The Pure, The Mixture, The Unknown Grade Nine

Transcription

1 Ohio Standards Connection: Physical Sciences Benchmark C Describe the identifiable physical properties of substances (e.g., color, hardness, conductivity, density, concentration and ductility). Explain how changes in these properties can occur without changing the chemical nature of the substance. Indicator 9 Investigate the properties of pure substances and mixtures (e.g. density, conductivity, hardness, properties of alloys, superconductors and semiconductors). Lesson Summary: This lesson involves classroom discussions, demonstrations and hands-on laboratory activities about pure substances and mixtures with a focus on common household materials. The students will investigate: What is a pure substance? What is a mixture? How difficult is it to obtain pure substances? How can one test for purity? What are some methods for purifying substances? The demonstrations and labs will introduce students to methods for determining freezing points, melting points, boiling points, density and, where equipment is available, conductivity. Students also will learn to use these data for assessing the purity of substances. Estimated Duration: Four hours Commentary: This lesson is designed to help students understand the differences between pure substances and mixtures. To begin the lesson, students will examine household materials and attempt to classify them by using their labels. They will then use the context of purity to examine the properties of materials and the methods for separating components of impure materials (mixtures). The lesson was field tested and reviewed by educators across the state. Some of their comments were: The hands-on nature of the lesson is great. All students get involved. Good connections (real world) by using household substances. This lesson was well received by students and improved their understanding of matter and its properties. 1

2 Pre-Assessment: On the day before the lesson is taught, ask students to consider and write their answers to the following questions as homework. See Attachment A, Pre-Assessment for a student handout. 1. What is a pure substance? 2. What is a mixture? 3. What types of mixtures are there? 4. How do you know if something is pure? 5. How are substances purified? 6. When is it important to have pure substances? 7. What are some examples of pure substances and mixtures that can be found at home, in grocery stores or hardware stores? Ask students to bring in labels from discarded containers of the pure substances and mixtures on their lists of examples, when possible. Caution them not to remove labels from household products which may be misidentified by family members. At the beginning of the lesson, have the students share and discuss their answers. Finally, ask students these questions: 1. What would scientists consider pure substances? 2. How do they determine when a substance is pure? Scoring Guidelines: The pre-assessment is not a graded activity. Its purpose is to bring out the varying views students have of purity and to stimulate their interest in the topic. From the early part of the discussion, assess how much students know and what their misconceptions are about pure substances and mixtures. For example, some students consider a substance to be pure if it is colorless, clear, safe, clean or free of bacteria. From student answers, note their differing views of purity, some of which may be fine for everyday usage, but not for all situations and not for use in science. From this information, decide when to provide scientific information about substances. See instructional procedures for key information and additional questions to use during the pre-assessment discussion. For additional answers, see Attachment B, Pre- Assessment Sample Answers. Post-Assessment: Part One Have students describe and/or draw diagrams of procedures for investigating the purity of two or more of the following substances and state the scientific bases of their procedures. Clearly communicate to students the level of detail that is expected in their write-ups and/or diagrams. Responses may include, details of the procedures, rational for using the procedures, properties of the materials that make the procedures possible, optional procedures, discussion of key points, and or listing of any problems. 1. Commercial bottle of vinegar 2. Aluminum screws that have no iron in them 3. Water from a tap 2

3 4. Water from a pop machine (bottled water) 5. Pond water 6. Distilled water 7. Table salt (iodized and non iodized) Part Two Have students describe and/or draw diagrams of procedures to separate the items in one of the following mixtures and state briefly the scientific basis for their procedures. 1. White sand, sugar and sawdust 2. Iron filings, aluminum filings and sugar 3. Powdered sulfur and table salt 4. Iron filings and coffee grounds 5. Vegetable oil and water 6. Crushed pumice, salt and sand Scoring Guidelines: See Attachment C, Post-Assessment Answers and Scoring Rubric. Instructional Procedures: 1. On the first day of the lesson, ask students to share and discuss their answers to the preassessment questions. Ask them to refer to the information on the labels they brought or that you provide. 2. As students share their examples of pure substances and mixtures, record their answers on the board in a table like the one students used for homework. Instruct students to add the information to their tables. 3. As the students respond, ask questions and provide information about the characteristics of pure substances, mixtures, types of mixtures, etc. 4. The following are suggested questions to ask and key information to provide during the discussion: Why do you think the substance is a pure substance or a mixture? What information do you have about the substance? What is the source of the information? What can you observe? What does the label tell you about the material? Why do companies add substances to purified bottled water? Scientists classify matter as pure substances and mixtures. a. A pure substance contains only one element or one compound. A combination of pure water and pure ice is a pure substance in two physical states. b. A mixture contains two or more elements or compounds that are physically mixed but are not chemically combined. Mixtures are classified as homogeneous or heterogeneous. a. A homogeneous mixture is uniformly distributed at the microscopic level. A heterogeneous mixture is not uniformly mixed at the atomic or molecular level. Close examination by eye or microscope will show the different parts. b. An alloy is a mixture of two or more solids. 3

