Can the solutions of ionic compounds be identified based on the results of their reactions with other solutions ?
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.
Table 1. Known Solutions Table 2. Unknown Solutions
|KI||NR||NR||NR||yellow ppt, small particle||E||cream ppt
|NR||bubbles, no color or odor||NR|
|wppt, light, a little, milky sln||wppt, curd-like, later purple||D||wppt, light,
|HCl||bubbles, odorless, colorless||NR||C||wppt, curd-like;
purple after some time
Solutions studied: AgNO3, Na2CO3, HCl, NaOH, Na3PO4, and FeCl3
Hydrochloric acid - corrosive and toxic; silver nitrate- highly toxic and oxidant
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.
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 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.......)
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).