Chemistry : Class XII 12th Chemistry Projects for 2009 Exams
Saturated Solutions: Measuring Solubility

Objective

The goal of this project is to measure the solubilities of some common chemicals: table salt (NaCl), Epsom salts (MgSO₄), and sugar (sucrose, C₁₂H₂₂O₁₁).

Introduction

A good part of the substances we deal with in daily life, such as milk, gasoline, shampoo, wood, steel and air are mixtures. When the mixture is homogenous, that is to say, when its components are intermingled evenly, it is called a solution. There are various types of solutions, and these can be categorized by state (gas, liquid, or solid). The chart below gives some examples of solutions in different states. Many essential chemical reactions and natural processes occur in liquid solutions, particularly those containing water (aqueous solutions) because so many things dissolve in water. In fact, water is sometimes referred to as the universal solvent. The electrical charges in water molecules help dissolve different kinds of substances. Solutions form when the force of attraction between solute and solvent is greater than the force of attraction between the particles in the solute. Two examples of such important processes are the uptake of nutrients by plants, and the chemical weathering of minerals. Chemical weathering begins to take place when carbon dioxide in the air dissolves in rainwater. A solution called carbonic acid is formed. The process is then completed as the acidic water seeps into rocks and dissolves underground limestone deposits. Sometimes, the dissolving of soluble minerals in rocks can even lead to the formation of caves.

If one takes a moment to consider aqueous solutions, one quickly observes that they exhibit many interesting properties. For example, the tap water in your kitchen sink does not freeze at exactly 0°C. This is because tap water is not pure water; it contains dissolved solutes. Some tap water, commonly known as hard water, contains mineral solutes such as calcium carbonate, magnesium sulfate, calcium chloride, and iron sulfate. Another interesting solution property is exhibited with salt and ice. Have you ever had the chore of throwing salt on an icy sidewalk? When the ice begins to melt, the salt dissolves in the water and forms salt water. What happens to the freezing point of water when salt is added to it? Even some organisms have evolved to survive freezing water temperatures with natural "antifreeze." Certain artic fish have blood containing a high concentration of a specific protein. This protein behaves like a solute in a solution and lowers the freezing point of the blood. Going to the other end of the spectrum, one can also observe that the boiling point of a solution is affected by the addition of a solute. Do eggs cook faster or slower when salt is added to the pot of water? These two properties, namely freezing-point depression and boiling-point elevation, are called colligative properties (properties that depend on the number of molecules, but not on their chemical nature). Exploring these properties and others of aqueous solutions are just some of the many ways that you could expand the scope of this project.

Finally, if you enjoy learning about solutions or other areas of chemistry, consider a career in the physical sciences. One example is working as an analytical chemist. Such chemists analyze the chemical composition of substances. They conduct many experiments to identify special characteristics of substances for a wide variety of reasons. Perhaps they are charged with testing municipal drinking water for its purity, or perhaps they must test a forensic sample for evidence in a trial. Whatever the reason, it is challenging work that requires precision and creative thought.

In this project you will measure the aqueous solubility of some common household chemicals: table salt (NaCl), Epsom salts (MgSO₄), and sugar (sucrose, C₁₂H₂₂O₁₁). How much of each chemical can dissolve in a given volume of water?

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Solution

• Solute

• Solvent

• Soluble vs. insoluble

• Chemical structure of water

• Polar molecule

• Force of attraction

• Concentration of a solution

• Dilute vs. concentrated

• Sodium chloride (NaCl)

• Magnesium sulfate (MgSO₄)

• Sucrose (C₁₂H₂₂O₁₁)

Questions

• What is a saturated solution?

• What is the difference between a saturated solution and an unsaturated solution?

Bibliography

• Any basic physical science text will have a chapter on solutions or solubility. Begin by reading a chapter on basic solution chemistry, such as Chapter 4 in:
  Haber-Schaim, U., R. Cutting, and H. G. Kirksey, 1999. Introductory Physical Science, seventh edition, Belmont, MA: Science Curriculum, Inc. (ISBN: 1-882057-18-X).
  

