The Density of an Unknown Liquid
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1 Name: 8 th Grade Science Essential Labs The Density of an Unknown Liquid Date: / / Scientific Question How do you determine the density and identity of an unknown liquid? Objectives SWBAT: Measure a liquid s volume & mass using appropriate tools (graduated cylinder, scale) Calculate density given the equation: Density = Mass/Volume (D = m/v) Calculate density by reading a line of best fit on a graph of Mass/Volume Background Information Scientists study the properties of matter for many reasons. One reason is to help them identify unknown substances. Density is often used to identify an unknown substance. The density of the unknown is compared to a list of known substances to determine its identity. It is very unlikely for two substances to have the same density. In this experiment, you will use density to figure out the identity of an unknown liquid. Safety Alerts Wear goggles when using glassware. Do not drink any substances in the lab including the unknown liquid that is provided. Measurements The unit of measurement for mass is grams (g) The unit of measurement for volume is milliliters (ml) Density s unit of measurement is g/ml or g/cm^3 Materials In a basket, you would place a 10mL graduated cylinder, a plastic pipette, a cup with the unknown liquid. Each lab bench would also need an electronic balance. Post-Lab Assignment 1. A liquid has a mass of 4.5 g and a volume of 5.5 ml. What is the density of the liquid? 2. In the previous problem, the student was given the mass of a liquid. Describe how you would find the mass of a liquid. Page 1 of 23 Lab Procedures: 1. Measure the mass of an empty 10mL graduated cylinder using an electronic balance. Record the mass of the graduated cylinder here and in your data table. Record the volume of the empty graduated cylinder as 0 ml in (Data Point 1 under volume ) since your graduated cylinder is empty. 2. Add 2 milliliters of the unknown liquid to your graduated cylinder using a pipette. Record the exact volume that you added in your data table here and in your data table. (Data Point 2). 3. Measure the mass of the graduated cylinder with the liquid. Record this mass in your data table (Data Point 2). 4. Do not discard, which means do not throw away, the liquid or empty out your graduated cylinder! 5. Add another 2 milliliters of the unknown liquid to your graduated cylinder. Record the exact volume that you added here and in your data table (Data Point 3). 6. Measure the mass of the graduated cylinder with the liquid. Record this mass here and in your data table. 7. Repeat steps #4 - #7 three more times to fill in the remainder of your Data Table.
2 Data Record your data table in the chart below. Remember no naked numbers! -add units to ALL your numbers! Data Point Table 1: Data Collected from Experiment Mass of Volume Volume of Graduated of liquid Liquid in Mass Cylinder + added Graduated Of Graduated Liquid Cylinder each time Cylinder g g ml ml Mass of Liquid in Graduated Cylinder g Density of Liquid g/ml d=m/v g 0mL ml Graph Create a graph of your data of each density from all 7 data points. Remember to put your independent variable on the x-axis, which is and the dependent variable on the y-axis, which is. Create a scatterplot by plotting all your data points on the graph no lines connecting them, just points scattered on the graph After you have plotted, draw a line of best fit this is the one straight line that seems to fit all the points you plotted in the scatterplot. Determine the slope of that line. Show your calculations in the box below. Calculation of Slope of the Line of Best Fit: HINT! Slope = rise/run Slope = ΔY/ΔX 8 th Grade Science Essential Labs Page 2 of 23
3 CFU Question #3: What things are needed on a proper and accurate graph? Analysis Questions 1. The table below contains different liquids and their densities. Water 1.00 g/ml Butane 0.60 g/ml Citric Acid 1.66 g/ml 1. Which liquid is most likely the same as your unknown liquid? Justify your answer in complete sentences. Support your answer by explaining your reasoning or showing calculations. 2. What is the average density of your unknown liquid? 3. Predict what would happen to the density of your liquid if you heated the liquid. Explain your reasoning. 8 th Grade Science Essential Labs Page 3 of 23
4 Conclusion Using complete sentences, answer your scientific question on page 1. Use specific data from your lab investigation as evidence. 2. Ricardo collected the following data in his experiment for the density of an unknown liquid. Volume of Liquid 9 ml Mass of Empty Graduated Cylinder Mass of Graduated Cylinder + Liquid 26 g 39 g 1. A scientist found a bottle without a label in her lab s chemical closet. The bottle contained a liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below. Based on his data, what is the mass of his liquid? a. 9 ml b. 3 g c. 13 g d. 26 g e. 39 g What is the volume of Vivian s rock? If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical? a. water b. vinegar c. carbonic acid d. ethanol e. isopropyl alcohol a. 10 ml b. 20 ml c. 30 ml d. 40 ml e. 50 ml 8 th Grade Science Essential Labs Page 4 of 23
