differently in solution. Ionic solutes
dissociate in solution, and therefore have van’t Hoff
factors greater than 1. (For example, one molecule of
sodium chloride in aqueous solution dissociates fully into
two particles, one Na+ ion and one Cl– ion, so sodium
chloride has a van’t Hoff factor of 2.) Some solutes, such
as sucrose in water, do not dissociate, and therefore have
van’t Hoff factors of exactly 1. A few solutes associate
in solution, yielding fewer individual particles than the
molality of the solution suggests. These solutes have
van’t Hoff factors of less than 1. Some solutes, such as
weak acids and bases in aqueous solution, dissociate
partially, giving fractional van’t Hoff factors, typically
greater than 1 but less than 2.
The van’t Hoff factor depends on both solvent and solute.
For example, when hydrogen chloride (HCl) gas dissolves
in water, it dissociates completely into H+ and Cl– ions,
giving a van’t Hoff factor of 2. Conversely, when HCl gas
dissolves in benzene, it remains in molecular form, giving
a van’t Hoff factor of 1 for that combination of solvent
and solute.EvdRE y Ay CoLLIGATIvE pRopERTIES
Although colligative properties may seem to be an abstract
concept with applications only in chemistry labs, in fact
colligative properties of solutions have everyday real-world
applications.- The blood of mammals, birds, fish, and insects
native to arctic, antarctic, and other cold regions
has been found to contain special proteins that
act colligatively to lower the freezing point of their
blood. Some studies suggest that humans who live
in cold environments have different blood chemistry
from people who live in warm climates. What’s
really interesting is that if someone from, say Nome,
Alaska, moves to Honolulu, Hawaii, after some years
he loses his “cold resistance.” Conversely, someone
from Honolulu who moves to Nome over some years
develops this “cold resistance.” - The antifreeze in your car’s radiator depresses the
freezing point of the water, preventing the radiator
from freezing up in cold weather. Conversely, it also
raises the boiling point, preventing the radiator from
boiling over during hot weather. - In winter, highway departments scatter road salt on
icy roads to melt the ice, a practical application of
freezing point depression. - Tears are salty for a reason. They keep your eyes
sterile by killing microbes by osmotic dehydration. - Millions of people have drinking water thanks to a
practical implementation of the colligative properties
of solutions. Many seawater desalinization plants
use reverse osmosis to remove salt from seawater,
producing potable water. In ordinary (forward)
osmosis, water molecules migrate from the
freshwater side of the semipermeable membrane
to the seawater side, which is exactly the opposite
of what’s needed to desalinize water. In a reverse
osmosis plant, the seawater side is placed under
pressure, which forces water molecules through the
semipermeable membrane to the freshwater side.