Colligative and Propeties of Solution
What Are Colligative Properties ?
The term of “colligative” comes from Latin word
“colligare” which means “to gather”. Colligative properties are properties of
solution that do not depend on the identity of the solutes, but on the number
of dissolved particles in the solution. There are two kinds of colligative
properties,namely :
-
Colligatigative properties of nonelectrolyte
solutions
-
Colligative properties of electrolyte solutions
What Properties belong to Colligative Properties of Solution ?
The formation of solution, from
solute and solvent, only gives a small effect to the chemical properties of
solute. Its physical property, however, will drastically change. Colligative
properties of solution is The vapor pressure of the pure solvent is higer then
that of solution, The boiling point of the pure solvent is lower thanthat of
the solution, The freezing point of the pure solvent is higer than that of the
solution. It means that the presence of a solute can cause : the pavor pressure
lowering, the boiling point evaluation, and the freezing point depression. The
properties are know as colligative properties of solution. The osmotic
pressure, which arises from osmosis between two solutions with different
concentrations, is also know as colligative properties of solution.
The Vapor Pressure Lowering
Vapor pressure : If a liquid is
placed in a closed container, it will evaporate. The liquid particles will
decrease while the gaseous particles will increase. As the gaseous particles
increase, they will bounce around and turn back into liquids or undergo
condensation. The evaporation and the condensation rates equilibirium is
reached. At equilibrium, the evaporation and the condensation rates will be the
same. When these particles hit the wall of the container, they exert a
pressure. This pressure in this equilibrium
is called the saturated vapor pressure of the liquid or vapor pressure
of the liquid.
The value of vapor pressure
depends on the identities of substance and temperature. Subtances with high
vapor pressure are easily evaporated, and they are called volatile substances,
such as ethanol and ether. While substances with low vapor pressure are hardly
evaporated (nonvolatile), for examples water,glycol, and glycerol.
The solute molecules and the
solvent molecules attract each other. It eans that the solute molecules it
causes the vapor pressure of solution is than that of the pure solvent. The
difference between the vapor pressure of the pure solvent and that of a solution
is called the vapor pressure lowering. This phenomenon was discovered for the
first time by a French chemist, Francois-Marie Raoult. Raoult stated that the
addition of a nonvolatile solute to a solvent led the lowering of vapor
pressure.
The Boiling Point Elevation
Have you ever seen boiling water
? A liquid boils when its saturated vapor pressure becomes equal to the
external pressure. The normal boiling point of a liquid is a temperature at
which the vapor pressure equals to 1 atmosphere. For example, the normal
boiling point of water is 100C. But at a different pressure, water will boil at
different temperature. For example, at the top mountain the pressure is lower,
so the noiling point of water will also be lower than 100 C. You have learned that
a nonvolatile solute in solution causes the lowering of vapor pressure.
The Freezing Point Depression
The freezing point or melting point
of a solution is the temperature at which solid and liquid are in equlibirium.
The most generic solid-liquid equilibrium is water and ice at 0C and 1
atmosphere . This dynamic equilibrium can be illustrated by a glass of ice that
is set in 0C. At this temperature, Ice starts melting. At the same time, water
also freezes and merges with the ice block. This equilibrium will change if
tempratute raises or drop.
The Osmotic Pressure
The process where a solvent
passes throught a asemipermeable membrance from one solution to another (or
from a pure solvent into a solution) is called osmosis. Osmosis process will
last until equilibrium is reached. It can be seen when the volume of a solution
stop changing. The volume difference between the two solutions at the
equilibrium generates a pressure called the osmotic pressure. The osmotic
pressure is defined as a pressure given to prevent osmosis process, which is to
prevent the flow of solvent molecules.
Colligatives Properties in Daily
Life
Colligatives properties,
especially the freezing point depression, have so many uses in daily life. The
applications of freezing point depression can survive many lives during cold
season. The application of freezing point depression :
1 . Ice
Cream Making .
What is he difference between ice block and ice cream ? ice
block is so hard and has no taste because ice block is only water that freezes.
Wheares, ice cream is soft and delicious. The ingredients of ice cream, such as
sugar, milk, chocolate, butter, and salt lower freesing point of water and make
ice cream cold and soft but no frozen.
2 . Antifreeze
in Car Radiator. In a cold region, people often apply an antifreeze agent such
as ethylene glycol to their car radiator. Do you know why ? In cold region,
water in radiator easily freezes. If it happens continuously, the car engine
will damage. A solution of ethylene glycol in the radiator water can lower the
freezing point, so the water will not freezing easly.
3 . Antifreeze
to melt snow. In certain regions, every winter, snow drops over the streets, of
course, it makes vehicles difficult to move. People often put salt, such as
Nacl and Cacl2,on the roads after a snowfall to melt the snow. The
more salt are spread, the more snow will melt (the less snow will freeze).
References (Yudistira,kimia for senior high school jilid xii)
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