LAB: 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 formation 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 undergone 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 Ca 2+ ions and SO42- 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 investigation.
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.
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.
·
l
3. Continue the solution combinations (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
X
MgCl2
X
X
X
X
X
X
X
X
Na2CO3
K3PO4
NaNO3
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)
a. CuS04(aq) + AgN03(aq)
b. MgCl2(aq) + AgNO3(aq)
c. Na2C03(aq) + AgNO3 (aq)
d. K3P04 (aq) + AgNO3 (aq)
e. NaNO3(aq)
+ AgNO3(aq)
f. CuS04(aq) + NaN03(aq)
g. MgCl2(aq) + NaNO3(aq)
h. Na2CO3(aq) + NaNO3(aq)
+ NaNO3(aq)
j. CuSO4(aq) + K3P04(aq)
k. MgCl2(aq) + K3PO4(aq)
l. Na2C03(aq) + K 3P04(aq)
i. K3P0 4(aq)
m. CuS04 (aq) + Na2CO3(aq)
n. MgCl2(aq) + Na2C03(aq)
o. 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.