Colligative Properties of
Solutions and
Heterogeneous Mixtures
Sections 15.3 & 15.4
What are Colligative Properties?
Physical Properties that depend on
the number of solute particles
Do NOT depend on the identity of
the solute particles
Historically used to determine molar
properties, especially molar mass
The Colligative Properties
Freezing Point Depression
Boiling Point Elevation
Vapor Pressure Lowering
Osmotic Pressure
Colligative Properties of Electrolytes
Freezing Point Depression
The freezing point of water is 0°C, but that melting
point can be depressed by the adding of a solvent such
as salt. The use of ordinary salt (NaCl) on icy roads in
the winter helps to melt the ice from the roads by
lowering the melting point of the ice because the solute
particles interfere with the attractive forces among the
solvent particles, preventing the solvent from entering
the solid state at its normal freezing point.
Boiling Point Elevation
The boiling point of pure water is 100° C, and is the
temperature at which its vapor pressure is equal to
atmospheric pressure. The boiling point can be
elevated by the adding of a solute such as salt. This
means the solution must be heated to a higher
temperature to supply the additional kinetic energy
needed to raise the vapor pressure.
Ethylene glycol in antifreeze protects against freezing
by lowering the freezing point and permits a higher
operating temperature by raising the boiling point.
Vapor Pressure Lowering
Vapor Pressure is the pressure exerted in a
container by liquid particles that have escaped the
liquid’s surface and entered the gaseous state.
When a solute is dissolved in a solvent, the number
of solvent molecules near the surface decreases,
and the vapor pressure of the solvent decreases.
This decreases the rate at which the solvent
molecules of the liquid escape into the gas phase.
The Vapor Pressure of a
Solution is Lower than that of
the Pure Solvent
Osmotic Pressure
Depends on the number of solute particles in a
given volume of solution. A solute is separated
from a solution as it crosses a semi permeable
barrier. Water can move in both directions across
the membrane, the solute cannot. The additional
water molecules on the solution side cause
osmotic pressure.
The particles will move from an area of higher
solvent concentration to
lower solvent concentration
Colligative Properties of
Electrolytes
Since colligative properties depend on
the number of particles dissolved,
solutions of electrolytes (which
dissociate in solution) should show
greater changes than those of
nonelectrolytes.
Heterogeneous Mixtures
Suspensions
A mixture containing particles that settle
out if left undisturbed (cornstarch and
water, fine sand and water)
Suspended particles can be filtered out
Heterogeneous Mixtures
Colloids
Mixture of intermediate size particles
Components cannot be separated by settling or
filtration – they do not settle because they have
polar layers surrounding them. The layers
repel each other and prevent the particles from
separating
examples: milk, beaten egg white, butter,
mayonnaise
Characteristic Overview
Type of
Mixture
Settle
Out?
Relative
Particle
Size
Tyndall
Effect
Suspension
Yes
Large
Yes
Colloid
No
Small
Yes
Solution
No
Very Small
(dissolved)
No
Brownian Motion
The jerky, random movements made by
colloid particles
The dispersing medium particles form
charged layers around the colloid particles.
These charged layers repel each other and
keep the particles from settling out.