Transcript UNDERSTANDING SOLUTIONS

UNDERSTANDING
SOLUTIONS
PART 1
SOLUTIONS & SUSPENSIONS
• A suspension is a mixture in which
particles:
– Can be seen with the eye
– Settle to the bottom
– Can be filtered out
Example: pepper in water.
• A solution has the same properties
throughout. Its particles…
– Can not be seen
– Will not settle to the bottom
– Can not be filtered out.
Example: Salt in water
SOLVENTS & SOLUTES
• SOLVENT- The part of the solution
present in the largest amount. The
substance that dissolves the other
substance.
• SOLUTE- The part of the solution present
in the smaller amount. The substance that
is dissolved.
Question: In salt water which is the solute?
WATER AS A SOLVENT
• Water is the solvent in
many common
solutions.
• Life depends on water
solutions:
– Sap in trees
– Nutrients in rainwater
through roots
– Saliva, Tears & even
Blood!
• Water has a simple molecular structure. It
is composed of one oxygen atom and two
hydrogen atoms. Each hydrogen atom is
covalently bonded to the oxygen via a
shared pair of electrons. Oxygen also has
two unshared pairs of electrons. Thus
there are 4 pairs of electrons surrounding
the oxygen atom, two pairs involved in
covalent bonds with hydrogen, and two
unshared pairs on the opposite side of the
oxygen atom.
• Water is a "polar" molecule, meaning that
there is an uneven distribution of electron
density. Water has a partial negative
charge (- ) near the oxygen atom due to
the unshared pairs of electrons, and partial
positive charges (+ ) near the hydrogen
atoms.
SOLUTIONS WITHOUT WATER
• Many solutions are made without water:
– Gasoline
– Acetone (Nail Polish)
– Hexane (Spot Remover)
– Ethanol (Perfume)
PARTICLES IN A SOLUTION
• Whenever a solution forms, particles of the
solute leave each other and become
surrounded by particles of the solvent.
IONIC SOLIDS IN WATER…
•
The positive and negative ions are attracted to polar water
molecules. Water molecules surround each ion as it leaves
the surface of the crystal. As each layer of the solid is
exposed, more ions can dissolve.
•
BREAK DOWN INTO INDIVIDUAL IONS
MOLECULAR SOLIDS IN
WATER…
• Molecular solids– don’t break into ions in water.
– Break up into individual neutral molecules. (like ions
they are completely surrounded by the water)
Example: Sugar in water,
•BREAK DOWN INTO MOLECULES!
SOLUTIONS & CONDUCTIVITY
• A solution of ionic compounds in water will
conduct electricity.
• A solution of molecular compounds in
water may not.
NiCl2(aq) +
KMnO4(aq)
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COLLOIDS
• A colloid is a mixture with small
undissolved particles that do not
settle out.
• It’s NOT a suspension or a solution!
Examples: Jello, Mayonnaise, Fog,
Shaving Cream, Whipped Cream.
A colloid contains particles large
enough to scatter a light beam.
EFFECTS OF SOLUTES ON
SOLUTIONS
• Solutes lower the freezing
point of a solvent.
– Example: Salt melts ice
because the salt molecule get
in the way of ice crystals
forming
• Solutes raise the boiling
point of solvent.
– Example: antifreeze keeps
cars from overheating.
CHECK FOR UNDERSTANDING 1
1. List 3 ways to tell the difference between a
solution & a suspension.
2. Describe what happens to the molecules of a
solid, such as a sugar cube, when it dissolves
in water. How does the process differ for an
ionic compound, such as table salt?
3. Why does salt sprinkled on icy roads cause the
ice to melt?
4. Why does ocean water freeze at a lower
temperature than fresh water?
1. The particles of a solution will not settle
out, are not visible to the eye and will pass
through a filter. The particles of a
suspension will settle out, are visible to the
eye, and will not pass through a filter.
2. The molecules of sugar separate from
each other and become surrounded by
water molecules, but the molecules do not
separate into ions or atoms. When an
ionic compound dissolves, it breaks up
into ions, which are surrounded by water
molecules.
3. The salt is a solute, which lowers the
freezing point of the water as the two
substances begin to mix.
4. Ocean water is a solution of salt and
water. The salt lowers the freezing point
of water below that of fresh water.
UNDERSTANDING
SOLUTIONS
PART 2
CONCENTRATION &
SOLUBILITY
CONCENTRATION
• The amount of one material dissolved in a
given amount of another.
– Think: how “strong” a solution is.
Dilute: Only a little
solute is dissolved in
the solvent.
Concentrated: A lot of
solute is dissolved in
the solvent.
MEASURING CONCENTRATION
• To measure concentration you compare
the amount of solute to the amount of
solvent.
-OR• You compare the amount of solute to the
total amount of solution.
SAMPLE PROBLEM
• SportMax is a mixture of fruit juice &
water. The concentration of a 500 ml
sample is 20% juice by volume. Find the
volume of juice in the solution
Think: What number is 20% of 500?
SAMPLE PROBLEM 2
• FruityPoo BBQ is a mixture of fruit juice &
vinegar. The concentration of a 300 ml
sample is 40% juice by volume. Find the
volume of juice in the solution
Think: What number is 40% of 300?
SOLUBILITY
• Solubility is a measure of how well a
solute can dissolve in a solvent at a given
temperature.
Think: is there a limit to how sweet you
could make a glass of iced tea by adding
sugar?
WHAT AFFECTS SOLUBILITY?
• TEMPERATURE
• TYPE of SOLVENT
And Pressure
Saturated and Unsaturated
Solutions
• A saturated solution contains as much
dissolved solute as possible at a given
temperature.
• An unsaturated solution does not yet hold
as much of a solute as is possible at the
given temperature.
SUPERSATURATED SOLUTIONS
• If you allow a heated saturated solution to
cool slowly, sometimes the extra solute
will remain dissolved.
• You’ve created a supersaturated solution:
a solution that has more dissolved solute
that you would predict based on its
temperature.
WORKING WITH SOLUBILITY
• Solubility is a physical property that helps
us identify substance.
CHECK FOR UNDERSTANDING 2
1. What quantities are compared when the
concentration of a solution is calculated?
2. Why would an ionic compound be more likely
to dissolve in water than oil?
3. How does a saturated solution differ from an
unsaturated solution?
4. When you heat tap water on the stove, you
can see tiny bubbles of oxygen form. They
rise to the surface long before the water begins
to boil. Explain what causes these bubbles to
appear.
1. The amount of solute is compared to the
amount of solvent or to the total amount of
solution.
2. Polar water molecules attract the positive
and negative ions. Nonpolar oil molecules
do not.
3. A saturated solution contains as much
solute as can dissolve in a solvent at a
given temperature. An unsaturated
solution does not (more solute could be
dissolved in it without heating it).
4. Oxygen dissolved in the tap water
becomes less soluble as the temperature
goes up. The bubbles form when the gas
comes out of the solution.