4 c. A solution is a homogeneous mixture because its parts are equally distributed at the atomic and molecular level. The solute is the material being dissolved. The solvent is the dissolving material. In a salt solution, salt is the solute and water is the solvent. What are the properties of the substance? Which properties could be used to separate the substance from another material? (Solubility, boiling points, magnetic properties, chemical activity, density, etc.) Examples of separating components of a mixture are: a. Iron filings can be removed from a mixture with a magnet. b. Salt (sodium chloride) dissolves easily in water and can be recovered from the solution by slow evaporation of the water and re-crystallization of the sodium chloride. c. Saw dust and pumice float on water and can be skimmed off of the surface of a mixture. d. Oil and water will separate into layers and the heavier (denser) layer can be drained out using a separatory funnel. The key point is that it is extremely difficult to obtain and maintain an absolutely pure substance. Use bottles of reagent-grade common chemicals, such as sodium chloride, sucrose and potassium chloride to demonstrate that even the reagent-grade substances that are used in labs are not 100 percent pure. Some are 99.4 percent pure or higher, but none is 100 percent pure. Pure substances have unique characteristics, such as melting points, boiling points, density and conductivity as well as characteristics, such as infrared light absorption patterns that can be obtained with modern instrumentation. These characteristics help judge the purity of the substances and, in some cases, help separate them from mixtures. 5. After the class discussion of the key points about mixtures and pure substances, describe the labs that students will do in rotations. 6. Demonstrate what is to be done at each station and describe the data that are to be recorded. Provide students with data sheets to record data and observations. See sample student handout in Attachment D, Data Table for Lab Rotations and Demonstrations. Ensure that students are wearing goggles and are taking other safety precautions. 7. Have students work in groups of two to three and proceed through the lab stations listed below. a. Separation of liquids: Use a separatory funnel to separate two immiscible liquids (e.g., cooking oil and water, or motor oil and vinegar). b. Filtration: Use filter paper to separate a mixture (e.g., sand and kosher salt or sugar and sawdust). c. Evaporation and re-crystallization: Use a watch glass to evaporate the water from a drop or two of a solution (e.g., salt and water or sugar and water). Use a hot plate or other safe source of heat. d. Liquid density determination: Use hydrometers to measure the densities of five solutions (e.g., distilled water, two different salt solutions, rubbing alcohol and cooking oil). 4

5 e. Solid density determination: Measure the mass and volume of pennies from different time periods (pre- and post-1982) and calculate the densities. f. Conductivity: If equipment is available, test water for purity using conductivity kits or swimming pool testers. g. Conductivity of semiconductors: If kits are available, test the conductivity of silicon lumps and silicon used in computer chips. 8. At the conclusion of the labs and demonstrations, have students write their results as laboratory reports with procedures, data, conclusions and discussions. Additional suggestions for materials and equipment can be found in the Materials and Resources section of this lesson. 9. If possible, demonstrate the determination of melting and boiling points. Have the students record observations on their data sheets. Emphasize that melting and boiling points are unique characteristics of pure substances that can be used to identify them and, in some cases, separate them. If the equipment is not available, video can be found on the Internet by searching for How to take a melting point. 10. If possible, demonstrate distillation as a technique used for purification. Have the students record observations on their data sheets. Again, if a live demonstration is not possible, use video. 11. After the labs and demonstrations are complete, guide the class through sharing and discussing the results. 12. Ask students to critique in positive ways other students lab reports, especially the conclusions, by asking questions or making suggestions. 13. Use the lab report to determine what should be stressed in the conclusion of the lesson. 14. Conclude the lesson by reviewing topics. The key points students should understand are: a. A pure substance can be purified by separating it from unwanted components of a mixture by various lab procedures, such as separation of immiscible liquids, filtration, evaporation and re-crystallization and distillation. b. A substance has unique characteristics that can be used to identify it, to judge its purity and to help isolate it (e.g., distillation uses the differences in boiling points to separate the components of a mixture). c. More than one technique may be needed to separate and purify a substance. d. Some testing for purity may require destructive techniques on small samples, (i.e., chemical analyses that change the substance chemically). Differentiated Instructional Support: Instruction is differentiated according to learner needs, to help all learners either meet the intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the specified indicator(s). In addition to or instead of written formats, have students use diagrams, pictures, flow charts, concept maps, other graphics or visual presentations. Place students in mixed groups to respond to the students differing skills, knowledge, experience, physical challenges, etc. Have students working beyond the standard use inquiry as well as further research in areas of student interest. See suggestions in the Extensions section of this lesson. 5

