Solutions
Solutions are homogeneous mixtures.
In everyday life we usually think of solutions as liquids,
but air is a gaseous solution, alloys like brass are solid
solutions.
Binary solutions - have 2 components only
Solvent - substance present in greater amount
When water is the solvent the solution is aqueous (aq).
Solute - substance present in lesser amount
Can have a solid, liquid or other substance
dissolved in a liquid solvent
Process of dissolving – a physical change
Solvation – Individual solute particles are surrounded by
solvent molecules.
Hydration- Solvation where water is the solvent.
Molecular compounds - Usually the molecules separate
from one another and are surrounded by solvent. The
molecules themselves stay intact. (There are exceptions to
this e.g. acids)
Most solutions of molecular compounds DO NOT conduct
electricity. Solutes are nonelectrolytes.
Example: sucrose (table sugar)
C12H22O11 (s) → C12H22O11 (aq) (by adding water)
Ionic compounds – One that dissolve in water separate into
individual ions that are surrounded by solvent.
NaCl (s)
 NaCl (aq)
(by adding water)
NaCl (aq) is equivalent to Na+(aq) + Cl-(aq)
Each individual sodium ion is surrounded by water.
Each individual chloride ion is surrounded by water.
There is a favorable (attractive) interaction between the
solvent and the solute particles in a solution. Otherwise,
you wouldn't get dissolving at all.
Watch how many ions you get.
MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) (by adding water)
1 formula unit of magnesium chloride yields
1 magnesium ion and 2 separate chloride ions.
Example:
Iron(III) fluoride
FeF3 (s) → Fe3+ (aq) + 3 F1- (aq)
Polyatomic ions stay intact.
Keep track of how many you get:
Mg(OH)2(s)  Mg2+(aq) + 2 OH1 formula unit of magnesium hydroxide yields
1 magnesium ion and 2 separate hydroxide ions
Hydroxide ions stay intact (the O and H don't separate).
Other examples:
Iron(III) nitrate Fe(NO3)3 → Fe3+ (aq) + 3 NO31- (aq)
Ammonium phosphate
(NH4)3PO4 (s) → 3 NH41+ (aq) + PO43- (aq)
In wires electric current is due to the flow of electrons.
In solutions electric current is due to the flow of IONS.
(Solutions are conductive.)
Absolutely pure water doesn't conduct electricity. Tap
water will due to minerals and other dissolved ions.
Electrolytes- .Solutes that yield conductive solutions
All ionic compounds that can dissolve in water are
electrolytes.
Strong electrolytes – dissociate completely to ions in
solution. Good electrical conductors.
Weak electrolytes – not all particles dissociate into ions in
solution (most remain intact). Poor electrical
conductors.
Some molecular compounds are electrolytes , many are
weak electrolytes(examples: some acids and bases).
Example: Acetic acid, CH3COOH ⇠acidic hydrogen
comes off as H1+
CH3COOH ⇄ CH3COO1- (aq) + H1+ (aq)
More proper to write: CH3COOH + H2O ⇄ CH3COO1- (aq) + H3O 1+ (aq)
Only a few % of the acetic acid molecules form ions.
Solubility
Soluble – (adj.) applies to a substance that can dissolve in a
certain solvent
Insoluble – (adj.) applies to a substance does not dissolve
in a certain solvent
Terms for liquids mixing with liquids:
Miscible – (adj.) means two liquids can mix
Immiscible – (adj.) means liquids do not mix (oil & water)
Solubility - The maximum amount of a solute that can be
dissolved in a given amount of solvent.
(At a certain temperature and pressure.)
Units: grams of solute per 100 g of solvent
g of solute/100 mL of water (recall dH2O=1 g/mL)
grams of solute per liter of solvent
moles of solute per liter of solute.
Many solubilities are tablulated.
CRC Handbook of Chemistry and Physics
Figure 5.3 gives some general solubility trends.
Temperature Dependence - Rules of thumb:
Solids – solubility often increases as temperature increases
(sugar in hot vs. cold tea)
Gases – solubility decreases as temperature increases
(Boiling water drives out dissolved gases.)
Carbonated beverages are best cold, to keep CO2 dissolved
so it will still be fizzy. Hot Coke is “flat” due to loss of
CO2.
There are exceptions to the rules of thumb.
3 classifications of solutions based on the amount of
dissolved solute.
1. unsaturated solution - less than the maximum amount of
solute is dissolved. More could be added and dissolved.
dilute - relatively small amount of a solute is
dissolved
concentrated - relatively large amount of solute is
dissolved
2. saturated solution - Exactly the maximum amount of
solute is dissolved in a given amount of solvent, no more,
no less.
3. supersaturated solution - an unstable solution that has
more than the usual maximum amount of solvent
dissolved.
To do this:
- In hot solvent dissolve a large amount of a solute
whose solubility is very temperature dependent.
- VERY carefully cool the solution
- Sometimes the extra solute will stay dissolved
- B bumping, stirring, putting in a crystal, etc. can
cause the excess solute to come out of solution and
form a solid precipitate.