Unit: Chemical Interactions
Chapter 8: Solutions
When substances dissolve to form solutions, the
properties of the mixture change.
8.1: A solution is a type of mixture
8.2: The amount of solute that dissolves can vary
8.3: Solutions can be acidic, basic, or neutral
8.4: Metal alloys are solid mixtures
8.2: The amount of solute that dissolves can vary
8.2: The amount of solute that
dissolves can vary
 Before, you learned
 Solutions are a type of mixture
 A solution is made when a solute is dissolved in a solvent
 Solutes change the properties of solvents
 Now, you will learn
 About the concentration of a solution
 How a solute’s solubility can be changed
 How solubility depends on molecular structure
8.2: The amount of solute that dissolves can vary
Review / warm-up
 True or False?
 Dissolving a solute in a solvent is a chemical change
 False: physical change
 Adding a solute to a solvent raises the solvent’s boiling point
 True
 The particles in a solution are larger than the particles in a
suspension
 False: smaller
8.2: The amount of solute that dissolves can vary
A solution with a high concentration
contains a large amount of solute
 Ocean water vs well water
 Both contain salt (solute)
 Ocean water contains more salt (solute) than the well water
 The more solute, the higher concentration of the solute
 Concentration: depends on the amount of solute dissolved in a
solvent at a particular temperature
 Adding lemonade mix to water
 More powder you add, the higher concentration of the drink
8.2: The amount of solute that dissolves can vary
Degrees of Concentration
 “Dilute solution”: a solution that has a low concentration of solute
 Well water is a “dilute solution” - there is salt dissolved in the
water but at a low concentration
 Add more solvent, or pure water, you can dilute the solution more
 A “concentrated solution” has a large amount of solute
 A “saturated solution” – when no more solute can be dissolved
into the solvent – it can’t “hold” any more at that temperature
 If it contains less than this maximum amount, it is a “unsaturated
solution”
8.2: The amount of solute that dissolves can vary
Supersaturated Solutions
 “Supersaturated Solution”: a solution that contains more dissolved
solute than is normally possible
 Can occur if more solute is added while the temperatures is raised, then the
solution is slowly cooled, the solute can remain dissolved

This solution is unstable
 If the solution is disturbed, or more solute is added in the form of a crystal,
the excess solute will quickly solidify and form a precipitate
 Ex: chemical heat pack – sodium acetate and water (1st)( 2nd –how)
 Bag contains a solution of sodium acetate (solute) and water (solvent)
 When heated, the excess sodium acetate can dissolve into the water
completely  supersaturated solution
 Heat pack is activated by bending it – disturbing the solution, solidifying
the sodium acetate and releasing a large amount of heat over a long period
of time (liquid  solid releases heat)
Link – saturated, unsaturated, supersaturated, crystallization
8.2: The amount of solute that dissolves can vary
Solubility
 The amount of a substance that will dissolve in a certain amount of
solvent at a given temperature
 Ex: household ammonia (solution) is ammonia (solute) and water (solvent)
 A large amount of ammonia can dissolve in water  ammonia has a high solubility
in water
 Ex: carbon dioxide can dissolve in water, but only a small amount
 Low solubility in water
 Oils do not dissolve in water at all
 Insoluble in water
 The amount of solute needed to make a saturated solution depends on the
solubility of a solute in a particular solvent
 If the solute is highly soluble, a lot of solute will be needed for a saturated
solution, and the saturated solution will be very concentrated
 If the solute has a low solubility, little solute is needed for a saturated solution,
and the saturated solution will be dilute
8.2: The amount of solute that dissolves can vary
The solubility of a solute can be changed
 …in two ways:
1.
Change the temperature
 Raise the temperature of the solute – most solids are more soluble
at higher temperatures
2.
Change the pressure
 The solubility of gases in a liquid solvent increases at high pressure
8.2: The amount of solute that dissolves can vary
Temperature and Solubility
 Increase temperature effects on most solid substances:
1.
Solid solutes will dissolve more quickly
2.
A greater amount of the solid dissolves in a given amount of solvent
 Opposite true for gases! Increase temperature:
 Gases become less soluble in water

Warming tap water – as it approaches boiling point air bubbles come out of solution
and rise to the top!
 Recall: to supersaturate a solution, must raise the temperature first, then it
cools slowly
 The solid is less soluble in the cooler solution, but has not yet formed a
precipitate
 Consequences in real life:
 a factory takes lake water for use as a coolant – returns warmer water to lake

Dissolved oxygen content drops  bad for some marine life that use oxygen to
breathe!
8.2: The amount of solute that dissolves can vary
Temperature and Solubility
Solute
Inc. Temp
Dec. Temp
Solid
Inc. in solubility
Dec. in solubility
Gas
Dec. in solubility
Inc. in solubility
8.2: The amount of solute that dissolves can vary
Pressure and Solubility
 Pressure change usually only affects gases
 Solubility of any gas increases at higher pressures
 Decreases at lower pressures
 Ex: Soda – CO2 is added at higher pressure than typical air
 When you open it – the CO2 bubbles fizz out
 Ex: the bends – normal air we breathe: 78% N2, 21% O2
 Under water pressure is higher, so N2 solubility into blood is higher
 Rise to surface, the N2 comes out of solution, forming bubbles in
diver’s blood vessels!
 Avoidable?
8.2: The amount of solute that dissolves can vary
Pressure and Solubility
(Generic Graph:)
Solute
Inc. Pressure
Dec. Pressure
Solid
No effect on solubility
No effect on solubility
Gas
Inc. in solubility
Dec. in solubility
8.2: The amount of solute that dissolves can vary
Solubility depends on molecular structure
 Ex: oil and water do not mix – they have different molecular
structures
 When a substance dissolves, its molecules or ions separate
from one another and become evenly mixed with molecules
of the solvent
 Water contains polar covalent bonds (negative and positive




regions)
Oil is non-polar
Therefore their molecules are not attracted to each other
Sugar is polar, and highly soluble in water
Ionic compounds are soluble in water, also (ions are charged)
•Polar solvents dissolve polar solutes
•Nonpolar solvents dissolve nonpolar solutes
•“Like dissolves like”