6 Extensions: Have students explore which properties of gold allow a practical test for the grade of gold in jewelry and the purity of gold bars. Encourage students to visit the Web sites of chemical, oil, manufacturing and pharmaceutical associations and companies. Have students research and prepare posters on how substances are separated and purified on large scales (e.g., water for scientific use; crude oil to yield gasoline and other products; sugar from sugar beets or sugar cane). Have students make a hydrometer from a Beral pipette and metal filings and design a method for using it to determine the purity of some liquids such as antifreeze. Prepare an investigation for using chemical tests on small samples. Prepare an investigation that uses quantitative techniques. Homework Options and Homework Connections: Have students collect labels from various household materials to determine if they contain pure substances or mixtures. Materials and Resources: The inclusion of a specific resource in any lesson formulated by the Ohio Department of Education should not be interpreted as an endorsement of that particular resource, or any of its contents, by the Ohio Department of Education. The Ohio Department of Education does not endorse any particular resource. The Web addresses listed are for a given site s main page, therefore, it may be necessary to search within that site to find the specific information required for a given lesson. Please note that information published on the Internet changes over time, therefore the links provided may no longer contain the specific information related to a given lesson. Teachers are advised to preview all sites before using them with students. For the teacher: Labels from common substances, particularly table salt, iodized table salt, kosher salt, milk (whole, skim and1% fat), bottled water (especially one whose label indicates that substances have been added after purification), carbonated cola, orange juice from concentrate, aspirin, white vinegar, household ammonia and powdered drink mix; distillation apparatus and melting point apparatus. For the students: Apparatus and materials for separation of liquids lab: glass separatory funnels, rings to hold them, ring stands, beakers, flasks for holding and transferring liquids, two immiscible liquids: cooking oil or motor oil and water or vinegar. If separatory funnels are not available, separations can be done with large plastic syringes without the needles, turkey basters or, for small amounts, eyedroppers. Apparatus and materials for filtration lab: ring stands, rings, funnels with matching filter paper, sand and kosher salt or sugar and sawdust. Apparatus and materials for evaporation and re-crystallization lab: watch glasses, pipettes, test tubes, distilled or de-ionized water, salt and/or sugar. 6

7 Apparatus and materials for liquid density lab: hydrometers (from chemistry labs, aquarium shops or made from Beral pipettes (see extensions)), cylinders, flasks and the following liquids in flasks labeled with a letter but without their names: Liquid A (distilled water), Liquid B (salt solution with about four parts water and one part salt), Liquid C (salt solution with about eight parts of water and one part salt), Liquid D (rubbing alcohol, preferably 70 percent), Liquid E (clear cooking oil or mineral oil). Apparatus and materials for solid density lab: pennies (50 or more) minted before 1982 and pennies (50 or more) minted after 1982, 100 ml graduated cylinders, water, balance and flask. Conductivity: conductivity kits, if available. Safety Equipment: goggles, aprons, gloves, shields, and other protective equipment. Instructional Tip: Confer with the chemistry chairperson and obtain from the chemical storage room a collection of chemicals with their assays or purities noted on the labels. Get bottles of technical-reagent-and analytic-grade chemicals, like sodium chloride, sucrose and potassium chloride, and use them to point out the different levels of purity in these lab materials. Vocabulary: assay crystallization evaporation distillation filtration precipitation conductivity density solution solvent solute homogeneous mixture heterogeneous mixture microscopic level atomic level molecular level colloid alloys pure substance mixture compound 7

