Introduction: (Initial Observation)
When salts dissolve in water, they become ionized. In other words they break down to the ions that formed them. For example a solution of common salt contains sodium and chloride ions.
Many organic and inorganic chemicals are made by a chemical reaction involving two or more aqueous ionic solutions. Resulting products may precipitate or be separated by crystallization.
By understanding the mechanism of the reaction in ionic solutions, we can predict the direction of reactions, resulting products, and produce new products.
Warning: Goggles must be worn throughout this experiment. This project involves working with hazardous chemicals.
Find out about ionic solutions and activity of ions. Read books, magazines or ask professionals who might know in order to learn about the direction of reaction when you mix two ionic solution. Keep track of where you got your information from.
Following are samples of information that you may find:
When two salts are mixed together, different ions may react and result new chemical products. This especially happens when one of the possible combinations are water insoluble and separate in the form of gas or solids. Knowing how do the ions react with each other can help us predict and control the reactions in a direction that is beneficial for us.
Before performing the experiment, you should be aware of the nature of ionic compounds, or, at least, know that many substances form ions in solutions. Many ionic compounds have limited solubility so they may precipitate in a saturated solution. The following equations describe the reactions that form precipitates:
- Fe2+(aq) + 2 OH-(aq) —-> Fe(OH)2 (s)
- Zn2+(aq) + 2 OH-(aq) —-> Zn(OH)2 (s)
- Mg2+(aq) + 2 OH-(aq) —-> Mg(OH)2 (s)
- Fe2+(aq) + CO32-(aq) —-> FeCO3 (s)
- Mg2+(aq) + CO32-(aq) —-> MgCO3 (s)
- 3 Zn2+(aq) + 2 PO43-(aq) —-> Zn3(PO4)2 (s)
- 3 Fe2+(aq) + 2 PO43-(aq) —-> Fe3(PO4)2 (s)
- 3 Mg2+(aq) + 2 PO43-(aq) —-> Mg3(PO4)2 (s)
You should write the overall and the net ionic equations. The distinction between reacting species and spectator ions should be understood. The experiment can be used as an introduction to solubility rules.
(aq) = is dissolved in water.
(s) = is a solid – insoluble in the water solution.
(cr) = is in crystal form – insoluble in the water solution.
(g) = is a gas.
(l) = is a liquid.
spectator ions = Ions in a solution that do not participate in a chemical reaction.
Strong acids and strong bases by definition ionize completely when dissolved in water. For example,
HCl + H2O —–> H3O+1 + Cl-1
NaOH ——-> Na+1 + OH-1
There is no back reaction. This implies that chloride ions and sodium ions are poor conjugate bases and acids, respectively. In fact, we make the blanket assumption that the anions of strong acids, and the cations of strong bases have no tendency to react with water, and are called spectator ions.
This activity is designed to allow students to observe the formation of precipitates in aqueous reactions. It also demonstrates the fact that certain combinations of ions do not form precipitates but remain in solution as spectator ions. Practice in the writing of ionic and net ionic equations is also provided.
The purpose of this project is to test the reaction of ionic solutions of some consumer chemicals for formation of precipitates.
Which combination of ionic solutions (of consumer chemicals) forms a new product in the form of a solid precipitate?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Independent variables (also known as manipulated variables) are the chemicals (or pair of chemicals) that we choose to test their reaction in an ionic solution.
Dependent variable (also known as responding variable) are the solubility and color of the resulting product.
Controlled variables are the environmental conditions including light and temperature, experiment method and concentration of solutions.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Construct your hypothesis in the form of one or more statements or in the form of a table similar to the results table. Your hypothesis must show which combination of ionic solutions you think will result precipitates. And why do you think so. Following is a sample hypothesis:
Carbonates and hydroxides of alkaline and earth alkaline metals are water soluble, while the carbonate and hydroxide of all other metals are water insoluble and will precipitate.
Sulfates, nitrates and chlorides of all metals will be water soluble.
My hypothesis is based on my previous observation during my past experiments.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Introduction: In this experiment you will gather varieties of consumer chemicals* and make strong solution of them in water. (Dissolve one tablespoon of solute in water and diluting to 100 mL). You will then test the effects of such solutions on each other as described below.
