EXPERIMENT # 8
DOUBLE DISPLACEMENT AND GAS FORMING REACTIONS -
IDENTIFYING SIX IONIC SOLUTIONS

Question
Can the solutions of ionic compounds be identified based on  the results of their reactions with other solutions ?

Background Information
Mixing a solution of an ionic compound with another solution can bring  valuable information through the observation of the results of the occurring reaction. This results may  include: precipitate formation,  its color, amount,  and texture, gas evolution, heat absorption or evolution, or simply no change (no reaction).
Some of the reactions are characteristic for certain ions only, thus facilitating the identification of the cation and/or  the anion in an unknown solution.  Moreover,   the information on the solubility in water of a certain ionic compound may be obtained by mixing two solutions,  one containing the cation of the compound in question and the other the anion. The formation of a precipitate as a result of a double displacement reaction will indicate that the compound in question is indeed insoluble.

About this experiment
The experiment consists of two distinct  parts:
In part I  you will study the interactions (reactions)  of six aqueous solutions of known ionic compounds by systematically  mixing them in pairs (total of 15 different pairs). You will  record as much of the information about the occurring reaction as possible (precipitate formed, color and texture of the ppt, color of the solution before and after the reaction, gas evolution and its smell, etc.) in a data table.
In part II, you will  study  a set of six unknowns which are  the same solutions as the knowns, but labeled with a letter code instead of the chemical name. You will mix the unknown  solutions in pairs and record the results in a separate  data table.
You may use abbreviations, such as  wppt for white precipitate, NR for no reaction, etc. The observations have to be  meticulously recorded. Some acceptable abbreviations used to record your observations are: ppt for precipitate, wppt for white precipitate or sln for solution. Make sure that you are able to understand your abbreviations when later analyzing the data collected!
Your goal is to identify all six unknown solutions, i.e. assign a chemical formula to each  unknown code (ex. A = NaCl).
The  identification is based on the comparison of the information collected in the “unknown” part of the experiment with  the data from the “known” part of the experiment.
Example
Table 1. Known Solutions                                                          Table 2. Unknown Solutions
Na2CO3 Na2SO4 HCl AgNO3 A B C D
KI NR NR NR yellow ppt, small particle E cream ppt
curd-like
NR bubbles, no color or odor NR
AgNO3 cream ppt
curd-like, heavy
wppt, light, a little, milky sln wppt, curd-like, later purple D wppt, light,
small amount
NR NR
HCl bubbles, odorless, colorless NR C wppt, curd-like;
purple after some time
NR
Na2SO4 NR B yellow ppt
small particle
Refer to Table 1 for an example of a data table for mixing 5 known solutions  in pairs and to Table 2 for the unknown solutions.
In this case you could confirm the identity of the solution A as AgNO3  (ibeacuse it produced the largest number of precipitates) . The yellow precipitate is formed when mixing AgNO3 and  KI as well as a result of mixing  solution B and solution A. A is silver nitrate, therefore, B is KI.
The similar deduction process (show complete process for all solutions in the report) leads you to the identification of solution C as HCl and solution E as Na2CO3. By elimination and comparison the texture and color of the formed ppt (show it ) with the known results D could be  identified as Na2SO4 .

PROCEDURE
Solutions studied: AgNO3, Na2CO3, HCl, NaOH, Na3PO4, and FeCl3
SAFETY
Hydrochloric acid - corrosive and toxic; silver nitrate- highly toxic and oxidant

GOOD ADVICE
Some of the reactions  will occur very  fast and some more slowly. Make sure that you observe the reaction from the start and  for about 1-2 minutes, then record all observations.  The gas evolution  is immediate.
Make sure that your recorded observations include types and textures of precipitates formed, not only the color (e.g. off-white ppt, cottage cheese-like curds, heavy, light, etc.). Describe  how fast the gases  are formed and their colors.
ALWAYS stir the mixed  pair of  solutions  with a  provided plastic stirrer.  After each use, rinse the stirrer using a wash bottle to squirt water onto it, and into the beaker; do not dip it into the water in a beaker. Dry the stirre using a paper towel.

PART I: KNOWN SOLUTIONS
1. Label six small test tubes to reflect the names of six known solutions.

2. Obtain about 1 ml of each solution in the labeled test tubes.

3. Obtain six plastic transfer pipets and label each. Place each labeled pipet in the test tube with the corresponding solution. As long as you are working with one solution at a time and replace the pipet in the test tube, you will not  need to label the pipets.

Steps 1-3: Alternatively, obtain six small scale plastic pipets already labeled  with the names of the solutions. Fill them and use  to deliver the solutions for the reactions. You will need to clean the pipets after the lab !

4. Obtain a  spot plate and label five wells with the formula of the first known solution on your data sheet.
You only need five wells (you will not be adding the solution that already is in the well to itself, e.g. you won’t be mixing NaCl with NaCl). You may also  use a  sheet  of plain  paper underneath the transparent glass plate to label.

5. Using the designated pipet, transfer about 3 drops of this first known  solutions on your data sheet  into each of  the five labeled wells.

6. Using a designated pipet, add 3 drops of the second solution from your data sheet to the first well.  Record your observations.

7.  Add  the third solution on your data sheet to the next  available well with the first solution. Record the observations. Repeat, using consecutive solutions on your data sheet, until all 5 wells containing the first solution have been used.

8. In the same manner, continue mixing the solutions until you have data for 15 different pairs of solutions.
You may need  to use two spot plates or, if  necessary, clean one spot plate between the several sets of reactions and relabel.

PART II: UNKNOWN SOLUTIONS
Obtain new set of six pipets. Repeat the steps from the KNOWN part but use the uknown solutions coded with  letters: A through F. Record all observations.

CLEANUP
When you are done, rinse all 12 pipets in the following manner:
1) Expel any remaining solution from the pipet into the waste container.
2) Pour distilled water to three small test tubes.
3) Draw some water from the first test tube with a pipet to be rinsed, invert it  and discard the water to the waste container.
4) Draw some water from the second test tube and discard, repeat with the third.
5) Repeat the process for all pipets, always placing the dirty pipet in the first test tube, then proceeding to the second and third.  Wash the plate(s) and the test tubes.  Return the plates and the  pipets to the instructor.

REPORT
Report should  include:
1.  The original  tables with observations for knowns and unknowns
2. The balanced net ionic equations for for all pairs of known solutions that produced precipitates  or gases.
Indicate a ppt on the product side with a (s) and the gas with a (g), following  the formula, e.g.
     Cl- (aq) + Ag+ (aq)  --->  AgCl (s)
Remember that :
- cloudiness, milkiness, or  turbidity is  observed after mixing two solutions  means that a  precipitate was formed
- bubbles  mean that a  gas  was formed.
3.  Discussion: reasoning  and experimental basis for the  identification of each unknown; this means: step-wise,  logical analysis of the information that lead  you to the identity of the unknowns.
4. The conclusion including the identity of each unknown ( e.g. The solution A was identified as.......)

PRELAB
POSTLAB
1. a. Based on the experience gained from the experiment performed think of  two solutions that you could you mix together to determine whether mercury (I) chromate (Hg2CrO4) is water soluble?  Explain  your choice using words, not equations.
 
 
 
 

b.  Assuming that Hg2CrO4 is not  soluble write  the net ionic equation for the reaction in (a).