CHEM 121
Introduction to Fundamental Chemistry
Summer Quarter 2008 SCCC
Lecture 11
http://seattlecentral.edu/faculty/lcwest/CHE121
CHEM 121
Introduction to Fundamental Chemistry
Summer Quarter 2008 SCCC
Lecture 8
http://seattlecentral.edu/faculty/lcwest/CHE121
Earlier we defined matter as being either a mixture or a pure
substance.
Can anyone remember the two kinds of mixtures possible?
So far we have been talking mostly about pure substances.
e.g. ions, molecules etc
Most matter in natural systems is present as a mixture.
Solutions are a kind of homogenous mixture.
They consist of a larger component called the solvent
and one or more smaller components called the solutes.
Can you think of examples of solutions?
Examples of solutions include:
Air:
what is the solvent in air ?
Nitrogen, N2
What is a solute in air?
Oxygen, O2
Examples of solutions include:
18 ct gold:
what is the solvent in 18 ct gold ?
Gold, Au
What is a solute in 18 ct gold?
Copper, Cu
Examples of solutions include:
Sea water:
what is the solvent in sea water ?
Water, H2O
What is a solute in sea water?
NaCl, salt
Some general properties of solutions include:
 Solutions may be formed between solids, liquids or
gases.
 They are homogenous in composition
 They do not settle under gravity
 They do not scatter light (like muddy water)
Solutions form when one substance dissolves in another.
Soluble substances are those that can dissolve in a given
solvent.
Insoluble or immiscible substances are those that cannot
dissolve in a given solvent.
Which of the following are soluble in water?
NaCl, sugar, cooking oil, alcohol, gasoline, motor oil
Which of the following are immiscible in cooking oil?
NaCl, sugar, alcohol, gasoline, motor oil, water
The maximum amount of a given solute a solvent can dissolve is
called the solubility. The solubility is dependent on the
temperature and pressure.
Solubility is often expressed in terms of grams of solute per mL
of solvent but may have other units.
When a solvent contains the maximum amount of a solute
possible the solutions is said to be saturated.
By forming a solution at a high temperature then slowly cooling it
we can form supersaturated solutions that contain more solute
than in a saturated solution.
These kinds of solutions are very unstable and tend to separate
out the excess solute with the slightest disturbance.
http://www.youtube.com/watch?v=uy6eKm8IRdI&NR=1
http://www.youtube.com/watch?v=aC-KOYQsIvU&feature=related
Solutions form when one a soluble solute is dissolved in a
solvent.
In biological systems aqueous (solutions where water is the
solvent) are of particular importance.
The solubility of most liquids and solids in water increases with
temperature.
The effect of pressure on the solubility of liquid or solid solutes
in water is negligible.
The solubility of gases in water decreases with temperature.
 Are cold carbonated drinks bubblier than warm
carbonated drinks?
The solubility of many gases in water is directly proportional to
the pressure being applied to the solution.
i.e. double the pressure, double the solubility
 What happens when the cork is removed from a
bottle of champagne?
 What is the origin of decompression sickness?
 Anyone heard of hyperbaric therapy?
How do solutions form?
Why do some substances leave one phase and enter the
solution and others don’t?
How can we use chemistry to predict solubilities?
Lets first look at the formation of a solution between an ionic
solute and a polar solvent such as H2O.
Ionic compounds are composed of oppositely charged ions
arranged in a repeating 3-d arrangement.
They are held together
by attractive forces
between oppositely
charged ions.
When we place an ionic solid in water there will be attractive
forces between the ions at the surface of the crystal and the
water molecules.
Water molecules orient such that the -ve end of the molecule is
oriented towards the +ve ions at the surface and vice versa.
e.g. for KCN
red is the region
where electrons
are found most
often and blue is
where electrons
are rarely found
If the attractive force between the surface ion and the solvent is
greater than the forces between the ion and the solid then the ion
will enter the solution phase.
H2O
The ion that has left the
solid and becomes
completed surrounded
by water molecules. It has
K+
become solvated or
hydrated.
The process continues as new water molecules approach the
crystal until the crystal has been fully dissolved.
Note the different
orientation of water
molecules around
the oppositely
charged ions.
In a solution of an ionic compound a solvated ion will
occasionally collide with the surface of the solid.
Sometimes when this happens the ion will “stick” to the surface
and become part of the solid phase again.
This will happen more frequently the more concentrated the
solution is.
When the rate of ions leaving the solid equals the rate of ions
going back to the solid the system is at equilibrium and the
solution is saturated.
