6.7 Design Your Own Experiment: Factors

Transcription

1 6.7 Design Your Own Experiment: Factors That Affect the Rate of Dissolving Page 158 PRESCRIBED LEARNING OUTCOMES measure substances and solutions according to ph, solubility, and concentration conduct investigations into the properties of matter KNOWLEDGE mixtures can be classified as solutions, suspensions, or mechanical mixtures solutions are mixtures that appear as a single substance SKILLS AND ATTITUDES demonstrate curiosity, skepticism, creativity, open-mindedness, accuracy, precision, honesty, and persistence as important scientific attributes ask questions and formulate hypotheses that are tentative and testable, and draw conclusions from results recognize that an experiment must be repeated and yield consistent results to be considered scientifically valid use appropriate tools and techniques to gather, analyze, interpret, and share information handle chemicals and equipment safely and responsibly ICT OUTCOMES access information using a variety of on-line information tools SCIENCE BACKGROUND The particle theory is based on the assumption that all matter is made up of many tiny particles (molecules) that are always in motion. In a hot object, the particles have a higher energy and thus move faster than the particles in a cooler body. Many factors affect the rate at which one substance dissolves in another. The surface area of the solute particles, the size of the solute particles, temperature, type of solvent, amount of shaking/stirring, and amount of solvent all affect how quickly substances will mix. The particle theory explains the factors that affect the rate of dissolving. Particles in a solution have an attraction for each other. The greater the amount of solvent relative to the solute, the greater the opportunity for interactions and attraction between the solute and solvent particles, and the more uniform the distribution of the particles. As temperature increases, the particles move faster and spread out, with greater chances for mixing of the solute and solvent particles. Breaking up the solute crystals into smaller pieces increases the overall surface area of crystals exposed to the solvent, thereby accelerating the dissolving process. Time min Key Ideas Solutions can be measured by concentration, solubility, and acidity. Skills and Processes Observing Communicating (sharing) Communicating (recording) Interpreting Observations Measuring and Reporting Interpreting Data Predicting Designing Experiments Fair Testing Controlling Variables Hypothesizing Lesson Materials per student sugar cubes water beakers any other materials that students may need Program Resources BLM Design Your Own Experiment: Factors That Affect the Rate of Dissolving Investigation Rubric 7: Design Your Own Experiment Investigation Rubric 8: Design Your Own Experiment Self-Assessment Science Skills and Processes Rubric 3: Hypothesizing Science Skills and Processes Rubric 4: Hypothesizing Self-Assessment Nelson Science Probe 7 Web site Chapter 6 Matter can be classified. 273

2 Related Resources Bath, John B. Kitchen Chemistry. Greensboro, NC: Carson-Dellosa Publishing, INVESTIGATION NOTES Student Safety Students will likely require the use of a hot plate for this activity. Review proper procedures for using a hot plate. Remind students not to heat to dryness, to disconnect the hot plate by pulling on the plug not the cord, and to handle all glassware with care. Consider setting up the hot plates in one area of the classroom for more effective supervision. Check for Misconceptions Identify: Students may think that individual particles expand or contract when heated or cooled. Clarify: Tell students that it is the space between the particles that increases or decreases. The transfer of heat causes increased movement of the particles, which causes the volume of the substance to increase. As heat is removed, activity decreases and the volume decreases. Ask What They Think Now: Ask students to think of particles requiring more space when they have more energy. Students should be able to visualize particles moving faster and further, and, therefore, requiring more space as they collide with one another. To assess students, you may want to use or adapt Investigation Rubric 7: Design Your Own Experiment and/or Investigation Rubric 8: Design Your Own Experiment Self-Assessment. Question Before allowing students to begin their experiments, discuss what factors might increase the rate at which a solute dissolves. Students may already know from Section 6.6 that temperature is one factor. Ask students to brainstorm other factors. They should think about crushing the sugar cube (surface area), increasing the rate of stirring, and increasing the amount of solvent. Direct students to use Reading and Thinking Strategies: Make Connections to help them plan their experiment. Hypothesis The students hypotheses should be stated as a cause-and-effect relationship. Students are to use If then the sugar will dissolve more quickly. Possible hypotheses might include the following: If the temperature of the water is increased, then the sugar will dissolve more quickly. If the sugar cube is crushed into crystals, then the sugar will dissolve more quickly. If the water is stirred more quickly, then the sugar will dissolve more quickly. If more water is added, then the sugar will dissolve more quickly. 274 Unit B: Chemistry

3 Materials Ensure that students list of materials is complete and that it includes quantities required to conduct the experiment. Students should have all necessary materials before beginning their experiment, and the materials should be checked to ensure they are safe. Additional materials that students may require for this experiment include hot plate something to crush sugar cubes (mortar and pestle, spoon and a bowl) stirring rod Procedure Help students identify all of the variables that may affect their results. If students are having difficulty, refer them to Conducting an Investigation in the Skills Handbook to review independent, dependent, and controlled variables. Ensure that students procedures address each question related to planning their experiments and that students are aware of all safety precautions. Ensure students have a controlled variable(s) in their experiment. Students will have to test one factor at a time and will, therefore, have to control all other factors. Students will probably recognize that, to have a fair test, the amount of solute will have to be controlled in all situations. This experiment is about the rate of dissolving. Therefore, time will have to be a variable. Students may consider time to be a controlled variable; that is, they will make their observations of both conditions (e.g., stirring and no stirring) after the same period of time. This is to ensure that they can be reasonably certain that it is the stirring and not some other factor (i.e., time) that determined the dissolving. If time is used as a controlled variable, students will have to subjectively determine which situation caused the most dissolving in the same time period by visually comparing the two set-ups. Alternatively, students may use time as the independent variable, and observe and record how much has dissolved in each situation independently after regular intervals. For example, they will observe the set-up with no stirring and record their observations after every five minutes for 30 minutes. Then they will observe the set-up with stirring, and record their observations at the same intervals over the same period of time. This method will also produce subjective observations, because students will have to judge or estimate how much of the solute has dissolved after each time interval. Although quite acceptable from a science inquiry perspective, this method is obviously more time consuming and will not likely produce results that are any more valid or reliable. Chapter 6 Matter can be classified. 275

