KEY CONCEPTS AND PROCESS SKILLS

Transcription

1 Soil Columns 40- to minute sessions ACTIVITY OVERVIEW 4 L A B O R AT O R Y Students investigate the composition of two different types of soil by separating the soils with water. By creating and observing a water-and-soil column, they gather evidence to show that soils are made from a mixture of different-sized materials. They learn how to use a graduated cylinder to quantify the height of the different sections. Comparison of this data shows that soils have different types and proportions of these materials. KEY CONCEPTS AND PROCESS SKILLS (with correlation to NSE 5 8 Content Standards) 1. One way to gather data is through direct observation. Accurate and complete observations are important for making conclusions about the natural world. (Inquiry: 1) 2. Scientists use common units of measurement to collect data. This system is known as the metric or international system (SI). (Inquiry: 1) 3. Soil consists of weathered rocks and decomposed organic material from dead plants, animals, and bacteria. (Earth Science: 1) 4. Soils from different environments have different compositions and properties. (Earth Science: 1) KEY VOCABULARY centimeters (cm) composition evidence milliliter (ml) observation A-33

2 Activity 4 Soil Columns MATERIALS AND ADVANCE PREPARATION For the teacher * 2 3 large containers or tubs 1 Science Skills Transparency 1, How to Read a Graduated Cylinder (optional) For each group of four students 1 sample of Soil A (labeled) 1 sample of Soil B (labeled) * 1 sample of local soil (optional) * 1 cup of water 1 SEPUP tray 2 3 large clear plastic tubes with caps 1 30-mL graduated cup 1 50-mL graduated cylinder 1 plastic spoon 1 ruler 1 magnifying lens (3x/6x, optional) For each student 1 Student Sheet 4.1, Soil Column Observations 1 Science Skills Student Sheet 1, Understanding Measurement: Length (optional) 1 Science Skills Student Sheet 2, Understanding Measurement: Volume (optional) *Not supplied in kit A local soil can be added to the two provided in the equipment kit as a third soil for student investigation. There is enough equipment and supplies for the inclusion of a third sample. The local soil can be introduced as a known basis of comparison or as an unknown soil that is part of the garden problem. Gather large containers or tubs for soil set up and clean up. Provide them in a common area where students can collect and return the soil. Have access to running water for cleaning out the soil tubes. You can reuse the soils in this activity with subsequent classes. The silt in the mixtures is easily washed away, so be on the lookour for a change in the composition of soils that have been mixed with water and dried. The soils from this activity will NOT dry out enough overnight to work well in the next activity, so you will need to reserve some soil for Activity 5, Soil Composition. Activity 6, Describing Soil Scientifically and Activity 10, Organic Matter Test require soil that has not yet been wet and dried, so you will need to reserve some of each soil sample for these activities as well. A-34

3 Soil Columns Activity 4 In the next activity, students are asked to reflect back on this activity and apply their new knowledge. If possible, keep at least a few soil columns from this activity available until the completion of Activity 5. Masters for the Science Skills transparency and student sheets can be found in the Science Skills section of Teacher Resources II: Diverse Learners. TEACHING SUMMARY Getting Started 1. Introduce soil composition. 2. Students predict what will happen when soil and water are mixed. Doing the Activity 3. Students construct and observe soil columns. Follow-Up 4. Let tubes stand overnight and make further observations. 5. Students revisit their predictions and construct a definition of soil.if t BACKGROUND INFORMATION Settling Many students know that large particles will settle to the bottom of a mixture. However, there is a common misconception that this is due solely to its weight or density. Settling patterns are due to many factors, but in particular, to the ratio of mass to frictional resistance as the particle travels through the water. The shape, or aerodynamic profile, of the particle affects the amount of friction. A higher mass to friction ratio will travel through the water faster and therefore settle down to the bottom faster. A-35

