Materials Included In Kit

Additional Materials Required

Prelab Preparation

Charcoal–salicylic acid mixture: To prepare the charcoal–salicylic acid mixture, mix measured amounts of powdered charcoal and salicylic acid in a beaker. (Record the mass percent of each solid in your notes.) Do not use more than 40 mass percent charcoal in the mixture (30–40% is best). Stir the mixture well to thoroughly mix the solids. The mixture should appear as a gray powder. There will likely be some small pieces of charcoal mixed in with the powder. This is acceptable—the students should observe that the original charcoal–salicylic acid mixture is heterogeneous in appearance, and their quantitative results for two trials should support this observation. The mixtures will probably analyze high for percent charcoal—do not grade on accuracy for the percent of each component in the mixture. Percent recovery, however, should be excellent (greater than 90%).

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. Charcoal is a flammable solid. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the 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 original charcoal–salicylic acid mixture and the recovered solids may be packaged for landfill disposal according to Flinn Suggested Disposal Method #26a. The filtrate remaining at the end of the experiment may be neutralized according to Flinn Suggested Disposal Method #24b.

Lab Hints

• The laboratory work for this activity can be completed within a typical 2-hour lab period. Students will need to dry the filter paper and recovered solids for at least two hours before weighing them. Depending on the lab schedule and the way the lab is used during the day, students may come in at prescribed times to weigh the products. Alternatively, the solids may be allowed to dry in a secure location until the next scheduled lab. Assigned lab drawers may be used to store and dry the products.
• For best results, use a soft “block” charcoal such as wood charcoal lumps (Flinn Catalog No. C0252). Do not use activated charcoal or a fine charcoal powder such as lampblack. These powders will clog the filter paper and slow down the filtration. Place the charcoal block in a zipper-lock plastic bag, and place inside a second, larger bag. Use a mallet or hammer to pulverize the charcoal.
• If time permits, allow students to investigate the physical properties (appearance and solubility) of the individual components of the mixture, charcoal and salicylic acid, before separating the mixture. Supply samples of the pure substances 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 60–70 °C .This is a nice demonstration of a phase change.
• 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—the “active” or acidic hydrogen is shown in boldface.
  {14027_Hints_Figure_3_Structure of salicylic acid}

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 fibers. 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 that 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 a mixture of charcoal and salicylic acid will be separated in this experiment.
   {14027_PreLabAnswers_Figure_4}
   *Note: The 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 may not understand the neutralization process.

Sample Data

Laboratory Report

Answers to Questions

Laboratory Report

1. For each trial, calculate (a) the original mass of the charcoal–salicylic acid mixture, (b) the mass of recovered charcoal, (c) the mass of recovered salicylic acid and (d) the total mass of recovered solids.
   {14027_Answers_Table_2}
2. Calculate the percent recovery of the charcoal–salicylic acid mixture for each trial.
   {14027_Answers_Equation_3}
   Percent recovery = (0.51 g/0.52 g ) x 100% = 98% (Trial 1)

   (0.46 g/0.51 g ) x 100% = 90% (Trial 2)

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

   Mass percent charcoal = (0.24 g/0.51 g) x 100% = 47% (Trial 1)

   (0.24 g/0.46 g) x 100% = 52% (Trial 2)

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

   Mass percent salicylic acid = (0.27 g/0.51 g) x 100% = 53% (Trial 1)

   (0.22 g/0.46 g) x 100% = 48% (Trial 2)

   Note: The actual percent composition of the prepared charcoal–salicylic acid mixture was 30% charcoal, 70% salicylic acid. The percent recovery results (Question 2) are generally excellent, but the percent composition results consistently give higher than expected values for charcoal. This is probably due to salicylic acid trapped (adsorbed) on the charcoal, since salicylic acid by itself dissolves completely in control tests under the process conditions.
5. Label each of the following as a physical or a chemical change:
   1. Salicylic acid dissolves in the sodium hydroxide solution. Chemical change
   2. The mixture is filtered to separate the charcoal. Physical change
   3. The filtrate is acidified to precipitate the salicylic acid. Chemical change
6. In comparing results obtained in this experiment with the actual or known percent salicylic acid in the mixture, a student found that the amount of salicylic acid isolated was 25% less than the calculated amount. Discuss possible sources of error that might account for this discrepancy, and describe a controlled experiment you could do to determine the source of the error.

   If the salicylic acid (SA–H) was not completely neutralized by reaction with NaOH, it would be trapped with the solid charcoal in the first filtration step and give a lower than expected value for percent SA. Alternatively, some of the neutralized salt Na⁺SA^– might be adsorbed on the surface of the charcoal. Carry out controlled experiments with just SA and NaOH to determine percent neutralization and percent recovery.

7. 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? Explain your reasoning.

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

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 sampled, its composition might be different from that 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 should not be altered by undergoing 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 charcoal, an activated form of carbon that is used to purify water, and salicylic acid, an organic compound used in pharmaceutical drug manufacture. Activated charcoal filters are used on water faucets to make drinking water taste better, in gas masks to absorb toxic gases, and in aquarium tanks to remove chemical and biological pollutants. 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 isolated in nature in 1828 from 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 that melts at 157–159 °C. Although salicylic acid is essentially insoluble in water, it will dissolve in water containing bases, such as sodium hydroxide. Charcoal is a black solid with a very high melting point. It is completely insoluble in water and in dilute solutions of acids or 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).

Experiment Overview

The purpose of this experiment is to separate a mixture of charcoal and salicylic acid and to determine the percent composition of each component in the mixture.

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 a mixture of charcoal and salicylic acid will be separated in this experiment.
   {14027_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. Charcoal is a flammable solid. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or 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 the same group.

1. Obtain about 0.6 g of the charcoal–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.
2. Weigh an empty 50-mL Erlenmeyer flask and record the mass.
3. Transfer about 0.5 g of the charcoal–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 charcoal–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 a filter funnel (steps 6–7).
6. Set up a funnel for filtration as shown in Figure 2. Place a clean 50-mL Erlenmeyer flask under the funnel to collect the filtrate.
   {14027_Procedure_Figure_2}
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 2). 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. 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 charcoal 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 from the funnel and spread it on a watch glass or on paper towels to dry the solid. Label the watch glass with your initials. Save the filtrate for step 12.
12. Using a Beral-type pipet, add one pipet-ful 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. Use no more than 6 mL total (three pipets) of hydrochloric acid.
13. Measure and record the mass of a second piece of filter paper and place it in 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 solid from the Erlenmeyer flask into the funnel using a small amount (3–5 mL) of distilled water from a wash bottle.
16. When the filtration is complete, carefully remove the filter paper from the funnel and spread it on a watch glass or on paper towels to dry the solid. Label the watch glass with your initials.
17. Allow the solids to dry on the filter papers for at least 2 hours at room temperature. The solids may also be placed in a lab oven at 50 °C or under a heat lamp (IR lamp) to speed up the drying process (15–20 minutes). Do not overheat.
18. Measure and record the mass of (a) the filter paper and charcoal and (b) the filter paper and salicylic acid.
19. The remaining filtrate should be neutralized if needed and may be rinsed down the drain with water.

Student Worksheet PDF

14027_Student1.pdf