4 Students can use BLM Design Your Own Experiment: Factors That Affect the Rate of Dissolving as they are working through this experiment. Once the experiment is complete, assign the Check Your Understanding questions. Data and Observations Have students prepare a data table. Remind students to set specific times to record their observations (e.g., every five minutes). Explain that making observations at specific and regular intervals is important when attempting to investigate the rate at which something is happening. Analysis Suggested Answers 1. Specific answers will vary depending upon variables selected. Ensure that students rely on both qualitative and quantitative observations when analyzing and evaluating the effect of the selected variable, and that responses are reflected in their data tables. Students should find that their results are similar to their classmates results. 2. Stirring or increasing the temperature increases the rate of dissolution. Increasing the temperature of the solvent causes the particles to move faster. The particles of the solute are able to move more quickly away from their solid form. Stirring causes greater movement of the solute particles and assists them in moving away from their solid form. Crushing or grinding the solute exposes more solute particles to the solvent (increasing its surface area), thereby increasing the rate of dissolving. Conclusion Ensure that the conclusions reached are based upon, and can be verified by, the data table. Students will likely conclude that stirring, crushing, and increasing the temperature all increase the rate at which a solute dissolves in a solvent. Applications Suggested Answers 1. Bouillon powder will speed up the soup-making process. The powdered bouillon dissolves faster because more bouillon particles are exposed to the soup than would be on the surface of a bouillon cube. The soup particles would have more opportunity to slip past the powdered bouillon, making the bouillon dissolve faster. 2. Students will know from experience that warm soda is more likely to fizz than cold soda. Warm soda contains less dissolved carbon dioxide than cold soda. When the cap is removed, the warm soda gives a loud whoosh as the gas escapes. Students may say that increasing the temperature of the liquid solvent decreases the amount of gas that can be dissolved the inverse of the effect of temperature on the solubility of a solid in a liquid. (Students may also mention the effect 276 Unit B: Chemistry

5 of shaking the pop bottle. When the pop bottle is shaken, the pressure builds up within the bottle. When the cap is removed, the pop will fizz much louder and, in some cases, will spray out. Increasing pressure in this manner results in more gas remaining dissolved in the soda, regardless of temperature. When the cap is opened, the pressure is released, allowing the escape of any gas that had been forced to remain dissolved.) 3. Student answers may vary. Some possible answers include the following: Making a bouillon base: Stirring or increasing the temperature will increase dissolving. Making Jell-O: increasing the temperature of the water will help Jell-O powder dissolve. Making iced tea from crystals: Using a bit of boiling water will help to dissolve the crystals faster before adding cold water. Timed release of some medications: A coating over medications will slow down the dissolution of pills in the stomach and ensure that the medication is available over longer periods of time. Chemical weathering of marble statues and limestone buildings: Coatings or other protection of these structures will slow down the dissolving process that occurs with acid precipitation. CHECK YOUR UNDERSTANDING SUGGESTED ANSWERS 1. Answers will vary. The independent variable is the variable selected to manipulate (temperature, amount of water, surface area of sugar cube, amount of stirring). The dependent variable is the rate of dissolving of the sugar cubes. 2. Accept any reasonable answers. Examples could include adding a third component to the mixture, using a larger beaker, etc. Chapter 6 Matter can be classified. 277

6 LEARNING TIP Make a web to show what you already know about substances that dissolve in water. Solubility You can make orange drink because the orange-drink crystals dissolve in water. Another way to say that a substance dissolves in water is to say that it is soluble in water. Can you think of some other substances that are soluble in water? If the orange-drink crystals did not dissolve in water, you would not be able to make the drink. An insoluble substance is a substance that does not dissolve. Can you think of some substances that are insoluble in water? Can a substance that is insoluble in water be dissolved in another solvent? TRY THIS: OBSERVE DIFFERENT SOLVENTS Skills Focus: predicting, observing, classifying Predict whether salt, sugar, butter, and wax will dissolve in water and in ethanol. Now try to dissolve each of these solutes in the two different solvents (Figure 2). Record your results. Reading and Thinking Strategies: Make Connections Before beginning this experiment, remind students of the information they learned in Section 6.6. They need to recall factors that affect solubility. They should also recall information that they may have from everyday experiences. For example, they may recall that stirring, or crushing something into a powder, helps it dissolve faster. Observe Different Solvents Dissolves in water? Dissolves in ethanol? salt sugar butter wax Figure 2 Adding sugar to ethanol to see if the sugar dissolves 154 Unit B Chemistry Meeting Individual Needs Extra Support Have students work in carefully selected pairs. ESL Provide language support to help students better understand what to do. Consider having ESL students work in small groups. Encourage group members to support each other. Extra Challenge Have students research how soda pop is made; specifically, how the gas is put into the pop. GO ASSESSMENT FOR LEARNING What To Look For in Student Work Evidence that students can explain how solutions can be measured design a fair test to determine the effect of the variable make a hypothesis to provide an answer to a testable question write a conclusion that refers back to the hypothesis interpret data accurately create products that are complete (e.g., a lab) Suggestions for Teaching Students Who Are Having Difficulty Help students identify specific variables for each of the factors that could affect the outcome of this experiment. For example: Time length of time sugar cubes are left in the beaker. Measurement temperature of the liquid. Quantity amount provided. 278 Unit B: Chemistry