Chemistry 100
Bettelheim, Brown, Campbell & Farrell
Ninth Edition
Introduction to General,
Organic and Biochemistry
Chapter 6
Solutions & Colloids
Solutions
Components of a Solution
•Solvent: The substance that is the dissolving medium,
usually present in largest quantity, has the same phase
as the solution.
•Solute: The substance or substances dissolved in the
solvent to make the solution, usually present in smaller
quantity than solvent, may have a phase different from
the solution before dissolution.
Examples: Liquid: Salt in water; sugar in water;
alcohol in water; oxygen in water.
Gas: Oxygen in nitrogen.
Solid: Copper in zinc.
Characteristics of Gas & Liquid Solutions
•The distribution of particles is uniform.
•The components do not separate on standing.
•The components cannot be separated by
filtration.
•For many solvent/solute combinations, solute
concentrations may vary widely.
•Solutions are transparent.
•Solutions can be separated into pure
components; the separation is a physical change,
not a chemical change.
1
Solubility
Solubility: the maximum amount of a solute that can
dissolve in a given amount of solvent at a given
temperature.
Solubility is a physical constant, each solid has a
different solubility in every liquid; those with low
solubility are said to be insoluble, those with higher
solubility are said to be soluble.
Some liquids are insoluble in each other, as for example
gasoline in water. Other liquids have limited solubility
in each other, as for example ether in water (6 g/100 g
H2O). Still other liquids are miscible, i.e. completely
soluble in each other, for example ethanol and water.
Solutions
•Saturated solution: A solution that contains the
maximum amount of a solute that can be dissolved at
equilibrium at a given temperature.
•Unsaturated solution: A solution that contains less than
the maximum amount of a solute that can be dissolved
at a given temperature.
•Supersaturated solution: A solution that contains more
than the maximum amount of a solute that can be
dissolved under equilibrium conditions at a given
temperature. When a supersaturated solution is
disturbed in any way, the excess solute will separate and
the equilibrium solubility is restored.
Solubility Rules
Like Dissolves Like
Polar Solvents dissolve Polar and Ionic
Solutes!
Water Ù sugar, alcohol or salt
Nonpolar Solvents dissolve Nonpolar
Solutes!
Kerosene Ù Oil or tar
2
How Water Dissolves Ionic Compounds
Ionic compounds are a regular array of positive and
negative ions. Water is a polar molecule, a dipole with
positive and negative ends.
The negative ions attract the positive end of the water
dipole, the positive ions attract the negative end of the
water dipole. Each ion, depending on its size, attracts
four to eight molecules of water.
Ions dissolved in water are said to be hydrated, i.e. they
are surrounded by water molecules.
Water of hydration: the attraction between ions and
water is so strong that water molecules are a part of the
crystal structure of many solids
How Water Dissolves Polar Compounds
In a few cases, molecular compounds dissolve in water
because they react with water. For example HCl(g).
HCl(g) +
H2O(l)
+
+ Cl-(aq)
H3O(aq)
Non-polar covalent molecules do not dissolve in water.
Polar covalent molecules dissolve because they are
solvated by hydrogen bonding, e.g. CH3CH2OH.
When the non-polar part of an organic molecule is
considerably larger than the polar part, the molecule
no longer dissolves in water.
When an organic molecule is strongly polar on one end
and non-polar on the other it is a detergent or soap.
Temperature Effects
Increasing Temperature:
• Solid or Liquid solutes: usually
increase in solubility, converse is also
true.
• Gaseous solutes: decrease in
solubility, converse is also true.
3
Pressure Effects
• Pressure has little effect on the
solubility of liquid or solid solutes.
• Gas Solubility is proportional to the
Partial Pressure of the gas in
equilibrium with the solution. This is
called Henry’s Law.
Concentrations of Solutes
Percent composition can be expressed three
ways. Each is unique and must be specified.
Weight of solute per volume of solution (w/v): A
solution of 10 g of table sugar in 100 mL of solution,
for example, has a concentration of 10 percent w/v.
Weight of solute per weight of solution (w/w): A
solution of 10 g of table sugar in 100 g of solution, for
example, has a concentration of 10 percent w/w.
Volume of solute per volume of solution (v/v): A
solution of 40 mL of ethanol in 100 mL of aqueous
solution is 40 percent v/v.
Concentrations of Solutes
Molarity, M : Moles of solute per liter of solution.
Delivering a fraction of a liter of the solution will
deliver that same fraction of the number of moles of
solute in a liter. A way to deliver moles of solute by
volume of solution. M1V1 = moles
Dilution of Solutions: If we dilute a solution, the
number of moles of solute remains the same after
dilution as before dilution.
We can then use this relationship: M1V1 = M2V2 to
calculate the new molarity.
4
Concentrations of Solutes
Blood serum is approximately 0.14 M NaCl, how much
salt does a pint ( 473 mL) of blood contain?