* See the list of chemicals and their sources in the material section.
Procedure: Mix solutions that you have prepared, two at a time, to determine which combinations form precipitates. Knowledge of which ions are present makes it possible to reduce which of the possible ion combinations are responsible for the formation of the precipitates.
Test each possible pair of solutions by combining 1 to 2 drops of each member of the pair on a sheet of plastic wrap.
Record observations, noting formation and color of any precipitate. A data table similar to the one below provides an efficient means of recording and comparing results.
Use the data obtained to write complete and/or net ionic equations for those combinations that result in a precipitate.
All solutions may be flushed down the sink with plenty of water. Since you use only a very small quantity of each solution, you can dispose of your samples by folding up the plastic wrap and placing it in with the solid waste.
Hydrochloric acid is a corrosive liquid. Keep away from skin and eyes. Avoid breathing vapors. When reacting the penny with this acid (as suggested in the material section), perform the procedure under a hood. If spilled, use baking soda to neutralize acid before cleaning up. Sodium hydroxide and trisodium phosphate are very caustic; avoid contact with skin. If spilled, immediately flush the area with water and clean up. Goggles must be worn throughout this experiment.
Materials and Equipment:
- Iron(II) sulfate
- sodium hydroxide
- sodium chloride
- magnesium sulfate
- sodium phosphate
- hydrochloric acid (dilute 500 mL concentrated HCl solution to 1.00 L with distilled or deionized water)
- distilled or deionized water
Note: All salt solutions can be made by dissolving one tablespoon of solute in water and diluting to 100 mL
- dropper bottles, small test tubes or small bottles
- eye droppers or disposable pipettes
- plastic wrap
- funnel and support
- filter paper
- 250-mL beakers
- 100-mL graduated cylinder
Consumer sources of all chemicals used in this experiment are available.
- Iron(II) sulfate is available at garden supply centers. Sodium hydroxide is available as lye, sodium chloride as table salt, and sodium carbonate as washing soda at grocery stores. Magnesium sulfate is available as Epsom salts at grocery or drug stores. Sodium phosphate is available as TSP at grocery or hardware stores.
- Hydrochloric acid is available at hardware stores as muriatic acid, 28% HCl.
- To obtain a stock solution of ZnCl2, two pennies (post-1983), having a zinc core are scratched to expose the core in several places, and are immersed in 10 mL of muriatic acid and left to react overnight. The resulting solution is diluted to 200 mL. Excess acid in this dilution may be minimized by adding small amount of sodium carbonate (enough to minimize the release of CO2 gas).
- The Fe+2 ion can be obtained from an FeSo4 solution. This solution is made by adding 5 #2d common nails to 50 mL of 10% sulfuric acid. After being allowed to react overnight, the solution is filtered, diluted to 100 mL and stored with a clean iron nail. The solution should be neutralized with Na2CO3 until almost neutral.
- For filtration purposes, Whatman #1 paper is recommended. However, several thicknesses of paper towel or coffee filter in a strainer clear the solution adequately for this experiment.
- Plastic straws may be used in place of eye droppers or disposable pipettes.
- Glass plates, spot plates, or transparencies may be substituted for the plastic wrap.
- Styrofoam cups may be used in place of beakers.
- A measuring cup may be substituted for the graduated cylinder.
- One of the benefits of using consumer products is the familiarity you may have with them. It is also possible that they contain some impurities. Read the labels that are on these products. The original containers may be used as a part of your display so that the labels can be read by the students for additional emphasis on the need for caution.
- Solutions can be prepared in advance.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
No calculation is required for this project.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
- During the formation of precipitates, some irregularities may be observed.
- Fe2+, usually green, may rapidly oxidize to orange Fe3+ therefore forming precipitates which obviously are different.
- The Zn+2 ion solution is fairly acidic since it was prepared with HCl. A very dilute solution is used to insure formation of the Zn(OH)2 precipitate.
Merrill, P., Parry, R.W., Tellefsen, R.L., and Bassow, H., Chemistry: Experimental Foundations, Laboratory Manual, Prentice Hall, Inc, Engelwood Cliffs, NJ, 1982, p. 50. This work contains an activity similar to that described here, but uses different chemicals and equipment.