Name: Date: Period:

CHEMISTRY

LAB: Precipitates and Solubility Rules

Introduction

In a general sense, solubility can be thought of as the tendency of a substance (the solute) to dissolve in another substance (solvent).

Ionic compounds dissolve in water by a process known as dissociation. In this process, the crystal lattice of the solid breaks down, and free ions move throughout the solution. The total number of positive charges is equal to the total number of negative charges in an ionic solution.

If aqueous (water) solutions of two different ionic compounds are mixed, one of two things will occur. If all of the ions remain free, “nothing” will happen. That is, the mixture will remain clear, or transparent. However, if two oppositely charged ions are attracted to each other strongly enough, they will bond together to form an ionic compound that is insoluble in water. In such a case, a precipitate forms.

In this investigation, aqueous solutions of several different ionic compounds will be used. Different combinations of solutions will be mixed and the reaction results observed. For those mixtures in which precipitates form, equations will be written for the double replacement (exchange of ions) reactions. You then will be able to state a hypothesis about the tendency of various ions to form stable, or insoluble, salts.

Pre-Lab Discussion

1. How many products are there in a double replacement reaction from which to choose the precipitate?

2. How can you recognize a precipitate when you see one?

3. Why is it necessary to use different micropipets for different solutions?

4. Why is distilled water, and not tap water, used for the solutions?

Focus Question of the Lab

What compounds, from the materials list, do you think are or will form precipitates?

Hypothesis

What is your answer to the focus question? Explain your answer.

Materials

Equipment: goggles, well plates, toothpicks, dropper pipette, distilled water.

Chemicals:

SET A: silver nitrate, barium chloride, sodium carbonate, potassium sulfate, calcium nitrate, sodium phosphate.

SET B: zinc nitrate, iron (III) chloride, sodium hydroxide, sodium iodide, calcium carbonate, sodium acetate.

Safety

Observe all normal safety precautions. Wear safety goggles and protective clothing at all times when working in the lab.

Procedure

1. Obtain two well plates . Mark the well plates with names of the solutions you will be mixing. (See data table). Add three to five drops of barium chloride to the first well in the first row of your well plate. Then add five drops of silver nitrate to this same well. Mix the solutions thoroughly with a clean toothpick. Record your observations in the Data Table. Pay particular attention to the color of any precipitate that forms. It may help to observe the well plate first on top of a piece of white paper.

2. Repeat step one for the next well in your well plate, except this time use silver nitrate and sodium carbonate.

3. Continue this procedure until al possible combinations have been tested. Refer to the data table for the combinations.

4. Discard the materials in the well plates. Wash and dry the well plates using distilled water.

Data/Observations

SOLUTION SET A

	AgNO3	BaCl2	Na2CO3	K2SO4	Ca(NO3)2
BaCl2
Na2CO3
K2SO4
Ca(NO3)2
Na3PO4

SOLUTION SET B

	Zn(NO3)2	FeCl3	NaOH	NaI	CaCO3
FeCl3
NaOH
NaI
CaCO3
NaC2H3O2

Equations: For each precipitate formed in the experiment, write a balanced molecular and net ionic equation for the double replacement reaction that occurred.

SET A

(Example)

• 2Ag NO₃ (aq) + BaCl₂ (aq) → 2AgCl (s) + Ba(NO₃)₂ (aq)
• 2Ag⁺(aq) + 2Cl^-(aq) → 2AgCl(s)

SET B

Critical Thinking: Analysis and Conclusions

1. Formulate a hypothesis about the relative tendency of the following ions to form soluble or insoluble salts within water.

Na⁺, K⁺, NH₄⁺, Ag⁺, NO₃^-, Cl^-, CO₃^2-, PO₄^3-, C₂H₃O₂^-

2. Compare your experimental results and conclusions with the solubility data in the table A-7?

Reflection:

In this section you will summarize the findings of the lab activity (should be closely tied to the purpose statement). Try to relate the data to the theories behind the results. You must also evaluate the information you gathered. What is the error (or errors) in your data? Are there any anomalies (unexpected results) and if so why are they there? Do you accidentally spill part of a reactant altering the volume needed? Did you wait too long during part of the lab procedure? It is not wrong to make mistakes during a lab procedure. However, it is wrong to falsify or misrepresent results!