Materials Included In Kit

Additional Materials Required

Safety Precautions

Hydrochloric acid solution is a corrosive liquid. Dilute sodium hydroxide solution is irritating to the skin and eyes. Salicylic acid is moderately toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash hands thoroughly with soap and water before leaving the laboratory.

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 original iron oxide–salicylic acid mixture and the recovered solids may be disposed of in the solid trash according to Flinn Suggested Disposal Method #26a.

Lab Hints

• The laboratory work for this activity can be completed in a 50-minute class period. Students will need to dry the filter paper and recovered solids for at least two hours before weighing them. Depending on the teacher’s schedule and the way the lab is used during the day, students may come in during their lunch hours or after school to weigh the products. Alternatively, the solids may be allowed to dry overnight in a secure location and students may weigh them before the next scheduled class period. Assigned lab drawers (if students have them) are an excellent place to store and dry the products.
• Review the filtration setup and procedure with your students. An overview of the filtration technique is included in the Supplementary Information in the Further Extensions section.
• The actual iron oxide–salicylic acid mixture used to obtain sample data consisted of 35% iron oxide and 65% salicylic acid by weight. The mixture should appear as a rust-colored powder. There will likely be some small white particles of salicylic acid in with the powder. This is acceptable—the students should observe that for the most part, the mixture appears homogeneous. The mixtures will probably analyze high for percent iron oxide—do not grade on accuracy for the percent of each component in the mixture. Percent recovery, however, should be excellent (greater than 90%).
• A small amount of carry-over of iron oxide occurs in the initial filtration. This will give a slight pink color to the recovered salicylic acid.
• For best results, demonstrate the rinsing–swirling–pouring techniques (steps 7 and 8) for students. Heat lamps (IR lamps) may be used to speed up the drying process (step 17).
• For an introductory-level experiment, it may be necessary to explain the meaning of the (s), (l) and (aq) terms in Equations 1 and 2.
• If time permits, allow students to investigate the physical properties (appearance and solubility) of the individual components of the mixture, iron oxide and salicylic acid, before separating the mixture. Samples of the pure substances are supplied for students to examine. Also, as an extension of this experiment, students may compare the melting point of the recovered salicylic acid versus that of the pure substance. The salicylic acid may be recrystallized from hot water (dissolve in a minimum amount of boiling water, then cool to room temperature). Salicylic acid, which melts at 157–159 °C, will sublime at 76 °C—this is a nice demonstration of a phase change.
• The extra amounts of iron oxide and salicylic acid can be used to prepare a second mixture that is 50:50 ratio.

Teacher Tips

• Salicylic acid (2-hydroxybenzoic acid) is used in acne-treatment and in wart-removal products. It is called a “keratolytic” agent, meaning it removes the top layer of skin. It is also a member of the class of compounds called the “hydroxy acids” that are used in (very expensive!) skin care lotions and creams. The structure of salicylic acid is shown in Figure 3—notice the “active” or acidic hydrogen which is shown in boldface.
  {13536_Tips_Figure_2_Structure of salicylic acid}
• An alternative, guided-inquiry activity using sand, iron, stearic acid and salt, “Separation of a Mixture,” AP7033, is available from Flinn Scientific, Inc.

Further Extensions

Supplementary Information
Two of the most basic and important techniques to master in chemical analysis are filtering and decanting. These two techniques of quantitative transfer occur as crucial steps in many analytical determinations. To make sure the filtering speed is as rapid as possible, the filter paper must be seated properly in the funnel. The filter paper should make complete contact with the sides of the funnel and drain with few, if any, air bubbles in the stem. This is done by folding the filter paper as shown in Figure 3 and tearing off the corner. This allows the subsequent cone of filter paper to be placed smoothly against the sides of the funnel. Place the filter paper cone in the funnel. Wet the filter paper thoroughly with distilled or deionized water and use your fingers to smooth the paper against the sides of the funnel until no air bubbles are visible in the stem.

Proper decanting technique ensures that the precipitate is transferred from the beaker to the filter paper and little, if any, is lost during the transfer. Start by holding a stir rod against the lip of the beaker and pour the liquid from the beaker into the funnel. The liquid should run down the rod and into the funnel without splashing (see Figure 4). Keep the level of the liquid in the funnel below the top of the filter paper. When all the liquid has been transferred to the funnel, begin transferring the remaining precipitate from the beaker to the funnel. Add a stream of distilled or deionized water to the beaker. Use a rubber policeman on the end of the stir rod to loosen any precipitate clinging to the beaker. Rinse the rubber policeman with a stream of distilled or deionized water, swirl the beaker to suspend the precipitate and transfer the suspension to the funnel using the technique outlined in Figure 4. Repeat this rinsing until nearly all the precipitate has been transferred. Rinse the beaker again and transfer the liquid to the flask. Hold the beaker and stir rod as shown in Figure 5 and rinse the sides and bottom of the flask with distilled or deionized water. Rinse at a rate that allows the liquid to flow down the stir rod into the funnel without splashing and doesn’t allow the liquid level to rise above the top of the filter paper. Repeat this rinse until no precipitate is visible in the beaker.

