Mixing Warm and Cold Water

Transcription

1 Mixing Warm and Cold Water A Continuing Investigation of Thermal Pollution By Kevin White 1

2 Context: This lesson is intended for students conducting an ongoing study of thermal pollution. Perhaps, students are inquiring about the environmental impact of heated water being dumped into a local river or lake by power plants. This lesson would likely follow three or four previous lessons that provide a basic understanding of how coal and nuclear plants use water to cool and, indirectly, produce energy. This lesson is intended to provide an opportunity for teachers to engage in explicit instruction of the nature of science and scientific inquiry. This lesson is based on an existing lesson developed by Vernier and Texas Instruments for use with their products. Possible Objectives for a teacher to pursue in this lesson: Scientific Inquiry Fundamental Abilities Necessary to do Scientific Inquiry: Use technology and mathematics to improve investigations and communications. Fundamental Understandings About Scientific Inquiry: Scientists rely on technology to enhance the gathering and manipulation of data. Nature of Science Fundamental Understandings the Nature of Science:: Subjective aspect of the nature of science: scientists often come to different conclusions even though they are using the same data. Subject Matter Content Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature. 2

3 Mixing Warm and Cold Water Mixing Warm and Cold Water OBJECTIVES In this investigation, you will: A Continuing Investigation of Thermal Pollution construct and use a simple calorimeter use a TI Graphing Calculator, a LabPro interface, and two Temperature Probes to measure temperature mix cold and warm water MATERIALS LabPro interface 2, styrofoam cups TI Graphing Calculator 2, 250-mL beaker DataMate program cold water 2, Temperature Probes warm water Graduated cylinder Figure 1 3

4 Investigations Start With A... Temperature of the cool water: C Temperature of the warm water: C 1. Make a prediction! What will happen to the temperature of the warm and cool water when the two are mixed together? Ten seconds before you mix? Ten seconds after you mix? Two minutes after you mix? Write your prediction below: (Shhh!! Don t share this yet!) Why do you believe this? In other words, what s your explanation for your hypothesis? Is your question testable? How will you know if your hypothesis was correct? Draw and label a graph that best represents the temperature of both the hot and cold water before and after mixing (i.e., temperature versus time). Hint: Each probe will have its own line in the graph. 4

5 Mixing Warm and Cold Water A Method & Tool that Might Help Answer Your Question.. Gathering data for graphical analysis using Vernier Probeware and TI-83 Plus Calculators: 1. Turn on the calculator, press the blue button labeled APPS, and start the DATAMATE program by pressing 2 on your calculator. [Something to keep in mind: Press CLEAR when you re at this menu to reset the program. This will often reset your sensors but it will also erase any data that you haven t saved at that point.] 2. Set up the calculator and interface for two Temperature Probes. [If the calculator shows the following then it s already been done and you re ready to go.] CH 1 : TEMP (C) CH 2 : TEMP (C) [However, if you don t read the above on your calculator s screen then you ll need to do the following:] a. Select SETUP from the main screen. b. If the calculator displays two Temperature Probes, one in CH 1 and another in CH 2, proceed directly to Step 5. If it does not, continue with this step to set up your sensor manually. c. Press ENTER to select CH 1. d. Select TEMPERATURE from the SELECT SENSOR menu. e. Select the Temperature Probe you are using (in C) from the TEMPERATURE menu. f. Press once, then press ENTER to select CH2. g. Select TEMPERATURE from the SELECT SENSOR menu. h. Select the Temperature Probe you are using (in C) from the TEMPERATURE menu. 3. Place one probe on the table and have one of your lab partners carefully wrap your fingers around the stainless steel portion of the probe. Look at the viewscreen on your calculator. Which channel s (CH1 or CH2) or probes numbers just increased. What does this mean to you? 4. Setting up the data-collection mode. a. At the first menu press 1 To select MODE, use to move the cursor to MODE and press ENTER. b. Select TIME GRAPH from the SELECT MODE menu. c. Select CHANGE TIME SETTINGS from the TIME GRAPH SETTINGS menu. d. Stop here and look up at your teacher. Do not press any buttons. We re doing to discuss what these numbers mean. e. Select OK again to return to the main screen. 5. Place a Styrofoam cup into a 250-mL beaker as shown in Figure 1. Use a graduated cylinder to get ml (100.0 g) of cold water. Be careful not to take any ice pieces. Pour the cold water into the Styrofoam cup and insert Probe 1. 5

6 7. Use a 100-mL graduated cylinder to get ml (100.0 g) of warm water from the container supplied by your teacher. Place Probe 2 into the warm water in the graduated cylinder. 8. Measure temperatures. 1. After the probes have been in the liquids for exactly 30 seconds, select START to begin collecting temperature data. 3. Let the lab pro collect data for exactly 60 seconds and then transfer the warm water and its probe to the to cup containing the cool water and its probe. 4. Use the warm water s probe to gently stir warm/cool water mixture. Continue stirring until the remaining 120-seconds of data collection is finished. 9. On the displayed graph of temperature vs. time, each point for Probe 1 is plotted with a dot, and each point for Probe 2 with a box. As you move the cursor right or left, the time (X) and temperature (Y) values of each data point are displayed below the graph. Record the temperature of this bath just prior to mixing on the next page (round to the nearest 0.1 C). Press to switch the cursor to the curve of temperature vs. time for Probe 2 (warm water). Examine the data points along the curve. Record the temperature of this bath just prior to mixing in the data section below (round to the nearest 0.1 C). Press the right arrow key on the calculator until you get to point at which you have the temperature for the mixed bath. Record this in the data section below. (round to the nearest 0.1 C). DATA Cold Water Warm Water Probe 1 Probe 2 Temperature of each bath just prior to mixing C C (Hint: you will need to interpret your graph here) The temperature of the mixed bath C PROCESSING THE DATA 1. Is the final temperature of the mixed bath the exact midpoint between the initial cold and warm water baths? 2. Does the graph created in your calculator match your prediction? Why or why not? 6

7 Mixing Warm and Cold Water 3. How did using the Vernier Probeware today improve your investigation? Hint: Compare the use of probeware to Alcohol thermometers. How would you need to change your procedure? Do you have any other sources of error when you use the thermometer? 4. What are some reasons you can think of as to why scientists rely on new technology to gather and manipulate their data? 7

8 A Sneak Peak for Tomorrow s Lesson: As we continue to investigate thermal pollution we need to have a better understanding of the movement of thermal energy. Heat can be defined as energy transferred between particles of matter because of differences in temperature. The ability of matter to transfer heat depends on its mass, temperature and specific heat capacity. The joule (J) is the SI unit we use to communicate the quantity or amount of energy. If you recall, the experts we read about in the Seattle Times article used a variety of numbers and figures to support their arguments. So far, we understand the basic idea of heat transfer in a qualitative way. Now we need to acquire some quantitative tools to make sense of these numbers from the article. Our lab tomorrow will get us started on that. We ll be using one equation that uses three ideas you re probably already familiar with (i.e., heat, temperature, and mass). We re going to build on these three ideas and add one new one, specific heat capacity. Here is an equation that puts those hour ideas together: H = t m C p where H = heat absorbed or released (in J), t = change in temperature (in C), m = mass (in g), and C p = specific heat capacity (4.18 J/g C for water). Your homework assignment for tomorrow is to be prepared to do the following: Identify one of the terms or variables (i.e., H, t, m, C p ) that remained constant (i.e., did not change) as we compared the cold and warm water baths? 8