8 Technology Connections: Use graphing calculators to record and plot data on melting points, heating curves and/or boiling points. Use spectrophotometers to demonstrate or analyze the purity of substances by their color characteristics. Use conductivity equipment or kits to demonstrate or to analyze the purity of substances. Use the Internet to learn about and look up properties of substances. Learn of other technological equipment that is used in science, industry and health services to determine the purity of substances. Use a spreadsheet for data tabulation and analyses. Research Connections: Marzano, R. et al. Classroom Instruction that Works: Research-Based Strategies for Increasing Student Achievement. Alexandria, VA: Association for Supervision and Curriculum Development, Cooperative learning has a powerful effect on student learning. This type of grouping includes the following elements: Positive interdependence; Face-to-face promotive interaction; Individual and group accountability; Interpersonal and small-group skills; Group processing. General Tips: Prior to the lesson, it may be helpful to talk with the chemistry chairperson for equipment setups, available chemicals and videotapes. Have on hand for this lesson appropriate references, including: 1. Handbook of Chemistry 2. Chemical supply company catalogs 3. Online resources about properties of substances 4. Merck Index 5. Material Safety Data Sheets (MSDS) Attachments: Attachment A, Pre-Assessment Attachment B, Pre-Assessment Sample Answers Attachment C, Post-Assessment Answers and Scoring Rubric Attachment D, Data Table for Lab Rotations and Demonstrations 8

9 Pure Substances and Mixtures 1. What is a pure substance? 2. What is a mixture? 3. What types of mixtures are there? 4. How do you know if something is pure? 5. How are substances purified? Attachment A Pre-Assessment 6. When is it important to have pure substances? 7. What are some examples of pure substances and mixtures that can be found at home, in grocery stores or hardware stores? Record them in the table below and complete as much of the information about them as possible. Name Properties Pure Substance or Mixture? Components; Homogeneous or Heterogeneous 9

10 Attachment B Pre-Assessment Sample Answers Name Properties Pure Substance Components; Homogeneous or Mixture? or Heterogeneous Milk, whole Mixture Heterogeneous mixture of water, proteins, fat Milk, skim Mixture Heterogeneous mixture of water, proteins, slight fat Carbonated drink (e.g., Cola) Sugar Mixture Nearly pure but not as pure as sucrose used in scientific labs Homogeneous mixture of sugar, coloring, flavor, carbon dioxide, water; solution while capped; heterogeneous mixture shortly after opening (CO 2 released) Homogeneous; sucrose Air of classroom Mixture Homogeneous mixture of oxygen, nitrogen, water, carbon dioxide and other gases; heterogeneous mixture when dust and other particles are taken into account. Orange juice freshly squeezed from oranges Orange juice from concentrate Mixture Mixture Heterogeneous mixture of water, protein, sugar, vitamins, etc. Heterogeneous mixture of water, protein, sugar, vitamins and, perhaps, additives Salt, iodized Mixture Heterogeneous mixture of sodium chloride, sodium iodide, silicates Salt, kosher Pure substance Homogeneous; sodium chloride; no silicate added; no iodide added Well water Mixture Homogeneous mixture (solution) of water and dissolved minerals and gases Tap water Mixture Homogeneous mixture (solution) of water and dissolved minerals and gases 10

11 Attachment B (continued) Pre-Assessment Sample Answers Bottled water Mixture Homogeneous mixture (solution) of water and dissolved minerals and gases Distilled water Pure substance in most cases Homogeneous; pure compound; homogenous mixture (solution) if dissolved gases are considered White vinegar Mixture Homogeneous mixture (solution) of acetic acid in water Ammonia, household Mixture Homogeneous mixture (solution) of ammonia gas dissolved in water; may contain detergents Copper penny Mixture Homogeneous mixture; alloy copper and zinc (pre-1982) Nickel Mixture Homogeneous mixture; alloy of copper and nickel Medicinal oxygen Very pure Homogeneous; pure element; homogeneous mixture (solution) if small amounts of other gases and water are present Powdered drink crystals Mixture Heterogeneous mixture of sugar, favoring, coloring, other substances Gasoline Mixture Homogeneous mixture (solution) of octane, other petroleum products, additives Aspirin Mixture Heterogeneous mixture of aspirin (acetyl salicylic acid), starch, maybe cellulose, glycol, other chemicals Coated pills also have gelatin, titanium dioxide, dye, glycerin. Coal Mixture Heterogeneous mixture of many chemicals, including water and heavy metal salts Plain chocolate candy bar Chocolate candy bar with nuts Mixture Mixture Homogenous mixture Heterogeneous mixture 11