• The Chem4Kids website is a good reference for basic chemistry concepts. Here is a link to their webpage on solutions:
  Andrew Rader Studios, 1997–2007. "Chem4Kids.com: Matter: Solutions," Chem4Kids.com [accessed October 3, 2007] http://www.chem4kids.com/files/matter_solution.html.
  

• This website has a collection of demonstration videos illustrating various properties of solutions:
  Maynard, J.H., 1998–2000. "General Chemistry Demonstrations: Properties of Solutions," University of Wisconsin-Madison, Chemistry Department, Demonstration Lab [accessed October 3, 2007]
  http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/11solutionspage/solutionsmain.htm.
  

• The experimental procedure is based on:
  

  • Gardner, R., 1999. Science Projects About Kitchen Chemistry. Berkeley Heights, NJ: Enslow Publishers.

  • Stretton, T., 2004. "Solubility of Compounds," Tom Stretton's Chemistry Pages, Upper Canada District School Board [accessed October 3, 2007] http://www.ucdsb.on.ca/tiss/stretton/CHEM2/compound_solubilities.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Distilled water

• Metric liquid measuring cup (or graduated cylinder)

• Three clean glass jars or beakers

• Non-iodized table salt (NaCl)

• Epsom salts (MgSO₄)

• Sugar (sucrose, C₁₂H₂₂O₁₁)

• Disposable plastic spoons

• Thermometer

• Three shallow plates or saucers

• Oven

• Electronic kitchen balance (accurate to 0.1 g)

Experimental Procedure

Do your background research so that you are familiar with the terms, concepts, and questions, above.

Determining Solubility: Method 1

1. Measure 100 mL of distilled water and pour into a clean, empty beaker or jar.

2. Use the kitchen balance to weigh out the suggested amount (see below) of the solute to be tested.

   1. 50 g Non-iodized table salt (NaCl)

   2. 50 g Epsom salts (MgSO₄)

   3. 250 g Sugar (sucrose, C₁₂H₂₂O₁₁)

3. Add a small amount of the solute to the water and stir with a clean disposable spoon until dissolved.

4. Repeat this process, always adding a small amount until the solute will no longer dissolve.

5. Weigh the amount of solute remaining to determine how much was added to the solution. Save your saturated solutions for the second method.

Determining Solubility: Method 2

1. Label the underside of each saucer with tape, one for each solution.

2. Weigh the empty saucer and record the weight.

3. Pour in 10–15 mL of the appropriate saturated solution (corresponding to the label on the saucer).

4. Weigh the saucer + solution and record the weight.

5. Repeat steps 2–4 for each of the three solutions.

6. Put the saucers in a warm place (e.g., an oven on low heat) and allow the water to evaporate.

7. Re-weigh the saucers + dry crystals.

   1. Tip: make sure all the water has evaporated by weighing each saucer several times, with an interval back in the oven in between, to make sure the weight is no longer changing.

Analyzing Your Results

1. To make sure that your results are reproducible, you should repeat your solubility experiment at least three separate times for each chemical.

2. For each solubility determined by Method 1, you will have the original volume of water, the total mass of the solute, and the remaining mass of the solute. You can calculate how much of the solute was dissolved.

3. For each solubility determination by Method 2, you will have the mass of the dry solid after evaporation, and the mass of the original solution. You can calculate the mass of the water that evaporated.

4. Calculate the average solubility, in grams of solute per 100 mL of water, as determined by each method.

5. More advanced students should also calculate the standard deviation of the solubility, as determined by each method.

6. Compare the results of the two methods.

7. Compare your results to published solubilities for the three chemicals.

Variations

• Let's say that instead of starting with pure water, you tried to dissolve Epsom salts (MgSO₄) in a saturated solution of NaCl. Do you think this would work? How much MgSO₄ would you expect to dissolve? Would it be more, less or the same amount as in an equal volume of distilled water? Design an experiment to find out.

• You could also try the experiment above with the other five pair-wise combinations of the three chemicals.

• Another variation you could try is an experiment on the how fast solutes dissolve. What can you do to increase the rate at which a solute dissolves in a solvent? How much more quickly does the solute dissolve, compared to when the solute is simply added to the solvent?