Mixtures and Solutions
Mixtures
• Heterogeneous vs. Homogeneous
• A heterogeneous mixture is a mixture that
does not have a uniform composition and
in which the individual substances remain
distinct.
• Suspensions are heterogeneous mixtures
containing particles that settle out if left
undisturbed.
• Examples: sand and silt in river water,
paint, etc.
Heterogeneous Mixtures (cont.)
• Colloids are heterogeneous mixtures of
intermediate sized particles (between 1 nm
and 1000 nm) and do not settle out.
• Consist of “dispersed particles” in the
“dispersion medium” (the most abundant).
• Milk is a great example of a colloid. Tiny
liquid butterfat globules stay suspended in a
water dispersion medium.
Heterogeneous Mixtures (cont.)
• Why don’t colloidal particles settle out?
• Polar groups on their surface attract
polar dispersion medium molecules or
dissolved ions.
• Charged layers around particles repel
each other and keep particles
suspended.
• Brownian motion is the jerky, random
movements of particles in a liquid colloid,
from the results of particle collisions.
• The Tyndall effect is when dispersed
colloid particles scatter light (does not
occur with solutions).
Heterogeneous Mixtures (cont.)
Homogeneous Mixtures
• Solutions are homogeneous mixtures that
contain two or more substances called the
solute and solvent.
• Term is typically used with liquids (water),
but gases and solids form solutions too.
• The solvent is the most abundant material
in the mixture. Water is our key solvent.
• Solutes are the less abundant material that
is mixed (“dissolved”) in the solvent.
• Can be liquids, solids or gases…all dissolve!
Homogeneous Mixtures (cont.)
Solutions
• A substance that dissolves in a solvent is
soluble.
• A substance that does not dissolve in a
solvent is insoluble.
• Two liquids that are soluble in each other in
any proportion are miscible.
• Two liquids that can be mixed but separate
shortly after are immiscible.
Solutions
• Electrolytes
– Compounds that dissociate into separate ions
in water and are good conductors.
– “Strong” electrolytes = 100% dissociation
• Ionic compounds are strong electrolytes
• Also includes the strong acids: HCl, HNO3, HBr
• Fully dissociate to form H+ ions in solution (H3O+)
– “Weak” electrolytes < 100% dissociation
• Nonelectrolytes
– Do not dissociate into ions in water.
– Nonpolar molecules
Section Assessment
The jerky, random movement of particles
in a liquid colloid is known as ____.
A. Brownian motion
B. Tyndall effect
C. Charles’s Law
D. kinetic energy
A.
B.
C.
D.
A
B
C
D
Expressing Concentration
• The concentration of a solution is a measure
of how much solute is dissolved in a specific
amount of solvent or solution.
• The level of concentration can be described as
concentrated or dilute.
• These are relative (qualitative) terms.
• Quantitative levels of concentration can be
described using a variety of ratios.
Expressing Concentration
(cont.)
Expressing Concentration
(cont.)
Expressing Concentration
(cont.)
• Molarity is the number of moles of
solute dissolved per liter of solution.
• Dilution equation: M1V1 = M2V2
Expressing Concentration
(cont.)
• Molality is the ratio of moles of solute
dissolved in 1 kg of solvent.
Expressing Concentration
(cont.)
• Mole fraction is the ratio of the number of
moles of solute in solution to the total
number of moles of solute and solvent.
where XA and XB represent mole
fractions of each substance
The Solvation Process
• Solvation is the process of surrounding
solute particles with solvent particles to
form a solution.
• Solvation in water is called hydration.
• Solvation of Ionic Compounds:
•
The attraction between dipoles of a water
molecule and the ions of a crystal are
greater than the attraction among ions of
a crystal.
•
Ion-dipole intermolecular force
The Solvation Process (cont.)
The Solvation of Molecules
• Sucrose molecules have several O–H
groups, which become sites for hydrogen
bonding with water molecules.
• Oil does not form a solution with water
because there is little attraction between
polar water molecules and nonpolar oil
molecules.
The Solvation Process (cont.)
• During solvation, the solute must separate
into particles and move apart. The solvent
must also move apart to make room for
solute particles. Both of these actions
require energy (endothermic).
• Then, when the solute and solvent mix
and attract each other, energy is released
(exothermic).
• The overall energy change that occurs during
solution formation is called the heat of
solution.
Factors That Affect Solvation
• Agitation: Stirring or shaking moves
dissolved particles away from the contact
surfaces more quickly and allows new
collisions to occur.
• Breaking the solute into small pieces
increases surface area and allows more
collisions to occur.
• As temperature increases, rate of solvation
increases.
Solubility
• Solubility = the maximum amount a solute
will dissolve in a solvent. It depends on the
type of solute and solvent and temperature.
• As concentration of solute in solvent
increases, more solute particles collide with
remaining crystalline solid and precipitate.
• Unsaturated solutions are solutions that
contain less dissolved solute for a given
temperature and pressure than a fully
saturated solution.
Solubility (cont.)
• Saturated solutions contain the maximum
amount of dissolved solute for a given amount
of solute at a specific temperature and
pressure. Equilibrium is reached between
solvation and precipitation.
• Solubility is affected by increasing the
temperature of the solvent because the
kinetic energy of the particles increases.
Solubility Graph (Water is the Solvent)
Temperature
can make a
big
difference!
Solubility (cont.)
• A supersaturated solution contains more
dissolved solute than a saturated solution
at the same temperature.
• To form a supersaturated solution,
a saturated solution is formed at
high temperature and then slowly
cooled. Adding more solid will
spark rapid precipitation (‘seeding’).
