Solutions
True Solutions:
A true solution is a homogeneous solution in which the solute
particles have diameters between 0.1 nm to 1 nm i.e., the solute
particles are of molecular dimensions.
Such dispersed particles dissolve in solution to form a homogenous
system. These do not settle down when the solution is left standing. The
particles are invisible even under powerful microscopes and
cannot be separated through filter paper ,parchment paper or
animal membranes. For example, sodium chloride in water is a true
solution. Most ionic compounds form true solutions in water. Organic
compounds like sugar and urea also form true solutions in water.
Suspensions
 Suspensions consist of particles of a solid suspended in a liquid medium.
Suspensions are systems with two distinct phases. The particles in
suspensions are bigger than 100 nm to 200 nm across. The particles of a
suspension may not be visible to the naked eye but are visible under a
microscope. Suspensions are heterogeneous systems.They stay only for a
limited period i.e. these are not stable as the particles have a tendency to
settle down under the influence of gravity. The particles of a suspension can
neither pass through ordinary filter paper nor through animal membranes.
 Examples of suspensions are sodium chloride in benzene, silver
chloride, barium sulphate or sand in water.
Colloidal solution or colloidal state or
colloidal dispersion
They represent an intermediate kind of a mixture between true solution
and suspension. The size of a colloidal particle lies roughly
between 1-100 nm. Colloids are also a two-phase
heterogeneous system consisting of the dispersed phase and
dispersion medium.
Colloidal solution or colloidal state or colloidal
dispersion
Since the dispersed phase in a colloidal system is uniformly distributed in
the dispersion medium, the colloidal state appears homogenous to the
naked eye or even an ordinary microscope (due to particles being
invisible). However it is a heterogeneous dispersion of two immiscible
phases and this is proved by viewing it under an ultra-microscope,
where the light reflected by colloidal particles can be seen. Colloidal
particles do not settle down under gravity: a colloidal solution of gold
prepared by Faraday over 125 years ago continues to be in excellent
condition even today. Colloids can pass through ordinary filter paper
but do not pass through animal membranes.
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Difference Between True Solutions,
Suspensions and Colloidal Solutions
http://academic.pgcc.edu/~ssinex/Solutions.ppt
< 1 nm
solutions
transparent
(clear)
molecular motion
never settle
> 100 nm
colloids
transparent with
Tyndall effect
Brownian motioncolloidal particles
moved by solvent
coagulation – can settle
suspensions
translucent
(cloudy)
movement by gravity
Colloids
Lecture 1
colloid (kŏl'oid) [Gr.,=glue like], a mixture in which
one substance is divided into minute particles (called
colloidal particles) and dispersed throughout a
second substance.
The mixture is also called a colloidal system, colloidal
solution, or colloidal dispersion.
Familiar colloids include fog, smoke, homogenized
milk.
Disperse systems
A system in which one substance (the disperse
phase), is dispersed as particles throughout
another (the dispersion medium).
Classification of dispersed systems
 Particle diameter
< 10-9 m
10-9 - 10-6 m
> 10-6 m
Homogenous
Colloid
Non homogenous
mixture
mixtures
 Particles are not resolved by ordinary microscope although they may
be detected under ultra-microscope
 Visible in the electron microscope
 Pass through filter paper but don’t pass through semi-permeable
membrane
 Diffuse very slowly
Colloid system can be classified as:
• Lyophobic sols: these sols are "solvent hating",
• are thermodynamically unstable, the particles tend to aggregate,
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they are irreversible systems.
Unstable in presence of even small conc. of electrolytes leading to
aggregation, depends on type, valency and concentration of
counter ion of electrolyte
Stability controlled by charge of particles
Little interaction between particles and dispersion medium.
Viscosity of the dispersion medium is not affected by the presence
of lyophobic colloidal particles.
Ex, water-insoluble drugs, kaolin and oils form lyophobic
dispersions
Stability is controlled by the charge of particles
Lyophilic sols: these sols are "solvent loving",
• are thermodynamically stable, they are reversible systems.
• Stable generally in presence of electrolytes, may be salted out by
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high concentration of very soluble electrolytes
Molecules of the dispersed phase are solvated
Stability controlled by charge and solvation of particles
Molecules disperse spontaneously to form colloidal solution
Viscosity of the dispersion medium increased by the presence of
dispersed phase
Ex, proteins, tragacanth and methylcellulose in water
Surfactant molecules
• Dispersion phase consists of aggregates (micelles) of small ions whose size
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individually is below the colloidal range.
Hydrophilic or lipophilic portion of the molecule is solvated , depending on
the dispersion medium
Because of their affinity for water and tendency to form micelles which are
of colloidal dimension, they form hydrophilic colloidal sol in water but are
usually classified separately as Association colloids
Colloidal aggregates are formed spontaneously when concentration of
amphiphile exceeds the critical micelle concentration (cmc)
Viscosity of the system increases as the conc. of micelles increase.
In aqueous solutions, the cmc is reduced by the addition of electrolytes
Properties of Colloids
Heterogeneity
A colloidal solution is heterogeneous system consisting of two
phases of dispersed phase (colloidal particles of a solid) and the
aqueous dispersion medium. Often a colloidal sol appears to be
homogeneous as the particles are small in size and not visible
to the naked eye. However, this is disproved when it is viewed
under electron microscope.
1.
2-Stable nature
Colloidal solutions are quite stable. The colloidal particles do
not settle at the bottom under the influence of gravity. This
is because of the constant motion of colloidal particles.
3-Filterability
Colloidal particles do not pass through ultrafilter papers,
animal and vegetable membranes. The large pore size of
ordinary filter paper enables colloidal particles to pass
through.