Dilution of Solutions
Make 500.0 mL of a solution of 0.12 molar acetic acid
from a stock solution of 6.00 M stock solution.
Concentrations of Very Dilute Solutions
For very dilute solutions, we sometimes express
concentration in parts per million (ppm), or even parts
per billion (ppb).
Parts per million:
May be either w/w or v/v; which ever quantities are
used, the units in which each is reported must be the
same:
For example, 1 mg of lead ions per 1 kg of water is
equivalent to 1 mg of lead per 1,000,000 mg of water;
the concentration of lead is 1 ppm.
Parts per billion is calculated the same way.
5
Ions in Solution Conduct Electricity
Ions in water can migrate throughout a solution, they
maintain their charge as they migrate. If electrodes are
present: Cations migrate to the negative electrode (the
cathode). Anions migrate to the positive electrode (the
anode). The movement of ions constitutes an electric
current.
An electrolyte is a substance that conducts electric
current when dissolved in water; a substance that does
not conduct electricity is called a non-electrolyte.
A strong electrolyte is a compound that dissociates
completely to ions in an aqueous solution.
A weak electrolyte is a compound that only partially
dissociates to ions in an aqueous solution.
Colloids
In a true solution, the maximum diameter of a solute
particle is about 1 nm.
Colloid: A suspension in solvent in which the solute
particle diameter is between 1 nm and 1000 nm.
Colloid particles have very large surface areas, which
causes colloidal systems to have two properties:
They scatter light (Tyndall Effect) and, therefore,
appear turbid, cloudy, or milky, not transparent.
They form stable dispersions; that is, they do not settle
out.
Colloids are intermediate between homogeneous and
heterogeneous mixtures.
Properties of Colloids
Tyndall effect: a characteristic of colloids in which
light passing through the colloid scatters.
When we see the beam of a flashlight through smoke,
dust or fog we are observing the Tyndall effect.
Brownian motion: the random motion of colloid-size
particles.
An example of Brownian motion is the motion of dust
particles in the air; what we see is not the dust particles
themselves but the flashes of scattered light passing
through the colloid due to the Tyndall effect.
6
Properties of Colloids
Why do colloidal particles remain in solution despite all
the collisions due to Brownian motion?
In liquids, most colloidal particles are surrounded by a
large solvation layer; if the solvent is water, as in the
case of protein molecules in the blood, the large number
of surrounding water molecules prevents colloidal
molecules from touching and sticking together.
In gases and liquids, because of their large surface area,
colloidal particles acquire static charges; for example,
they all may become negatively charged. When a
charged colloidal particle encounters another particle of
the same charge, they repel each other.
Colligative Properties
Colligative property: any property of a solution that
depends only on the number of solute particles, and not
on the nature of the particles. These include:
Freezing-point depression – One mole of particles
dissolved in a kilogram of water lowers the freezing
point by 1.86 EC.
Boiling Point Elevation - One mole of particles
dissolved in a kilogram of water raises the boiling point
by 0.52 EC.
Vapor Pressure Lowering – Lowered to degree solute
particles replace water molecules in the solution.
Osmosis – Most important biologically.
Colligative Properties
Freezing-point depression – One mole of particles
dissolved in a kilogram of water lowers the freezing
point by 1.86 EC.
CH2OHCH2OH (l)
NaCl(s)
H2O(l)
H2O(l)
Na+(aq)
CH2OHCH2OH (aq)
+ Cl-(aq)
one mole =
one mole
one mole = two
moles particles
H2O(l)
2 K+(aq) + SO4-2(aq) one mole = three
moles particles
Ethylene glycol is commonly use in antifreeze in car
radiators. Salts are commonly used to melt ice on roads
and sidewalks. The solutions they form freeze much
lower than the pure water in ice.
K2SO4(s)
7
Osmotic Pressure
Semipermeable membrane: A membrane with tiny
pores that are big enough to allow solvent molecules to
pass through them, but not big enough to allow the
passage of large solute molecules. Example: a cell wall!
Osmosis: the movement of solvent particles through a
semipermeable membrane from a region of lower solute
concentration (higher solvent concentration) to a region
of higher solute concentration (lower solvent
concentration).
Osmotic pressure: the pressure necessary to prevent
osmosis. Osmolarity (Osmol): the molarity multiplied
by the number of particles produced by each formula
unit of solute.
Osmotic Pressure
Isotonic solutions: solutions with the same osmolarity.
Isotonic solution: a term used primarily in the health
sciences to refer to a solution with the same osmolarity
as blood plasma and red blood cells.
Hypotonic solution: a solution with lower osmolarity
than blood plasma and red blood cells
Hemolysis: the swelling and bursting of red blood cells
because they cannot resist the increase in osmotic
pressure when put into a hypotonic solution.
Hypertonic solution: a solution with higher osmolarity
than red blood cells.
8