Answers to Prelab Questions

1. In filtration, why doesn’t the filter paper trap the dissolved solids the same way as it does the undissolved ones?

   The dissolved solids are extremely small (molecular-sized) particles. The filter paper acts as a screen or sieve. There are very small channels or pores in the filter paper fiber. Very small particles (the dissolved solids) pass through these pores. Undissolved solids, however, are relatively large particles, where each “particle” is billions and billions of molecules. These particles will not pass through the pores or channels in the filter paper and are thus “trapped” on the filter paper.

2. What is the filtrate?

   The filtrate is the liquid which passes through the filter paper and the funnel and collects in the filtration flask.

3. Read the entire Procedure and the accompanying Safety Precautions. Complete the flow chart to show how the mixture of iron oxide and salicylic acid will be separated in this experiment.
   {13536_PreLabAnswers_Figure_4}
   *Note: The final solution will be acidic—it will contain sodium chloride (from neutralization of sodium hydroxide and hydrochloric acid) and a small excess of hydrochloric acid. Students may answer that the solution also contains sodium hydroxide and hydrochloric acid. This is an introductory-level experiment—the students should not be expected to understand the neutralization process.

Sample Data

Answers to Questions

1. For each trial, calculate (a) the original mass of the iron oxide–salicylic acid mixture, (b) the mass of recovered iron oxide, (c) the mass of recovered salicylic acid and (d) the total mass of recovered solids.
   {13536_Answers_Table_2}
2. Calculate the percent recovery of the iron oxide–salicylic acid mixture for each trial.
   {13536_Answers_Equation_3}
   Percent recovery = (0.50 g/0.51 g) x 100% = 98% (Trial 1)

   (0.52 g/0.52 g) x 100% = 100% (Trial 2)

3. For each trial, divide the mass of recovered iron oxide by the total mass of recovered solids and multiply the result by 100. This is the mass percent of iron oxide in the mixture.

   Mass percent iron oxide = (0.21 g/0.51 g) x 100% = 41% (Trial 1)

   (0.21 g/0.52 g) x 100% = 40% (Trial 2)

4. In a similar manner, calculate the mass percent of salicylic acid in the mixture.

   Mass percent salicylic acid = (0.29 g/0.51 g) x 100% = 57% (Trial 1)

   (0.31 g/0.52 g) x 100% = 60% (Trial 2)

   Note: The actual percent composition of the prepared iron oxide–salicylic acid mixture was 35% iron oxide, 65% salicylic acid. The percent recovery results (Question 2) are generally excellent, but the percent composition results consistently give higher than expected values for iron oxide. This is probably due to salicylic acid trapped (adsorbed) on the iron oxide, since salicylic acid by itself dissolves completely in control tests under the process conditions.
5. Label each of the following as a physical change or a chemical change. (a) Salicylic acid dissolves in the sodium hydroxide solution. (b) The mixture is filtered to separate the iron oxide. (c) The filtrate is acidified to precipitate the salicylic acid.
   1. Salicylic acid dissolves in the sodium hydroxide solution. Chemical change
   2. The mixture is filtered to separate the iron oxide. Physical change
   3. The filtrate is acidified to precipitate the salicylic acid. Chemical change
6. Salicylic acid may be crystallized from hot water by dissolving the solid in a minimum amount of boiling water and then cooling the mixture to room temperature. Is this a physical or a chemical change?

   Physical change—the composition of salicylic acid is not altered when it dissolves in hot water and then crystallizes at room temperature.

References

This experiment has been adapted from Flinn ChemTopic™ Labs, Volume 2, Elements, Compounds and Mixtures, Cesa, I., Flinn Scientific, Batavia, IL, 2005

Introduction

Most of the matter around us consists of mixtures, or physical blends, of many substances. The main characteristic of a mixture is that it has a variable composition—the components of the mixture may be mixed in varying proportions. The substances in a mixture retain their distinctive chemical identities as well as some of their unique physical properties. How are the properties and composition of a mixture affected by physical and chemical changes?

Concepts

• Mixture vs. Pure substance
• Homogeneous vs. Heterogeneous
• Physical and chemical changes
• Filtration

Background

Mixtures can be classified as either heterogeneous or homogeneous. A heterogeneous mixture is a mixture that is not uniform in composition. If one portion of the mixture were to be sampled, its composition would be different from the composition of another portion. Soil, containing bits of decayed material along with sand, silt or clay, is a heterogeneous mixture. A homogeneous mixture (e.g., a solution) is a mixture that has a completely uniform composition. The components of the mixture are evenly distributed throughout the sample. Air, saltwater and brass are examples of homogeneous mixtures. Air is a gaseous solution consisting of a mixture of nitrogen, oxygen and carbon dioxide. Saltwater is a liquid solution containing sodium chloride dissolved in water, and brass is a solid solution of two metals, copper and zinc.

Many mixtures, both homogeneous and heterogeneous, can be separated into their components using physical separation techniques such as filtration, evaporation or distillation. The properties of each component before mixing and after separation will not be altered by undergoing the physical separation. Consider, for example, a homogeneous mixture (a solution) of sugar in water. The sugar can be recovered by evaporation of the water; the water can be recovered by condensation. The sugar has the same properties before mixing and after separation. The same is true of the water.

