Chapter 18
Big Idea : Most everything is a mixture
18.1 Most materials are mixtures
 Mixture- a combination of two or more substances in which
each substance retains its own properties.

 Most materials we encounter are mixtures: mixtures of
elements, mixtures of compounds or mixtures of elements
and compounds.
 Examples:
 stainless steel- mixture of iron (Fe), chromium (Cr),
nickel (Ni) and carbon (C).
 our atmosphere (air)- mixture of nitrogen (N2),
oxygen (O2), argon (Ar), carbon dioxide (CO2) and
water vapor (H2O)
Mixtures vs. compounds

There is a difference between the way
materials combine to form mixtures and
the way elements combine to form
compounds
Mixture

Each substance in a mixture retains its
chemical identity.
Ex- if you mix sugar into a cup of tea, the tea
keeps its properties and the sugar keeps its
properties. They merely mix together. The
sugar molecules just spread out uniformly
throughout the tea.
So, the formation of a mixture is a physical
change, not a chemical change.
Compound

 In contrast when you form a compound there is a
chemical change that occurs.
 Recall that when sodium (a soft, highly reactive
metal) combines with chlorine gas (a green,
poisonous gas) to form sodium chloride ( a white,
salt that we put on french fries), the elements lose
their chemical identities and form something
completely different.
 Compound formation is a chemical change, not a
physical change.
Mixtures can be separated by
physical means

The components of mixtures can be separated from
one another by taking advantage of differences in
the components’ physical properties.
ex- you can filter solids out of a mixture of solids
and liquids. (this is how coffee is made)
This process is called filtration – takes advantage of
size differences between components.
ex- you can take advantage of a difference in
boiling and melting points.
Saltwater (ocean water/sea water has more stuff
in it) is a mixture of water and salt.
To separate the salt from the water, you can boil
off the water leaving only salt crystals behind.
This process of boiling off liquids at their boiling
points is called distillation.
It is a very effective method in separating liquids
from each other.
18.2 The Chemist’s
Classification of Matter

 Pure Substance- material that consists of only one type of
element or one type of compound
 Mixture – contains two or more elements or compounds mixed
together.
 Mixtures can be heterogeneous or homogeneous.
 Heterogeneous mixture- the different components can be seen
as individual substances- ie, pulp in orange juice, sand in
water, oil in vinegar, granite
 Homogeneous mixture- have the same composition
throughout. Uniformly mixed so that you cannot see the
individual components. 14 kt gold, sugar in tea, cough syrup.
Suspensions

 Suspension- homogeneous mixture in which the different
components are in different phases – ie solids in liquids, or
liquids in gases.
 The mixing is so thorough that the different components are
not easily distinguished
 Ex- milk, blood, clouds.
 One way to tell is by shining a beam of light through themthe path of the light will be visible
 One way to distinguish a suspension from a solution is to
run it through a centrifuge which will separate the parts of a
suspension but not the parts of a solution.
18.3 Making solutions

 When you dissolve a solid in a liquid you are
creating a solution.
 The solid separates from each other and spreads out
evenly throughout the liquid so it appears to
disappear. It is only spread out, however.
 Terms:
 Solvent- the component present in the largest amount
(what does the dissolving)
 Solute(s)- other components present in smaller
amounts (what is being dissolved)
Types of solutions

 To make a solution a solute must dissolve in a solvent
 Whether something dissolves in another is a function of
their electrical attraction for each other. The stronger the
attraction, the greater the ability to dissolve.
 There is a limit to how much of a given solute can be
dissolved in a given solvent. At that point it is said to be
saturated with solute.

 Saturated solution- a solution containing the
maximum amount of solute that will dissolve in its
solvent.
 Unsaturated solution- a solution that is capable of
dissolving additional solute.
18.4 Concentration and the Mole

 The quantity of solute dissolved in a solution is
described in mathematical terms by the solution’s
concentration- which is the amount of solute
dissolved per amount of solution
 Concentration = amount of solute/amount of solvent
18.5 Solubility-
the ability of solute to dissolve in a solvent

 Solubility depends on the submicroscopic attractions
between the solute particles and the solvent particles
 The greater the attraction, the greater the solubility.
 If a solute has any appreciable solubility in a solvent, it is
said to be soluble in that solvent
 Solubility is also dependent on the attraction of solute
particles to each other.
 Ex- if you were to dissolve sugar in water, the water must
first pull the sugar molecules away from each other and
get in between them. This limits how much sugar can be
dissolved in water. Eventually you run out of water
molecules.
A material that does not dissolve in a solvent to
any appreciable extent is said to be insoluble in
that solvent. ie- pepper in water
Just because a solute doesn’t dissolve in one
solvent doesn’t mean it won’t dissolve in another.
ie- sand and glass won’t dissolve in water but will
dissolve in hydrofluoric acid (used to etch glass).
Styrofoam won’t dissolve in water but will
dissolve in acetone (active ingredient in fingernail
polish remover)
Solubility changes with
temperature

 For many solids, their solubility increases with an
increase in temperature. This does not hold true for
all solids but most will have increased solubility
when a solvent is heated.
 Ex- you can dissolve more sugar in a glass of hot tea
than a glass of cold tea.
 Other solids aren’t affected as much (the solubility of
sodium chloride- table salt- isn’t really affected by
temperature)
Solubility of gases

 The solubility of gases is actually the opposite of the
solubility of solids.
 The solubility of gases goes down when the
temperature rises and up when the temperature
drops. You can dissolve more oxygen in a stream
when it’s cold.
Rate of solubility – how quick
something dissolves

 Making solids smaller increases the surface area of
the solids exposing more to the solvent increasing
the rate or speed of solubility.
 Stirring solids forces the solute and solvent particles
to come in contact with each other more thereby
increasing the rate of solubility