4 Activity 4 Soil Columns TEACHING SUGGESTIONS GETTING STARTED 1. Introduce soil composition. Review the definitions of soil developed in the last activity. Let students know they will revise their definitions after this and other activities as their knowledge and understanding of soil continues to expand. Explain that in this activity they will further investigate the composition of the two soils previously examined. Clarify the meaning of the term composition and relate it to the observations of Soils A and B. Simply define composition as what something is made of or its specific ingredients. It may help to use the term composition in an everyday example such as, The composition of chocolate milk is milk, chocolate, and sugar. 2. Students predict what will happen when soil and water are mixed. Ask students to predict what will happen when water is mixed with the soils and is left to stand for a while. Provide a framework for the prediction by describing, in general terms, the procedure of creating the soil columns. Have students write down their prediction and ask them to explain. DOING THE ACTIVIT Y 3. Students construct and observe soil columns. Before starting the activity, review how to read the markings on the graduated cylinder by using Science Skills Transparency 1, How to Read a Graduated Cylinder. The SEPUP 50-mL graduated cylinder is marked by a short line (starting at 4 ml) at every milliliter, a long line at every 5 milliliters, and is numbered every 10 milliliters. Show students how to interpolate the measurement when the liquid falls in between lines. If appropriate, let students practice reading a graduated cylinder and ruler with optional Science Skills Student Sheet 1, Understanding Measurement: Length and/or Student Sheet 2, Understanding Measurement: Volume. Note that although the Student Sheets show a meniscus, students may have difficulty identifying the meniscus for this activity, as the water and soil mixture may not form a clearly visible curve. In this activity, students measure soil using a 30-mL graduated cup (instead of a balance, which is what most scientists would use). This is done for convenience because the amount of soil used may be difficult to accurately measure on a balance. The soils provided in this kit are reasonably consistent. In an actual test, however, soils have different levels of compactness and mass is a more accurate way to measure it. If students are curious about this, ask them why they think the experiment uses a measure of volume (ml) instead a measure of mass (g) for measuring soil amounts. Use the discussion to review the use of units in measurement. You may want to review some other aspects of the Procedure, such as using the SEPUP tray to keep the two tubes separated and identified, e.g., the tube containing Soil A is in Cup A, the tube containing Soil B is in Cup B. Have students complete the activity. After mixing with water, the soils should split into differentiated strata in about 5 minutes. Teacher s Note: Because the different strata in the tubes are not correlated to the soil layers found in the earth, this activity refers to differentiated soil particles in the soil columns as sections (and not layers). Soil layers are introduced in the next activity. One misconception related to this activity is the idea that the soil sections created by the settling action in the tubes corresponds to soil layers found under the surface of the earth. This is not true; the columns merely reflect varying settle rates of the different particles. Without getting into a lesson on the physics of fluid dynamics (see Background Information), avoid perpetuating this misconception with students. A sample student response to Student Sheet 4.1, Soil Column Observations, is shown on the next page. Actual student results may be slightly different as the soils in the kit may vary. Use Analysis Question 2 as an opportunity to discuss the different particle sizes found in soil. Without getting into the details of why particles have different fall rates, discuss the fact that the columns give clear observational evidence that soil is composed of different-sized particles. The next activity introduces sand, silt, and clay (and their differing size) as one of the major aspects of describing and differentiating soil composition. A-36

5 Soil Columns Activity 4 Sample Response to Student Sheet 4.1, Soil Column Observations Observations: Top getting darker Bottom getting lighter Particles moving downward Particles floating Splitting into sections FOLLOW-UP 4. Let tubes stand overnight and make further observations. Let the tubes stand overnight and have students make observations the following day. If it is not possible to keep individual tubes set up, keep a few for groups or the class to share. Give students an opportunity to compare their observations to the ones they made the first day. They should find even more separation of the soil components on the second day; the water should appear clearer and the sections more distinct. 5. Students revisit their predictions and construct a definition of soil. After completing the investigation, have students revisit their prediction from the start of the activity. Ask them to compare their first-hand observations to their predictions. Students should be able to articulate which parts of their predictions were accurate and which parts were not. For example, a student might have predicted that the water will get all muddy, when, in fact, observations indicate that the water was muddy for only a short time and that the water and soil were clearly observable as separate entities after settling occurred. Ask students to analyze the inconsistencies between predictions and observations. An accurate observation would note that a lot of the soil settled to the bottom of the tube over time, while some particles floated into various sections. This icon indicates questions for formative assessment. See Teacher Resources III: Assessment for more information. Lead a short discussion about the definition of soil that students constructed in the previous activity. Ask students to add to their initial ideas about soil. At A-37

6 Activity 4 Soil Columns this point, let students offer their ideas and words, since they are still building knowledge about soil composition. The reading in the next activity will further develop their understanding of soil. At that time, they will be asked again to revise their working definition of soil and will be assessed on a more complete understanding of soil composition. EXTENSION Students can estimate the percentage of the overall composition by taking the section height and dividing it by the overall column height of soil. A sample calculation is as shown below: This calculation is an estimate in that it does not account for the varying amount of water between the soil particles. In particular, the volume of floating organic material in column B is likely to be overestimated. SUGGESTED ANSWERS TO QUESTIONS 1. How were the sections in Tube A similar to or different from the sections in Tube B? Support your answer with your observations. The sections are similar in that both soils have more than one section and that the particles in the bottom section of Soil B look identical to the second from the bottom section of Soil A. The bottom section of soil A is different because it does not look like any particles in Soil B. Likewise, the material floating on the top only appears in Soil B. 2. Were the particles on the bottom of Tube A larger, smaller, or the same size as the particles at the top? The particles on the bottom looked larger than those above it. This is because the larger particles settle to the bottom first. Students may try to explain this because they are heavier but this is not entirely accurate. The reason is complicated and due to a number of factors (see Background Information). Because the physics behind different settling rates is difficult to understand, focus the response of this question on the resulting sections and not the actual mechanics of settling. 3. Is the composition of Soil A the same as that of Soil B? Support your answer with evidence from this activity. Students should be able to point out that since some of the soil sections revealed different types of particles when separated, the soil compositions are not the same. Encourage students to cite evidence to support their conclusions (for example, by describing the height and composition of a particular section and comparing the sections between the two soils). Students may also cite similarities between the soils (for example, they both contained silt). A-38

7 Name Date Soil Column Observations Observations 2012 The Regents of the University of California Issues and Earth Science Student Sheet 4.1 A-39