In this experiment, the components of a mixture will be separated using a combination of chemical and physical changes. The mixture to be separated consists of iron oxide, more commonly known as rust, and salicylic acid, an organic compound used in drug manufacture. Salicylic acid is the parent compound of a class of drugs called the salicylates. The most important drug in this class is aspirin (acetylsalicylic acid), which is made by reacting acetic acid and salicylic acid. Salicylic acid was first obtained in the early 19th century from compounds present in the bark of the willow tree. (The curative powers of willow tree bark had been known since the times of the ancient Greeks.)

Salicylic acid is a white solid which melts at 157–159 °C. Although salicylic acid is essentially insoluble in water, it will dissolve in water containing bases, such as sodium hydroxide. Iron oxide is a red-orange solid with a very high melting point. It is completely insoluble in water and in dilute solutions of bases. When salicylic acid (abbreviated SA–H) dissolves in sodium hydroxide solution (NaOH), it loses a hydrogen ion (H⁺) and is converted to an ionic form (Na⁺SA^–) that is soluble in water (Equation 1).

Adding hydrochloric acid to the resulting solution reverses the process—the SA^– anion picks up an H⁺ cation, reforming the neutral compound, SA–H, which then precipitates from solution (Equation 2).

Materials

Prelab Questions

1. In filtration, why doesn’t the filter paper trap the dissolved solids the same way as it does the undissolved ones?
2. What is the filtrate?
3. Read the entire Procedure and the accompanying Safety Precautions. Complete the flow chart to show how the mixture of iron oxide and salicylic acid will be separated in this experiment.
   {13536_PreLab_Figure_1}

Safety Precautions

Hydrochloric acid solution is a corrosive liquid. Dilute sodium hydroxide solution is irritating to the skin and eyes. Salicylic acid is moderately toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Form a working group with three other students and divide into two pairs. Each pair of students will complete one trial (steps 1–18) and then share their data with the other pair of students to answer the Post-Lab Questions.

1. Obtain about 0.6 g of the iron oxide–salicylic acid mixture in a weighing dish. Observe the physical appearance of the mixture using a magnifying glass—does the mixture appear to be homogeneous or heterogeneous? Record observations in the data table.
2. Weigh an empty 50-mL Erlenmeyer flask and record the mass.
3. Transfer about 0.5 g of the iron oxide–salicylic acid mixture to the Erlenmeyer flask. Measure and record the combined mass of the flask and the solid mixture.
4. Using a graduated cylinder, add 15 mL of 0.2 M sodium hydroxide solution to the iron oxide–salicylic acid mixture in the Erlenmeyer flask.
5. Gently swirl the flask to dissolve as much of the mixture as possible, and then allow the mixture to stand for a few minutes while setting up the filter funnel (steps 6–7).
6. Set up a funnel for filtration as shown in Figure 1. Place a clean 50-mL Erlenmeyer flask under the funnel to collect the filtrate.
   {13536_Procedure_Figure_1}
7. Measure and record the mass of a piece of filter paper. Fold the filter paper into a cone and place it in the funnel (see Figure 1). Wet the paper with a few drops of distilled water from a wash bottle.
8. Using a stirring rod to direct the stream of liquid, slowly pour the mixture from the Erlenmeyer flask (step 5) into the funnel. Note: Gently swirl the flask as you pour to transfer as much of the solid and liquid together as possible.
9. When most of the liquid has passed through the funnel, rinse any remaining iron oxide from the Erlenmeyer flask into the funnel with a small amount (no more than 3–5 mL) of additional sodium hydroxide solution.
10. Rinse the solid on the filter paper with 2–3 mL of distilled water.
11. When the filtration is complete, carefully remove the filter paper with the iron oxide from the funnel and spread it on a watch glass or on paper towels to dry. Label the watch glass with your initials. Save the filtrate for step 12.
12. Using a Beral-type pipet, add one pipet-full of 1 M hydrochloric acid at a time to the filtrate until no more white solid precipitates out. Swirl the flask to mix the contents. Note: Use no more than 6 mL total (three pipetfuls) of hydrochloric acid.
13. Measure and record the mass of a second piece of filter paper and place it in the funnel. Place the initial 50-mL Erlenmeyer flask under the funnel. Wet the paper with a few drops of distilled water from a wash bottle.
14. Slowly pour the mixture obtained in step 12 into the funnel.
15. When most of the liquid has passed through the funnel, rinse any remaining salicylic acid from the Erlenmeyer flask into the funnel with a small amount (3–5 mL) of distilled water from a wash bottle.
16. When the filtration is complete, carefully remove the filter paper with the salicylic acid from the funnel and spread it on a watch glass or on paper towels to dry. Label the watch glass with your initials.
17. Allow the solids to dry on the filter paper for at least 2 hours (overnight is best). Measure and record the mass of the filter paper and iron oxide and the mass of the filter paper and salicylic acid.
18. The remaining filtrate may be rinsed down the drain under cold running water.

Student Worksheet PDF

13536_Student1.pdf