• Supersaturated solutions are
unstable.
Solubility of Gases
• Gases are less soluble in liquid solvents at
high temperatures. Higher K.E. allows more
gas molecules to escape from solution.
• Solubility of gases increases as its external
pressure is increased. Carbonated sodas!
• Henry’s law states that at a given
temperature, the solubility (S) of a gas in
a liquid is directly proportional to the
pressure (P).
Section Assessment
For a given amount, which type of
solution contains the LEAST amount of
solute?
A. solvated
B. saturated
C. supersaturated
D. unsaturated
A.
B.
C.
D.
A
B
C
D
Section Assessment
At a given temperature, the solubility of a
gas is directly proportional to what?
A. volume
B. mass
C. molarity
D. pressure
A.
B.
C.
D.
A
B
C
D
Colligative Properties of Solutions
• Colligative properties are physical properties
of solutions that are affected by the number of
particles but not by the identity of dissolved
solute particles.
• Types of colligative properties:
• Vapor pressure lowering
• Boiling point elevation
• Freezing point depression
• Osmotic pressure
Colligative Properties of Solutions
• Strong electrolytes have a greater colligative effect
since they put more particles into solution than weak
electrolytes or molecules.
• The relationship between moles of solute and moles of
particles in solution is the van’t Hoff factor:
“ i ” = (moles of particles) / (moles of solute)
• Molecules always have an i = 1
• Strong electrolytes have i = 2 or more depending upon
the number of ions in formula (CaCl2 has i = 3).
• Assume all electrolytes are “strong” (i.e. 100%
dissociate) unless told otherwise.
Vapor Pressure Lowering
• Adding a nonvolatile solute to a solvent
lowers the solvent’s vapor pressure.
• Volatility is tendency of a substance to
vaporize (change from liquid to gas).
• When a solute is present, a mixture of solvent
and solute occupies the surface area, and
fewer particles enter the gaseous state.
• The greater the number of solute particles,
the lower the vapor pressure.
Vapor Pressure Lowering (cont.)
• Vapor pressure lowering is due to the
number of solute particles in solution.
Vapor Pressure Lowering (cont.)
• Raoult’s Law:
P(solution) = X(solvent) x Po (solvent)
where:
X(solvent) = mole fraction of the solvent
Po (solvent) = vapor pressure of pure solvent
Boiling Point Elevation
• When a nonvolatile solute lowers the vapor
pressure of a solvent, the boiling point is
also affected.
• Recall that the boiling point is when the
vapor pressure of the liquid is equal to
the atmospheric pressure.
• More heat is needed to supply additional
kinetic energy to raise the vapor pressure to
atmospheric pressure.
Boiling Point Elevation (cont.)
• The temperature difference between a solution’s
boiling point and a pure solvent's boiling point is
called the boiling point elevation.
ΔTb = iKbm
• where ΔTb is the boiling point elevation, i is
the van’t Hoff factor, Kb is the molal boiling
point elevation constant, and m represents
molality.
Boiling Point Elevation (cont.)
Freezing Point Depression
• At a solvent's freezing point temperature,
particles no longer have sufficient kinetic
energy to overcome interparticle attractive
forces.
• The freezing point
of a solution is
always lower than
that of the pure
solvent.
Freezing Point Depression (cont.)
• Solute particles interfere with the attractive
forces among solvent particles.
• A solution's freezing point depression is the
difference in temperature between its freezing
point and the freezing point of the pure
solvent.
ΔTf = iKfm
• where ΔTf is the freezing point depression, i is
the van’t Hoff factor, Kf is the freezing point
depression constant, and m is molality.
Freezing Point Depression (cont.)
Osmotic Pressure
• Osmosis is the diffusion of a solvent
through a semipermeable membrane.
Osmotic Pressure (cont.)
• Osmotic pressure is the amount of
additional pressure caused by water
molecules that moved that moved into the
concentrated solution.
• Osmotic pressure of a solution is always
greater than its pure solvent.
Section 14.1 Types of Mixtures
Key Concepts
• The individual substances in a heterogeneous
mixture remain distinct.
• Two types of heterogeneous mixtures are suspensions
and colloids.
• Brownian motion is the erratic movement of colloid
particles.
• Colloids exhibit the Tyndall effect.
• A solution can exist as a gas, a liquid, or a solid,
depending on the solvent.
• Solutes in a solution can be gases, liquids,
or solids.
Section 14.2 Solution Concentration
Key Concepts
• Concentrations can be measured qualitatively and
quantitatively.
• Molarity is the number of moles of solute dissolved per
liter of solution.
• Molality is the ratio of the number of moles of solute
dissolved in 1 kg of solvent.
Section 14.2 Solution Concentration
Key Concepts
(cont.)
• The number of moles of solute does not change during a
dilution.
M1V1 = M2V2
Section 14.3 Factors Affecting
Solvation
Key Concepts
• The process of solvation involves solute particles
surrounded by solvent particles.
• Solutions can be unsaturated, saturated, or
supersaturated.
• Henry’s law states that at a given temperature, the
solubility (S) of a gas in a liquid is directly proportional
to the pressure (P) of the gas above the liquid.
Section 14.4 Colligative Properties
of Solutions
Key Concepts
• Nonvolatile solutes lower the vapor pressure of a
solution.
• Boiling point elevation is directly related to the
solution’s molality.
∆Tb = Kbm
• A solution’s freezing point depression is always lower
than that of the pure solvent.
∆Tf = Kfm
• Osmotic pressure depends on the number of solute
particles in a given volume.