Materials Included In Kit

Additional Materials Required

Safety Precautions

The materials in this lab activity are considered relatively non-hazardous. Observe all normal laboratory safety procedures. Wear chemical splash goggles whenever chemicals, glassware or heat are used in the chemistry laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The iron filings can be recovered and recycled for future use. Salt, stearic acid, and sand can be discarded for solid waste disposal in a landfill according to Flinn Suggested Disposal Method #26a. All solutions can be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b. 

Lab Hints

• Set up a filtration apparatus and demonstrate the filtration setup and procedure for the students during the prelab.
• An overview of the filtration technique is included in the Supplemental Information section of the teacher PDF.
• This experiment is designed as a beginning inquiry- or discovery-based lab activity. The results from Part A should provide all of the practical experience the students need in order to successfully complete Part B. To ensure a safe lab environment, however, we recommend that the teacher provide some process to check the students’ proposed procedures for Part B.
• Depending on the learning styles and ability levels of your students, you may decide to give them more or less direction in writing the flow chart and designing a procedure for the separation process in Part B. Beginning-level students may benefit from working on this lab in two 50-minute classroom periods, during which the teacher can moderate a cooperative class discussion to complete the flow chart. For more advanced students, the amount of lab time may be similar, but the extent of teacher direction can be reduced. Students of all ability levels will prosper if a collaborative approach is used to encourage them to work together and to learn from each other.
• Is the experiment too similar to exercises provided in textbook or classroom discussions? Add some spice to the experiment, literally. Include a fifth component, such as poppy seeds, that will float in water.

Teacher Tips

• Separation of a mixture is not an abstract topic. One of the most vital applications of separation science is the process of removing salt from seawater to obtain drinking water. More than $25 billion has been invested worldwide in “desalination” technology over the past quarter century, and that amount is expected to increase dramatically. The majority of the world’s desalination plants are in the Middle East, where freshwater sources are scarce. In the United States, many desalination plants are located in California.

Answers to Prelab Questions

1. Think about the separation process outlined in Figure 1 and complete the flow chart to reveal the identities of the solid, liquid and vapor components.
   {13535_Answers_Figure_2}

Sample Data

Data Table A. Physical Properties of Substances

1. Sample Flow Chart—Part B. One Possible Procedure
   {13535_Data_Figure_3}
2. Mixture Code ___B___
   {13535_Data_Table_2}

Answers to Questions

1. The chemical formulas of iron, salt and stearic acid are Fe, NaCl and C₁₈H₃₆O₂, respectively. Are these substances elements or compounds?

   NaCl and C₁₈H₃₆O₂ are compounds. Fe is an element.

2. Are any of the substances in Part A magnetic? Is magnetism a physical or chemical property? Explain.

   Only one of the substances, iron filings, was attracted to the magnet. When the magnet was passed over the iron sample, the iron became attached to the plastic bag holding the magnet. The other solids did not move at all when a magnet was passed over them. Magnetism is a physical property—it does not change the composition of the sample.

3. Which substance(s) in Part A dissolved in water? Is solubility a physical or chemical property? Explain.

   Only one of the substances, salt, dissolved in water. The solid quickly disappeared and a colorless, clear solution was obtained. When water was added to iron, the iron remained as a separate layer of solid at the bottom of the test tube. When water was added to sand, the sand particles became suspended in the liquid. Most of the sand remained at the bottom of the test tube. Solubility is a physical property.

4. Which substance(s) in Part A have low melting points? Is melting point a physical or chemical property?

   Stearic acid has a low melting point. Melting point is a physical property.

5. Is the combination of salt and sand obtained in Part B, step 8, a new compound or a mixture? Explain.

   When salt and sand are mixed, there is no evidence of any kind of change. The solids retained their individual appearance and physical properties. This suggests that mixing produces a simple, “physical” mixture, not a new compound.

6. Describe the results of the filtration experiment in Part A. Which substance remained on the filter paper after filtration? Is the filtrate (the liquid that passed through the funnel) a pure substance? Explain.

   The sand particles remained on the filter paper after filtration. The salt remained dissolved in the filtrate. The filtrate consists of a homogeneous mixture of salt dissolved in water—it is a solution, not a pure substance.

7. Based on your observations in Part C, what are the components of the mixture?

   The particles removed in step 1 are iron. The solids filtered out in step 2 can only be either sand or stearic acid or both. Salt is soluble in water and would be carried away in the filtrate. In step 3, the melted solid can only be stearic acid and the solid particles are sand. Since only salt dissolves in water, the white crystals remaining in step 4 after evaporation are salt.

Teacher Handouts

13535_Teacher1.pdf

References

This experiment was adapted from Flinn ChemTopic™ Labs, Introduction to Chemistry, Volume 1, Cesa, I., Editor, Flinn Scientific, Batavia, IL, 2002.

