Precipitates and Solubility
Rules
Intro
What do geothermal vents have in common with a bathtub ring? The
vents spew clouds of mineral-rich water from deep inside Earth into
the ocean near mid-ocean ridges. A bathtub ring is a deposit formed
from hard water and soap. Both involve the process of precipitation,
the forma- tion of insoluble or slightly soluble solids. When oppositely
charged ions come in contact, they attract each other, and if that
attraction is stronger than the ions' attraction to water, they form
crystalline solids.
When two different ionic solutions with concentrations below their
saturation points are combined and a precipitate forms, they have
under- gone a double replacement reaction in which one of the
products is insoluble. The reaction of aqueous solutions of calcium
chloride and zinc sulfate, for example, combines Ca2-r ions and SO/ ions in a concentration above the saturation point of calcium sulfate.
The formation of the precipitate is described by the following
equation:
CaCl2(aq) + ZnSO4(aq)  ZnCl2(aq)
+ CaS04(s)
Insoluble salts can be identified by their low Ksp values
(equilibrium dissociation constants). The identity of precipitates can
also be deduced from the results of combining pairs of salt
solutions, as you will do in this investigation. A comparison of the
products from the combinations allows for the identification of any
precipitates that form. Trends, called solubility rules, can also be found
for some ions that tend to form precipitates more readily than others.
In this investigation, you will combine pairs of six given salt solutions
and look for precipitates. After you write a chemical equation for
each combination, you will attempt to deduce which products are
precipitates, and also discover some common solubility rules.
Pre-Lab
Discussion
Read the entire laboratory investigation and the relevant pages of
your textbook. Then answer the questions that follow.
1. How many products are there in a double replacement reaction
from which to choose the precipitate?
2. How can you recognize a precipitate when you see one?
3. Why is it necessary to use different droppers for each of the
different solutions?
4. Each of the solutions made today were made using distilled water (water
that has been put through a filtering system that takes out all of the
impurities and ions that would otherwise be in the water). Why is this a
necessary step?
Problem
What are the precipitates that form from the reactions of salt solutions?
Safety
Wear your goggles and lab apron at all times during this investi gation.
Silver nitrate (AgNO3)causes stains to skin and clothing.
Procedure
Put on your goggles and lab apron. Obtain micropipets of each
solution and label them if necessary. Mark the well plates with
the names of the six solutions in the manner shown in the
Data Table.
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In the upper left well of the well plate, combine the first
pair of solutions, ten drops each, using the micropipets.
Note the appearance or absence of a precipitate and record
your observation in the Data Table. Write NR if
there is no reaction.
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3.
Continue the solution cornbinations (15 total) until each of the
solutions has been combined with all of the others. Record the
results in the Data Table.
4. Dispose of any solutions containing silver compounds in a labeled
container.
.
5. Wash_ the well plate with soapy water and rinse thoroughly
and finally r i n s e w i t h distilled water. Clean up your work
area and wash your hands before leaving the laboratory.
Observations
DATA TABLE (the ‘X’ indicates duplicate boxes that do not need
to be filled in)
AgN03
NaNO3
K3PO4
Na2CO3
MgCl2
CuSO4
MgCl2
X
Na2CO3
K3PO4
NaNO3
X
X
X
X
X
X
X
X
X
Critical Thinking: Analysis and Conclusions
1.
Complete AND balance the double replacement reaction equations
for each combination. Leave blank spaces for the phase symbols.
You will fill them in for Question 2. (Applying concepts)
CuS04(aq) + AgN03(aq)
MgCl2(aq) + AgNO3(aq)
Na2C03(aq) + AgNO3 (aq)
K3P04(aq) + AgNO3 (aq)
+ AgNO3(aq)
CuS04(aq) + NaN03(aq)
MgCl2(aq) + NaNO3(aq)
Na2CO3(aq) + NaNO3(aq)
K3P0 4(aq) + NaNO3(aq)
CuSO4(aq) + K3P04(aq)
MgCl2(aq) + K3PO4(aq)
Na2C03(aq) + K 3P04(aq)
CuS04(aq) + Na2CO3(aq)
NaNO3(aq)
MgCl2(aq) + Na2C03(aq)
CuS04(aq) + MgCl2(aq)
2. Find those equations in Question 1 that have no precipitate in the
products. The products in these equations are salts that must be
soluble. Label each of these salts with (aq), like the reactants that are
soluble. Search for these same soluble salts in the products of the
reactions that did produce precipitates. Where they occur, label them
(aq), and note that the other product must be the precipitate. Label
the precipitates with the symbol (s) for "solid." Refer to the solubility
rules t a b l e for help.
3. Fill in the conclusions for the following list based on your lab results
a. List all metal ions that are not part of any precipitate.
b. List all the negative ions that are not part of any precipitate.
c. List all metal ions that occur only in products that are precipitates.
d. List all metal ions sometimes found in a precipitate.