12 Attachment C Post-Assessment Answers and Scoring Rubric Answers: Sample answers are provided, but student answers will vary. Part One: Investigating the purity. 1. Commercial bottle of vinegar: Determine the boiling point(s) and compare it to established values. Pure substances have specific boiling points. 2. Aluminum screws that have no iron in them Determine the melting point and/or density and compare them to established values. Pure substances have specific melting points and densities. 3. Water from a tap Determine the boiling point and compare it to the established value; evaporate a sample and examine the container for residue. Pure substances have specific boiling points. Pure water should evaporate completely and leave no residue. A residue indicates the presence of a contaminant. 4. Water from a pop machine (bottled water) Determine the boiling point and compare it to the established value; evaporate a sample and examine the container for residue. Pure substances have specific boiling points. Pure water should evaporate completely and leave no residue. A residue indicates the presence of a contaminant. 5. Pond water Determine the boiling point and compare it to the established value; evaporate a sample and examine the container for residue. Pure substances have specific boiling points. Pure water should evaporate completely and leave no residue. A residue indicates the presence of a contaminant. 6. Distilled water Determine the boiling point and compare it to the established value; evaporate a sample and examine the container for residue. Pure substances have specific boiling points. Pure water should evaporate completely and leave no residue. A residue indicates the presence of a contaminant. 7. Table salt (iodized and noniodized) Determine the melting point and compare it to established values. Pure substances have specific melting points. Part Two: Separating Components of a Mixture 1. White sand, sugar and sawdust Add water and stir thoroughly. Wait a few minutes to allow the mixture to settle. Scoop the sawdust from the surface of the water. Pour the remaining mixture through filter paper and recover the sand in the filter paper. Evaporate the water from the remaining 12

13 mixture to recover the solid sugar. Sawdust is not soluble in water and has a density that is less than water, so it will float on top of the water. Sand is not soluble in water and has a density that is greater than water, so it sinks to the bottom. Sugar dissolves in water. It can be separated from water by evaporating the water. 2. Iron filings, aluminum filings and sugar Separate the iron filings from the mixture with a magnet. Add water to dissolve the sugar. Pour the remaining mixture through filter paper and recover the aluminum filings in the filter paper. Evaporate the water from the remaining mixture to recover the solid sugar. Iron is attracted to a magnet, but aluminum is not. Aluminum is not soluble in water and is denser that water, so when it is mixed with water, it sinks to the bottom. Sugar dissolves in water. 3. Powdered sulfur and table salt Add water and stir thoroughly. Wait a few minutes to allow the mixture to settle. Pour the mixture through filter paper to recover the solid sulfur. Evaporate the water from the remaining mixture to recover the solid salt. Sulfur is not soluble in water; table salt is soluble. 4. Iron filings and coffee grounds Separate the iron filings from the mixture with a magnet. Iron is attracted to a magnet, but coffee grounds are not. 5. Vegetable oil and water Use a separatory funnel to separate the two immiscible liquids. Oil and water do not mix. The less dense oil rises above the water to form a separate layer. 6. Crushed pumice, salt and sand Add water and stir thoroughly. Wait a few minutes to allow the mixture to settle. Scoop the pumice from the surface of the water. Pour the remaining mixture through filter paper and recover the sand in the filter paper. Evaporate the water from the remaining mixture to recover the solid salt. Neither pumice nor sand dissolves in water; salt does dissolve in water. Rubric: Use the following rubric to score the post-assessment. Criteria Description Depth of Understanding Evidence of Inquiry Communication Level 4 Level 3 Level 2 Level 1 Scientific information and ideas are accurate. Scientific information and ideas are accurate, thoughtfully explained. Procedures are clearly identified and formulated in a manner that can be replicated. Scientific information is communicated clearly and precisely. Procedures are clearly identified. Scientific information is communicated clearly. Scientific information has occasional inaccuracies or is simplified. Procedures are implied. Scientific information has some clarity. Scientific information has major inaccuracies or is overly simplified. Procedures are unclear or absent. Scientific information is unclear. Adapted from Council of Chief State School Officers State Collaborative on Assessment and Student Standards Science Project, April

14 Attachment D Data Table for Lab Rotations and Demonstrations Separating and Identifying Characteristics of Components of a Mixture For each lab station and demonstration: 1. Briefly describe the procedure; 2. Record all observations and data. Station Procedure Data and Observations 1. Separation of Liquids 2. Filtration 3. Evaporation and re-crystallization 4. Liquid density determination 14

15 5. Solid density determination 6. Conductivity 7. Melting point determination 8. Boiling point determination 9. Distillation 15