Introduction

Most of the substances that we come in contact with every day—from the air we breathe to the water we drink and the foods we eat—are mixtures. How can the components of a mixture be separated?

Concepts

• Mixture vs. pure substance
• Physical changes
• Physical properties

Background

A mixture is a combination of two or more pure substances that retain their separate chemical identities and properties. Since the amounts of each substance making up a mixture can be changed, the physical properties of a mixture depend on its composition. In contrast, the composition of a pure substance is constant, and thus pure substances have characteristic physical properties that do not change. Examples of physical properties that can be used to describe pure substances include solubility, conductivity, magnetism, density, boiling point and melting point.

By taking advantage of the unique physical properties of individual components within a mixture, it should be possible to separate a mixture into its components. For example, if one component in a mixture of two solids dissolves in water, while a second component does not, the components can be separated by adding water to the mixture and then filtering the residue. Subjecting the mixture to a physical change in this way would change the ratio of components in the mixture. This leads to one of the definitions of a mixture—a substance whose composition can be altered by a physical change. Physical changes that can be used to separate the components of a mixture include filtration, evaporation, crystallization and distillation.

A flow chart is often used to illustrate the steps involved in the separation of a mixture. In a flow chart, the substances in the mixture are listed inside boxes that are connected by arrows. The actual physical steps that must be carried out to separate the components are listed next to the arrows. Imagine a sample of seawater that has been collected at the beach. There is a liquid layer, consisting of dissolved salt in water, and solid sand particles suspended in the liquid. How can these components be separated? The flow chart in Figure 1 shows one possible method for the separation and recovery of the components in a sample of seawater.

Experiment Overview

The purpose of this experiment is to study the physical properties of salt, sand, iron and stearic acid and then to use this information to design and carry out a procedure for separating a mixture of these substances.

Materials

Prelab Questions

1. Think about the separation process outlined in Figure 1 and complete the flow chart to reveal the identities of the solid, liquid and vapor components.

Safety Precautions

The materials in this lab activity are considered relatively nonhazardous. Observe all normal laboratory safety procedures. Wear chemical splash goggles whenever working with chemicals, glassware or heat in the laboratory.

Procedure

Part A. Physical Properties of Substances

1. Set up a boiling water bath for use in step 7. Half-fill a 400-mL beaker with water. Place the beaker on a hot plate and add a few boiling stones. Set the hot plate on a high setting to heat the water to boiling.
2. Using a spatula, transfer teaspoon-sized amounts of salt, sand, stearic acid and iron filings to four separate plastic weighing dishes.
3. Observe the physical appearance of each substance. Record all observations in Data Table A.
4. Insert a magnet into a plastic bag so that no surface of the magnet is exposed. Move the wrapped magnet through each substance in the dishes. If a substance is magnetic, it will stick to the bag that covers the magnet. Note: Do not place the bare magnet directly into the materials—it will be difficult to remove the magnetic material from the surface of the magnet. Record observations in Data Table A.
5. To remove the magnetic material, carefully open the bag and remove the magnet. The magnetic material will fall off the bag and back into the weighing dish when the magnet is removed.
6. Determine whether each substance is soluble in water by adding a small amount of each substance to separate test tubes containing about 2 mL of water. Gently swirl or shake each test tube and record your observations in Data Table A. Rinse out and dry each test tube for use in step 7.
7. Determine which substances have a low melting point. Add a very small amount of each substance to a separate test tube. Use the test tube clamp to place the test tube in a boiling water bath for two minutes. Record the results.

8. Combine the dry samples of sand and salt. Mix together and observe the combination. Describe its physical appearance in Data Table A.
9. Transfer this combined sample to a 50-mL beaker and add 10 mL of water. Stir the sample with a stirring rod and record any changes that occur.
10. Clamp a ring to a ring stand to support a filter funnel. Position the funnel above a beaker. Fold a piece of filter paper into quarters and place the paper in the funnel with three quarters placed to one side so that the paper forms a cone.
11. Wet the filter paper slightly with distilled water from a wash bottle and then slowly pour the mixture from step 8 into the funnel, using a stirring rod to guide the flow of liquid.
12. Using a wash bottle, rinse all of the solid remaining in the sample beaker into the funnel. Use a minimum amount of water (no more than 25 mL).
13. After all the liquid has passed through the funnel, gently remove the wet filter paper, open it half-way, and place it on a hot plate at a low setting to dry.

14. Design a flow chart to indicate how a mixture of these four substances can be separated and write a procedure outlining the exact steps that need to be followed.
15. Review the procedure, including any necessary safety precautions, with your instructor.
16. Obtain an unknown mixture and record the code number of the mixture in Data Table B. Transfer about 2 g of the mixture to a clean weighing dish.
17. Separate the components of the mixture using your procedure. Record your observations in Data Table B.

Student Worksheet PDF

13535_Student1.pdf