Name_______________________________
Block_____________________
PRECIPITATION REACTION LAB
Introduction:
The majority of ionic solids like salts (KCl and NaCl) are soluble in water because the
polar water molecules surround the individual ions of the salt. Those that do not dissolve
and go into solution form solid products called precipitates. These precipitates have
many colors and often help scientists identify what the precipitate is present. In
meteorology the term precipitation refers to meteorological phenomena such as rain or
snow. Precipitation is chemical solutions occurs when two chemicals react to form a
product that is insoluble in water and falls out of solution like rain or snow. A precipitate
is a solid substance that separates from solution during a chemical reaction. A precipitate
can be identified by the cloudy, milky, gelatinous, or grainy appearance it gives to the
mixture.
A barium sulfate precipitate can be produced by the reaction of barium chloride and
sodium sulfate. A chemical equation to describe the reaction is written and balanced like
this:
BaCl2 (aq) + Na2SO4 (aq)  2 NaCl (aq) + BaSO4(s)
Barium sulfate, BaSO4 (s), is a common precipitate used as an X-ray contrast medium
because it is insoluble in water an opaque to x-rays. Typically a patient drinks an aqueous
slurry of barium sulfate just before he is x-rayed. The precipitate coats his stomach and
intestines. These organs then show up on the x-ray film in vivid contrast, aiding the
doctor’s diagnosis. Notice the reaction that forms BaSO4 is a double-replacement reaction
in which the cations and anions of
the reactants trade partners to form the products.
To write a total ionic equation, rewrite all aqueous substances as their component ions
and keep all solid
substances unchanged.
Ba+2(aq) + 2Cl- (aq) + 2Na+(aq) + SO4-2(aq)  2Na+(aq) + 2Cl-(aq) +
BaSO4(s)
Cancel all components of the reaction that are identical on both the reactant and product
side of the reaction. These are called spectator ions. Write the net ionic equation by
rewriting the reaction equation including only those reactants and products
that change in the reaction.
Ba+2(aq) + SO4-2(aq)  BaSO4(s)
Always check to see that your net ionic equation is balanced.
When doing these reactions involving ionic compounds remember that they are made of
positive and negative ions held together by the attractive, electrostatic forces that occur
between oppositely charged particles. In water, soluble ionic compounds break apart
completely into their respective ions. Example: NaCl (s) when put into water yields Na+
Ag+(aq)
(aq) and Cl (aq), and AgNO3 also dissociates in water to form these respective ions
and N03-(aq). It turns out that when these two solutions of sodium chloride and silver
nitrate are mixed a solid falls out and precipitation occurs. The new mixture still contains
Na+(aq) and N03- (aq), and the newly formed precipitate is AgCl (s). How do we know that
AgCl forms a solid precipitate? Remember to reference the solubility pyramid I use that
helps you decide what will and won’t go into solution (fig 1) .
Fig 1
In this experiment, you will mix 7 different ionic solutions in all possible combinations
to determine which combinations result in precipitate formation. Based on your results,
you will write complete ionic equations and net ionic equations for each reaction that has
takes place. Use your solubility chart to determine which compounds yield
precipitates. While the lab focuses on the solubility of different compounds, my purpose
in having you do this activity is to give you more practice looking at chemical reactions,
writing formulas, naming inorganic compounds, and describing chemical changes with
equations.
Procedure: In this experiment, you will mix 7 different ionic solutions in a well plate
in all 49 possible combinations to determine which combinations result in precipitate
formation. Based on your results, you will write complete and net ionic equations for
each reaction that creates a precipitate. Use the solubility chart that is on the wall and
represented by Fig 1 above to compare your lab results with the expected results. You
will be combining solutions containing the dissolved ionic substances below. In each test,
you will need to add2 drops (Don’t make a Mess) of the first solution to a well
plate. Then add 2 drops (Don’t make a Mess) of the second solution a drop at a
time. When/if you see the formation of a precipitate, you have witnessed a reaction and
will need to write a balanced chemical equation for that reaction. You will be required to
(a) describe the precipitate (b) find the correct formula for the ionic products, (c) write
and balance the entire equation for each reaction you witness and (d) write and balance
the net ionic equation.
Chart out your chemical tests in an organized table similar to the fig 2 and then write the
reactions underneath. The table the written reactions, and answers to the two essay
questions constitute your lab report.
Solutions you will have available:
sodium sulfate (Na2SO4)
sodium chloride (NaCl)
silver nitrate (AgNO3)
sodium iodide (NaI)
lead nitrate (Pb(NO3)2
ammonium carbonate (NH4)2CO3
calcium chloride (CaCl2)
Presenting your data: Make a table which has columns and rows headed by the
formulas of each of the solutions to be tested. As you mix chemicals, you will report the
results from each of those combinations in the “box” on the table which lies at the
intersection of the column and row labeled with those two substances. If a precipitate
forms, describe its physical characteristics in the box.
Na2SO4 NaCl AgNO3 NaI Pb(NO3)2 (NH4)2CO3 CaCl2
Na2SO4
NaCl
AgNO3
NaI
Pb(NO3)2
(NH4)2CO3
CaCl2
Fig 2
Make sure you include in your lab a description of any precipitates and the chemical
compounds that make up the reactants, products, the full ionic equation and the net ionic
equation with (aq) and (s) designations
Write up!
Submit a full lab and include the graph of the chemical reactions as per fig 2, and include
BOTH word equations (with names of reactant and product compounds written out) and
chemical formula equations for every reaction which produced a precipitate. If no
precipitate formed, there was no chemical change (no reaction) and you write NR (No
reaction) in the grid space. If a precipitate forms, determine what compound is the
precipitate (and what ions are still floating around in solution). I will scrutinize your
equations to see that all formulas and equations are properly written, and that the
equation syntax (ie (aq) (s) is correct. If you have questions about compound or formula
names, ask before you leave the lab for the day. Print out your report and turn it in next
class and make sure you include a conclusion paragraph that includes a personal
reflection about precipitates. Also make sure you answer the post lab questions in your
conclusion. I do not want to see these questions answered 1, 2, 3 , 4 but want to see them
answered along with other thoughts and observations in the body of the conclusion.
Post Lab questions
1) What conclusions can be drawn from your observations? List any similarities
and differences in the physical behavior (ie color, texture) of the precipitates as
compared to their formula.
2) What is a precipitate?
3) Some heavy ions that precipitate out of solutions are used for specific purposes
because of this insolubility. Barium sulfate is an example. It is used as a darkening
compound for x-rays, since it shows as a dark outlined area that might help locate
abnormalities in the upper and lower gastrointestinal tracts. Explain how silver’s ability
to make a precipitate when activated by light is used in our every day life.
4) Make sure you include your one paragraph personal reflection about
precipitates in your conclusion.