Precipitates and Solubility Rules
Introduction
What do geothermal vents have in common with a bathtub ring? The vents spew clouds of mineral-rich
water from deep inside the 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 the 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 Ca2+ 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) à ZnCl2 (aq) + CaSO4 (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 deduct which products are precipitates, and also
discover some common solubility rules.
Problem
What are the precipitates that form from the reactions of salt solutions?
Materials
Chemical splash goggles
Latex gloves
6 micropiptes & wellplate
Marking pen & White paper
Distilled water
0.1 M sodium carbonate (Na2CO3)
0.1 M magnesium chloride (MgCl2)
0.1 M Copper (II) sulfate (CuSO4)
0.1 M sodium nitrate (Na2NO3)
0.1 M Potassium phosphate (K3PO4)
0.1 M Sodium hydroxide (NaOH)
Safety
Wear your goggles at all times during the investigation. Silver nitrate (AgNO3) and Sodium hydroxide (NaOH)
causes stains to skin and clothing. Wear gloves while handling silver nitrate.
Procedure
1.
Put on your goggles. Obtain micropipets of each solution. Mark the white paper with the names of the six
solutions in the manner shown in the Data Table.
2.
Put on your gloves. 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.
3.
Continue the solution combinations (15 total) until each of the solutions have been combined with all the
others. Record the results in the Data Table.
4.
Dispose of any solution containing silver compounds in the waste container located on teacher’s
laboratory desk.
5.
Wash the well plate with soapy water, then rinse with tap water and finally distilled water. Clean up
your work area and wash your hands before leaving the laboratory.
Data
Sheet
AgNO3
NaOH
K3PO4
NH4NO3
CuSO4
MgCl2
Na2CO3
MgCl2
CuSO4
NH4NO3
K3PO4
NaOH
NR= No Reaction (solution mixture remained clear)
PPT= a cloudy precipitate formed
Critical Thinking: Analysis and Conclusions
1.
Complete the double replacement reaction equations for each combination. Leave blank spaces for the
phase symbols (aqueous (aq) and solid (s) ). You will fill them in for Question 2. (Applying concepts)
2.
Find those equations in Question 1 that have no precipitation 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 product precipitates. Where they
occur, label them (aq), and note that the other products must be the precipitate. Label the precipitates with the
symbol (s) for “solid.”
3.
There should still be equations for which no precipitate has been identified. To deduce solubility rules for
use in identifying precipitates in these cases, fill in conclusions for the following list based on your lab results.
(Make predictions)
a.
List all metal ions that are not part of any precipitate. b. List all 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. e.
Use the list of generalities in a-d above to find which salts are the
precipitates in the remaining cases.
Critical Thinking: Applications
1.
Which metal ions of those encountered in this investigation would you expect to find contributing to
precipitates formed on the ocean floor around geothermal vents? Explain your answer.
2.
Soaps often contain sodium stearate. (Stearate is a complex ion derived from fatty acids or fat.) If a
precipitate forms when soap is dissolved in hard water (water containing Ca2+ and Mg2+ ions, what ions from the
soap would you expect to find in the precipitate? Why?