Progressive Precipitation
Discussion: Solubility or insolubility of a substance varies from compound to compound. If a product of a chemical
reaction is more insoluble than an insoluble reactant, then the reaction will proceed to the right forming the product.
The driving force for these reactions will be the formation of a precipitant that is more insoluble than the reactants. In
this experiment, several increasingly insoluble compounds are prepared in succession to show that the course of each
reaction is determined by the relatively insolubility of one of the products. Evidence for a transformation is in the
contrasting colors of successively precipitate products.
Purpose: to learn about the relative solubility of various compounds and to predict the products formed.
Materials:
6-inch test tube
0.25 M Na2SO4 solution
Rubber stopper
1.0 M AgNO3 solution
0.60 M NaCl solution
0.25 M K2CrO4 solution
Deionized water
Procedure:
Note: quantities of solutions should be within a mL of prescribed volumes.
1. Create a data table, have a column for what is in the test tube, a second for what is added, a third for observations
of reaction, a fourth for what precipitated, and fifth for what is left (the fourth column will then be transferred to
the next row as what is in the test tube).
2. In your test tube pour 5 mL of silver nitrate solution and add 10 mL of sodium sulfate solution. Stopper the test
tube; shake to mix the contents thoroughly. Then let the reaction mixture stand undisturbed for at least two
minutes. While waiting for the settling process to be complete, answer questions 1 and 2.
3. After the settling of the precipitate formed in part 1, carefully decant and discard the supernatant liquid (the liquid
above the precipitate). Retain essentially all of the precipitate in the test tube. Retain essentially all of the
precipitate in the test tube. To this precipitate add 8 mL of potassium chromate solution. Then stopper the test
tube and thoroughly mix the contents. Note any evidence of reaction. Then let the mixture stand undisturbed for
at least two minutes. While waiting for the settling process, answer questions 3 and 4.
4. After the settling of the precipitate in part 2, carefully decant (pour off) and discard the supernatant liquid. Retain
the precipitate in the test tube. Now add 3 mL of deionized water, shake with the precipitate in order to wash it,
and allow the solid to settle. Then decant and discard the wash water. Add 7mL of sodium chloride solution to the
washed precipitant. Again stopper the test tube and thoroughly mix the contents. Note any evidence of reaction
and set the mixture aside to settle as before. While waiting for settling process, answer questions 5,and 6.
5. Clean up your laboratory station. Continue to answer questions.
Questions:
1. State evidence of a reaction observed when solutions of silver nitrate and sodium sulfate were mixed. Consider the
two new possible compounds that could be formed, decide which of these compounds actually appeared as a
precipitate. Defend the decision. Give the formula and the name of the precipitate.
2. As a result of the reaction between sliver nitrate and sodium sulfate, which two ions are most abundant in the
supernatant liquid?
3. State evidence of a reaction between the first precipitate and the solution of potassium chromate. Considering the
two new possible compounds formed. Give the formula and the name of the precipitate.
4. As a result of the reaction just observed, (step 3), which two ions are most abundant in the supernatant liquid? Is
this liquid more or less intensely color than the added potassium chromate solution? What do you think explains
any color change intensity?
5. State evidence of a reaction between the second precipitate and the solution of sodium chloride. As in (1) and (3),
determine which compound appeared as a precipitate. Give the formula and name of the precipitate.
6. Describe the color of the supernatant liquid. How did the solution become colored? (remember that distilled
water was used to wash away unreacted soluble compounds before adding sodium chloride).
7. Write molecular and then net ionic equations for each of the reactions (you should have three of each).
Conclusion:
I have learned ___________ about solubility. I have learned this from ______. I know that the final precipitate in the
series of reactions is _______. My evidence for this conclusion is _______.