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Solutions
A solution is formed when a substance is dissolved in a solvent.
Any solution is formed of 2 phases; the solute which is also called
dispersed phase and solvent which is also called dispersion phase
Types of solutions
Solutions can be classified according to the concentration or the size
of solute particles
A- According to the concentration of solute particles,
solutions may be:
1- Normal solution
It is a solution, which contains the equivalent weight of the
solute in grams dissolved in one liter of the solution.
2-Molar solution
It is a solution, which contains the molecular weight of the
solute in grams dissolved in one liter of the solution.
3-Molal solution
It is a solution, which contains the molecular weight of the
solute in grams dissolved in one kilogram of the solvent.
B- According to the size of solute particles, solutions may be:
1- Crystalloids (True solutions)
They are solutions in which the size of solute particles is less
than 1 mµ (millimicron) e.g. sodium chloride solution.
2- Colloids
These are solutions in which the size of solute particles ranges
from 1 to 100 mµ. e.g. plasma proteins
3- Suspensions
They are solutions in which the size of solute particles is more
than 100 mµ e.g. suspension of sand in water.
Colloids
These are solutions in which the size of solute particles ranges from 1
to 100 mµ. e.g. plasma proteins.
Types of colloids
According to their ability to take up the solvent, colloids are classified
into emulsoids and suspensoids
1- Emulsoids
They are lyophilic (solvent loving) colloids. If the solvent is water
they are called hydrophilic colloids.
They are more viscid.
They are more stable and not easily precipitated as the solute particles
are surrounded by 2 stability factors:
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i- Negative or positive charge.
ii- A shell (layer) of solvent.
They can be precipitated by dehydration followed by neutralization of 2
the charge.
charg
Examples for emulsoids include protein, starch and egg white a layer of solvent
solutions.
2- Suspensoids
They are lyophobic (solvent hating) colloids. If the solvent is water
they are called hydrophobic colloids.
They are less viscid than emulsoids.
They are less stable and easily precipitated as the
solute particles are surrounded by one stability
factor that is negative or positive charge.
They can be precipitated by neutralization of the charge.
Examples for emulsoids include colloidal gold and colloidal iron
solutions.
The charge on the colloidal particles may be due to
1- Ionization of some groups on the surface of colloidal particles.
2- Ions from the solvent may be adsorbed on the surface of the
colloidal particles.
Stability factors of colloids
1- Presence of negative or positive charge on the surface of the
colloidal particles.
2- Presence of a shell (layer) of solvent surrounding the
particles of emulsions.
3- The size of colloidal particles; the smaller the size, the more
stable the colloid.
Separation of colloids
1- Ultracentrifugation, which means centrifugation of the
solution at a very high speed.
2- Ultrafiltration, which means forcing the solution, under
pressure, through membranes of varying porosity.
3- Electrophoresis, which means migration of colloidal particles
in electric field.
Precipitation of colloids
Colloids can be precipitated by:
1- Dehydrating agents.
2- Strong agitation, freezing and heating.
3- Colloids of opposite charge.
Viscosity
Viscosity means the resistance offered by a fluid to flow. It is reported
in units called poise
It is measured by comparing the time needed for certain amount of the
fluid to flow through capillary tube at a definite temperature, with the
time which is taken by an equal volume of water to flow through the
same capillary tube at the same temperature.
Factors affecting viscosity
1- Temperature, viscosity decreases with the rise of
temperature.
2- Solute concentration, viscosity increases with the increase in
solute concentration.
3- Size of particles of the solute, viscosity increases with the
increase in the size of particles.
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Diffusion
Diffusion means passage of substances in the solution in which they
are suspended. It occurs in many fluids of the body within cells during
secretory activity.
Water
Copper sulphate
solution
Water
Prussian blue
solution
Diffusion of copper sulphate and prussian blue
Factors affecting diffusion
1- Size of particles, diffusion decreases with the increase in the size
of particles.
2- temperature, diffusion increases with the increase in temperature.
An example of diffusion is the diffusion of copper sulphate and
prussian blue in water. Copper sulphate diffuses in water more rapidly
than prussian blue does because copper sulphate has smaller particle
size.
Dialysis
Dialysis means separation of colloids from crystalloids using a
semipermeable membrane, crystalloids can pass this membrane but
colloids can not because of the large size of their particles.
Dialysis is used in cases of renal failure. Blood passes through
dialyzing machine to get red of waste products (as urea and
creatinine), which are crystalloids, and preserving plasma proteins,
which are colloids.
Surface Tension
It is the force of attraction between molecules in liquids that holds the
surface molecules of a liquid together and attracts them towards the
body of the liquid. It is caused by unequal attraction of the surface
molecules by the inside molecules.
Below the surface of the liquid, the force of cohesion (sticking
together) between molecules is the same in all directions. Molecules
on the surface of the liquid, however, are subjected to a net
unbalanced force of attraction that pulls them towards the body of the
liquid. As a result, the liquid tries to take on the shape that has the
smallest possible surface area (the shape of a sphere).
Salt solutions have higher surface tension than organic solutions.
Surface tension decreases with the increase in temperature.
Surfactants are substances that lower surface tension. They include
soaps, bile salts and phospholipids
.
Hydrotropy
It is the capacity of certain substance to make water insoluble
substances more soluble in water.
Hydrotropic factors are substances, which make water insoluble
substances more soluble in water.
Examples of hydrotropic factors include bile salts, phospholipids and
glucuronic acid.
Adsorption
It is the capacity of substance to make other substance closely attached
to its surface (adsorbed on it) due to presence of attractive forces on its
surface.
Examples of adsorbents include charcoal, kaolin, talc powder and zinc
oxide.
Importance of adsorbents
1- Charcoal tablets are used to adsorb gases in the intestine in
gastric distention.
2- Kaolin is used in treatment of diarrhea as it adsorbs bacteria
and their toxins.
Elution
Elution means recovery of adsorbed material from adsorbing agent.
The solvent used in elution is called eluent and the extract obtained
from elution is called eluate.
Chromatography is a technique that depends upon adsorption followed
by elution. It is used for separation and purification of important
compounds such as many hormones, enzymes, neurotransmitters,
peptides and amino acids.
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