5 Teacher Notes for the Density of an Unknown Liquid General Description In 6 th grade, students were exposed to basic density calculations and focused on determining the density using water displacement. For our 8 th graders, we want to look at a different example, the density of the liquid. The goal is for them to determine the density and ultimately its identity from a list of known liquids. The other component is to begin to expose students to the idea that the slope of the line is often significant in science. This is an important concept for physics and AP science courses. Consumable Materials Materials Amount Per Group Cost Vendor Item Number food coloring (optional) 4-5 drops varies Wal-Mart Scientific Equipment Equipment Amount Per Group Cost Vendor Item Number Graduated Cylinder, 10 ml 1 $15.20 Flinn Scientific GP2005 Electronic Balance 1 $95.95 Flinn Scientific OB th Grade Science Essential Labs Page 5 of 23
6 Student Completion Time Pre-Lab Assignment: 15 minutes o If you want to review the pre-lab assignment before starting the lab, you can assign the pre-lab assignment as the homework assignment before the lab. Data Collection: 15 minutes o If you are worried about time, you could potentially use electronic balances to expedite this process. Calculations and Graphing: 20 minutes o Before your students begin with the analysis and conclusion questions, it is recommended that you go a quick check of their data, calculations and graph. Analysis and Conclusion 25 minutes o Students may not be able to finish their conclusion in this time frame; however, they can finish the rest for homework. o Your other option would be to give students more time and have the students who finish early work on density practice problems or an extension activity. Common Mistakes or Misconceptions Procedural Mistakes Most 10mL graduated cylinders are marked every 0.2 ml. Students often assume that markings represent 1 ml or 0.1 ml. One quick CFU would be to place a 10 ml graduated cylinder at each table and fill it with a known amount of liquid. You could then let each table read the graduated cylinder at their table. You could then get them to share out their reading and check against the reading that you took. This will tell you who needs more help reading the volume from a graduated cylinder. When a procedure asks students to add 2 milliliters of a liquid, they often assume that the amount of liquid that they put in the graduated cylinder is 2 milliliters. Be sure that students are adding the liquid and then using the graduated cylinder to measure the actual volume. You may want to provide students with a plastic pipette, this may make data collection more efficient because students will not fast time dumping excess water and then adding water back because they dumped out too much water. Students may dump the liquid out after measuring its mass, so they end up with a data points that all measure the same (or very similar) volumes of the liquid. When walking around the room, check student data to ensure that they are getting data for different volumes of liquid. Content Based Mistakes Students have been taught independent and dependent variables in 6 th grade. However, you may want to plan a 10 minute quick review of the concept prior to completing the lab. Students will be creating a scatter plot. They have created graphs in math class, and we want to be sure that we are holding them to same expectations. One common mistake is the scaling for the x- and y-axes. You want to ensure that students have a consistent scaling. 8 th Grade Science Essential Labs Page 6 of 23
7 Students will not have been introduced to the concept of a line of best fit. Plan to introduce this concept to your students either before the lab or after data collection. (project or print out the visuals to help explain these concepts) Teacher Preparation Instructions Preparation To help prevent students from knowing the unknown is water. You can add a small amount of food coloring, and it won t significantly change the density. Labs can be chaotic especially with middle school students moving around one classroom. Consider putting all of the materials at their lab benches. In a basket or small plastic tub, you would place a 10mL graduated cylinder, a plastic pipette, a cup or beaker with the unknown liquid. Each lab bench would also need a balance. Ideas for Differentiation For students who struggle, you can: o Eliminate a data point. This will reduce the amount that they have to collect and the amount they have to graph while still getting the full scope of the experiment. o Provide students with one or both scales for the axes. For students who need more of a challenge, you can: o Have them support or refute their prediction about the heated liquid. You can have them explore with warm water. o Have them explore how the addition of salt affects the density of a liquid. o Develop a reading about how density of the ocean affects ocean currents. Comment [PI1]: I d like to provide teachers more information than this about how to introduce this; however, I have no idea what should be taught nor how it should be taught. Disposal Since the unknown liquid is water, it can be disposed of down the sink. Answers to Pre-Lab Assignment 1. Would you classify density as a physical property or a chemical property? Justify your answer. Density is a physical property. To determine the density of an object, you do not have to change its composition or make-up. 2. A liquid has a mass of 4.5 g and a volume of 5.5 ml. What is the density of the liquid? 