When a solution is at equilibrium with its solute macroscopically
there will be no change occurring.
However, at the molecular level lots is happening, just in equal
and opposite directions.
Supersaturated solutions can form because there are no sites
for solute ions to collide with.
When we place a “seed” crystal in a supersaturated solution this
provides the needed sites and the excess solute crystallizes very
quickly.
In the you tube video we
watched you can just see
the tiny seed crystals on
the persons finger.
Polar but non-ionic solutes dissolve in water via a similar
mechanism as for ionic compounds.
A solute will be insoluble in a solvent if:
1. Forces between solute particles are greater than the
forces between solute particles and the solvent.
A solute will be insoluble in a solvent if:
2. Forces between the solvent particles are stronger than
forces between the solvent and the solute.
e.g. The only attractive force between oil and water will is
dispersion forces. These are weak compared to hydrogen
bonds between water molecules.
In a polar solvent there will be
attraction between the
oppositely charged ends of the
molecule.
A good “rule of thumb” that works especially well for non-ionic
compounds is:
“Like dissolves like”
i.e. Polar solvents dissolve polar solutes well and non-polar
solvents dissolve non-polar solutes well.
There are some more specific rules that allows us to better
estimate the solubility of ionic compounds.
You will be given these if you need them.
The rate of dissolution is dependent upon:
1. The surface area of the solute.
i.e. how finely divided it is.
Increasing rate
The rate of dissolution is dependent upon:
2. How hot the solution is.
i.e. the kinetic energy of solute and solvent.
3. The rate of stirring.
Typically when we are
preparing a solution in
the lab we will both
heat and stir.
When a solute dissolves in a solvent heat can be released or
absorbed.
When heat is absorbed the process is endothermic and the
solution becomes cooler.
This effect is used in instant cold packs for sporting injuries and
first aid.
More commonly dissolution is an exothermic process and heat
is released when a solute is dissolved.
Sometimes when we make a solution it will get so hot it boils!!
What is the safest way to prepare a solution?
Many reactions take place in solution:
e.g.
2AgNO3(aq)
+ Na2CO3(aq)

Ag2CO3(s)
+
2NaNO3(aq)
The co-efficients in equations allow us to determine the relative
amounts of products and reactants.
# moles of Ag2CO3 = ½(# moles of AgNO3)
# moles of Na2CO3 = ½(# moles of AgNO3)
# moles of NaNO3 = # moles of AgNO3
The amount of solute in a given amount of solution is defined by
the concentration.
The solution concentration can be defined in a variety of ways.
The most useful is the molarity (M).
The molarity of a solution is defined as:
“The number of moles of solute in 1 L of solution”
and is given the formula:
M = n/V
If I wanted to prepare 0.5 L of 1 molL-1 NaCl aqueous solution
how would I do it?
M = 1 molL-1
V = 0.5 L
n=?
M = n/V
n = MV
1molL-1 x 0.5 L = 0.5 moles of NaCl required
m = n x MW = 0.5 mol x (22.99 + 35.45) gmol-1
m = 29.22 grams of NaCl required.
To make solution take 29.22 g of NaCl dilute to a volume of 0.5L
with H2O
In the lab we would use a piece of glassware called a
volumetric flask to prepare this solution.
An alternative way of expressing the concentration of a solution is
in percent. Which is defined as:
“parts of solute per 100 parts of solution”
or:
parts solute
%
 100
total parts
Three different percent concentrations are commonly used:
1. % weight/weight (w/w)
2. % volume/volume (v/v)
3. % weight/volume (w/v)
%(w/w) is calculated using the formula:
mass of solute
%(w/w) 
 100
mass of solution
%(v/v) is calculated using the formula:
volume of solute
%(v/v) 
 100
volume of solution
%(w/v) is calculated using the formula:
%(w/v) 
mass of solute
 100
volume of solution
Often we will want to make a dilute solution from a more
concentrated one.
To determine how to do this we use the formula :
C1V1 = C2V2
Where:
C1 = concentration of more concentrated solution
V1 = volume required of more concentrated solution
C2 = concentration of more dilute solution
V2 = volume of more dilute solution
We can use any units in this equation but they must be the same
on both sides.
How would I prepare 50 mL of a 10 mgL-1 solution of NaOH
using a 100 mgL-1 stock solution?
C1V1 = C2V2
V1 = C2V2/C1
V1 = (10 mgL-1 x 50 mL)/100 mgL-1
V1 = 5 mL
Take 5 mL of stock NaOH solution and dilute to 50 mL