E2 Precipitation and Water Purity
“Will we do
the entire
experiment?”
“E2 is a one
session lab!”
Goals for Experiment 2
One session three hour lab
 Complete Parts 1 and 2A and 2B (all).
 Complete part 3 OR 4 as assigned.
 Omit part 5.
Experiment 2
Pre-lab correction
 Question 3 refers to Part 2A (not Part 2)
Precipitation
 A solid comes out of solution
Discussion: first hour of next session
 Prepare discussion abstract and presentation.
 Complete team report.
Salt solutions before mixing
Background: SALTS
After mixing
Background: Water and Salt Solubility
 Ionic compounds with cations (+) and anions (-).
 In the solid, the salt ions are
fixed in a rigid lattice.
“I’m a cation.
Note my eyes!”
 The simplest ratio of the
ions in the solid is represented
by the formula of the salt.
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Salt Formulas
Background: Water and Salt Solubility
 In solution the ions are mixed
with water molecules and free to
move about in solution.
Formula ?
Cr+3 O-2
= Cr2O3
Background: Water and Salt Solubility
Salt Solubility in Water
 Water (H2O) is polar.
 Polar water molecules reduce the effective charges of the ions
in the solid and thus salts dissociate and dissolve.
DEMO
NaCl(s) = Na+(aq) + Cl-(aq)
BaCl2(s) = Ba2+(aq) + 2 Cl-(aq)
Precipitation reactions
Salt Solubility
• Aqueous salt solutions are electrolytes.
Hg2+(aq) + 2Cl-(aq) +
K+(aq) + I-(aq)
KCl
HgI2
1. The positive ion of a dissolved salt combines with
the negative ion from a different dissolved salt.
DEMO
2. The recombined ions may stay in solution or come
out of solution in the form of a solid called a
“precipitate”.
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REFERENCE BLANK TEST
Part 1. What is the precipitate?
 Test mixture designed to identify reactants
HgCl2 (aq) +
KI (aq) →
(forming the precipitate) and spectators (non-reactants).
__?__ (s)
clear and colorless salt solutions
Test Mixture
__?
 Omit a species (ion) from the reaction mixture.
 Substitute a known SPECTATOR ion for the
ppt.
__?__ ↓
omitted ion such as Na+ or K+ or NO3 Compare the products of the test and original
reaction mixture
DEMO
Reference Blank Tests
Reaction:
HgCl2 (aq) + KI (aq) →
Clear and colorless
Reference Blank Test Design
HgCl2 (aq) + KI (aq) →
?
Reference blank test:
Hg(NO3)2 + KI →
Clear and colorless
Same
properties
Conclusion?
DEMO
Cl-
?
colorless solutions
Valid Reference Blank Test
 produces the identical products or no
reaction.
is a spectator ion.
Reference Blank Test Design
BaCl2 (aq) + AgF (aq) → white (s)?
HgCl2 (aq) + KI (aq) →
Test Observations:
1. FeCl3 (aq) + AgF (aq) → green (s)
?
Invalid test:
CuCl2 (aq) + KI (aq) →
DEMO
• The test produced a different reaction with different products
and is therefore invalid.
2. NaCl (aq) + AgF (aq) → white (s)
3. Ba(NO3)2 (aq) + AgF (aq) → no reaction
Q. What do you know about the white (s)?
1. Nothing.
2. Ba2+ is a spectator.
3. Cl- is a reactant.
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Part 2A. Precipitation Studies
Discussion questions 1-3, p.67
 Is the solubility of a metal ion predictable from the
position of its element in the Periodic Table?
 Is precipitate color predictable from the position of
the metal ion’s element in the Periodic Table?
Nitrate salts 
Sodium Salts
Cations
Cl-
Hg2+

CrO42
I
C2O42
S2
SO42
 Is there a relationship between ion charge and
solubility? Compare the solubility data of multiply
charged ions and….
Part 2A. Table, page 44; Cation assignments, page 216
Part 2B. Can I identify it?
 Identify an unknown metal ion based on
precipitation observations from part 2A:
Caution: Save your acetate sheet results from Part
2A for use in Part 2B
Concentration and Salt Precipitation
Part 3. Concentration and Precipitation.
Discussion (Question 4, page 67)
Examine the relationship between concentration and
amount of precipitate. Organize the class data…
Laboratory
 Combine assigned ion combinations at different
concentrations
 Observe and record the relative amounts of
precipitate formed at different concentrations.
Concentration and Salt Precipitation
Saturated solution = contains the maximum amount of
salt that can be dissolved in a given solution volume.
DEMO
Supersaturation
and
precipitation.
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Concentration and Precipitation
• Precipitation (ppt.) is dependent on concentration
and occurs only if the solubility limit of the salt is
exceeded.
Concentration and Precipitation
• Precipitation reactions are equilibrium systems and
thus there are always reactants and products present
when precipitation occurs
DEMO
0.10 M KI(aq) + 0.10 M HgCl2 (aq) →
ppt.
10-3 M KI(aq) + 10- 4 M HgCl2 (aq) →
NO ppt.
Concentration and Precipitation
 Precipitation reactions are equilibrium systems
 When precipitation occurs, some reactant ions
remain in solution:
Hg2+ and 2I- →
X HgI2(s)
Solvent Polarity
H
Laboratory
 Combine assigned salts in different solvents (water,
acetone, hexane)
 Compare salt solubility and amount of precipitate
formed by the salt mixtures in the different solvents.
Solvent Polarity
 Polar and non-polar solvents are immiscible
O
/
Part 4. Solvent Pollution and Precipitation
Discussion Question 5, page 67
What is the relationship, if any, between salt solubility,
precipitation, and solvent polarity?
“In reality, it is understood there is
both a forward and reverse arrow!”
 Water (H2O) is very polar.
DEMO:
1.Remove the ppt. by
filtration
2. Check for presence of
reactant ions in the filtrate.
\
H
 Hexane (C6H14) is nonpolar
CH3 CH2 CH2CH2CH2CH3
 Acetone (CH3COCH3 ) is moderately polar.
 Salts will not dissociate in non-polar solvents.
O

C
/
\
CH3 CH3
DEMO
5
Salt solubility and solvent polarity
Solvent Polarity and Solubility
Acetone is less polar than water.
Acetone is added to a saturated
solution of CuSO4(aq).
Q1. Will the solubility of the CuSO4
Increase?
Decrease?
Remain the same?
Q2. What will you observe?
B = Acetone is added to A = CuSO4(aq)
DEMO
After mixing
CuSO4(s) forms
 The less polar the polar solvent environment, the
lower the solubility of the salt.
Any Questions?
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