Ch. 14 Mixtures and Solutions
14.1 Types of Mixtures
Mixtures can be either heterogeneous or homogeneous.
Heterogeneous Mixtures
1. suspensions
2. colloids
suspension – a mixture containing particles that settle out if left undisturbed.
Ex.: muddy water
colloid- a heterogeneous mixture of intermediate sized particles (between atomicscale size of solution particles and the size of suspension particles).
Ex.: milk
Brownian motion – erratic movement of colloid particles. The dispersed
particles of liquid colloids make jerky random movements.
Tyndall effect – the scattering of light by colloidal particles. See Fig.3 pg478
Homogeneous Mixtures (solutions)
Types of solutions: See Table 2 pg479 Solutions can be gaseous, liquid or solid.
Common examples:
Gas: Air: the solvent is nitrogen, the solute is oxygen.
Liquid: Carbonated water: the solvent is water, the solute is CO2.
(Water is the most common solvent among liquid solutions)
Forming solutions:
soluble – a substance that dissolves in a solvent
miscible – 2 liquids that are soluble in each other
insoluble – a substance that does not dissolve in a solvent
immiscible – 2 liquids that can be mixed together but separate shortly after
14.2 Solution Concentration
You need to know 4 ways of quantitatively expressing concentration – measure of
how much solute is dissolved in a specific amount of solvent or solution.
1. percent by mass
2. percent by volume
3. molarity
4. molality
Concentration can be expressed qualitatively using the words concentrated or dilute.
Concentrated solutions contain large amounts of solute. Dilute solutions contain
small amounts of solute.
Percent by Mass
=
Percent by Volume =
mass of solute x
mass of solution
100
volume of solute x
volume of solution
100
Molarity (M)
=
moles of solute
liters of solution
Molality (m)
=
moles of solute
kg of solvent
Diluting a Molar Solution :
M1V1
=
M2V2
14.3 Factors Affecting Solvation
solvation – the process of surrounding solute particles with solvent particles to
form a solution.
In general, “like dissolves like”. Since water is a polar molecule it will tend to
dissolve other polar molecules and ionic compounds (extreme version of polar).
Aqueous solutions of ionic compounds: Water can dissolve most ionic
compounds. One exception is gypsum which is insoluble in water because the
attractive forces between the ions in gypsum are so strong that they cannot be
overcome by the attractive forces of the water molecules.
Aqueous solutions of molecular compounds: Sugar (sucrose) is an example of
a molecular compound that will dissolve in water. As soon as the sugar crystals
contact the water, water molecules collide with the outer surface of the crystal. Each
O-H bond on the sugar becomes a site for hydrogen bonding with water. Sucrose
molecules contain 8 O-H bonds and are polar. Polar water molecules form hydrogen
bonds with the O-H bonds, which pulls the sucrose into solution. (See Fig12 pg491)
Heat of solution- the amount of heat gained or lost in the solution formation
process.
There are 3 common ways to help solute and solvent particle collide with each other
and therefore dissolve faster:
1. Agitation (stirring)
2. Surface Area (increase by crushing)
3. Temperature (most solids dissolve more and faster in hot liquids)
unsaturated solution- one that contains less dissolved solute for a given
temperature and pressure than a saturated solution – contains the max amount of
solute for that temp and pressure.. In other words, more solute can be dissolved in
an unsaturated solution.
Solubility is affected by temperature and is often measured in g solute/100 g H2O.
See Fig15 pg493. Also see Table 4 pg494.
supersaturated solution – contains more dissolved solute than a saturated
solution at the same temperature
Solubility of gases: gases dissolve better in cold temperatures and under high
pressure.
Pressure and Henry’s Law: The solubility of a gas in any solvent increases as its
external pressure (the pressure above the solution) increases. Carbonated
beverages depend on this fact. Carbonated beverages contain carbon dioxide gas
dissolved in an aqueous solution. In bottling or canning the beverage, CO2 is
dissolved in the solution at a pressure higher than atmospheric pressure. When the
beverage container is opened, the pressure of the CO2 gas in the space above the
liquid decreases. As a result, bubbles of CO2 gas form in the solution, rise to the top
and escape.
Henry’s law – 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. See Fig18 pg496. When
the cap is on, the pressure above the liquid keeps the gas from escaping. When the
cap is removed, the pressure decreases, decreasing the solubility of the gas.
S1 = S2
P1
P2
14.4 Colligative Properties – physical properties of solutions that are affected by
the number of particles but not by the identity of dissolved solute particles. The 3
colligative properties are:
1. vapor pressure lowering – the greater the number of solute particles in a
solvent, the lower the vapor pressure
2. boiling point elevation- the temperature difference between a solution’s
boiling point and the boiling point of the pure solvent. Because a nonvolatile solute
lowers a solvent’s vapor pressure, it also affects the boiling point. When the
temperature of a solution containing a nonvolatile solute is raised to boiling point of
the pure solvent, the resulting vapor pressure is still less than the atmospheric
pressure and the solution cannot boil. The solution must be heated to a higher
temperature to supply the additional kinetic energy needed to raise the vapor
pressure to atmospheric pressure.
∆Tb
=
Kb m
(Boiling point elevation = constant x molality)
3. freezing point depression – difference between a solution’s freezing
point and that of its pure solvent.
∆𝑇 f =
Kfm
Osmotic Pressure
Osmosis – the diffusion of a solvent through a semipermeable membrane. See
Fig23 pg504. Due to osmosis, solvents diffuse from areas of lower solute
concentration to areas of higher solute concentration in an attempt to even out the
concentrations.
Assignment: 14.2 #9-19 and 24-28 pgs. 481-487, 14.3 #36-38 pg. 497 and #91 pg
509, And 14.4 #45-47 pg 503