4.5 g D m 0. V 5.5 ml In the previous problem, the student was given the mass of a liquid. List the steps you would take to find the mass of a liquid. 1. Measure the mass of an empty container. 2. Record the mass of the empty container. 3. Add the liquid to the empty container. 4. Measure the mass of the empty container and the liquid. 5. Record the mass of the empty container and the liquid. 6. Subtract the mass of the empty container from the combined mass of the empty container and the liquid. 4. Find the slope of a line in the graph below. g ml 8 th Grade Science Essential Labs Page 7 of 23
8 Slope rise run Sample Data Data Point Volume Mass ml 27.0 g ml 28.7 g ml 30.2 g ml 31.7 g ml 33.9 g ml 36.1 g Sample Calculations 1. For your second data point, calculate the mass of the liquid. Show your work in the space below g 27.0 g = 1.7 g 2. In the chart below, record your volume and the mass of the liquid. You will have to do the same calculation that you performed in the previous question. However, you don t have to show your work for the other data points. Table 2: Data from Your Calculations Data Point Volume of Liquid Mass of Liquid ml 0 g ml 1.7 g ml 3.2 g ml 4.7 g ml 6.9 g ml 9.1 g 8 th Grade Science Essential Labs Page 8 of 23
9 Mass (g) Sample Graph Density of an Unknown Liquid Volume (ml) Answers to Analysis Questions 1. What does the slope of your line represent? The slope of the line represents the density of the liquid. 2. What is the density of your unknown liquid? Support your answer by explaining your reasoning or showing calculations. Answers will vary but should match experimental data. You also want students to use the slope of the line by using two data points closest to their line of best fit. 3.2 g 1.7 g Density 0.94 g/ml 3.6 ml 2.0 ml 3. The table below contains different liquids and their densities. Water 1.00 g/ml Butane 0.60 g/ml Citric Acid 1.66 g/ml Which liquid is most likely the same as your unknown liquid? Justify your answer in complete sentences. Answers will vary, but you want students to answer base on their answer to the previous question. Here is an example. The unknown liquid is water because the density of the unknown liquid is the closest to the value for the density of water. 8 th Grade Science Essential Labs Page 9 of 23
10 4. Your lab partner, Julian, spilled some of the unknown liquid on the balance. He never cleaned it up. a. What part of your data would this affect: the mass or the volume? Explain. It would affect the mass of the liquid since the liquid was on the balance but not in the graduated cylinder. b. How would this affect your experimental value for your density? Justify your reasoning. The mass that I would read from the balance would be greater than what it should be. Since the mass would be bigger, the calculated density would be bigger as well.. 5. Predict what would happen to the density of your liquid if you heated the liquid. Explain your reasoning. The liquid s density should decrease because as you heat the liquid, the particles will gradually spread apart and occupy a bigger volume. Since the mass will be the same and the volume will be bigger, this would make the value of the density decrease. 8 th Grade Science Essential Labs Page 10 of 23
11 Mass (g) Mass (g) Visual Aids Density of an Unknown Liquid (blue) Volume (ml) Scatterplot data points graphed with no lines Density of an Unknown Liquid (blue) Volume (ml) Line of best fit line that is implied by the points 8 th Grade Science Essential Labs Page 11 of 23
12 slope of a line steepness of a line 8 th Grade Science Essential Labs Page 12 of 23
13 Practice Graphs 8 th Grade Science Essential Labs Page 13 of 23
14 Name Date Period Assessment on Lab #1: Density of an Unknown Liquid 8 th Grade Science Essential Labs Page 14 of 23
15 Objective #2 Question #s % Mastery Mastered? Objective Summary # Correct #1 I can calculate density given the equation: Density = Mass/Volume. /1 #2 I can measure an object s volume & mass using appropriate tools. /1 #3 I can calculate density to identify an unknown substance. /1 #4 I can calculate density by reading a line of best fit on a graph of Mass/Volume. /2 % Mastery Mastered? Directions: Do the best you can to answer the following questions. Take your time and read Objective #1 Question #s % Mastery Mastered? RETAKE? I can calculate density given the equation: Density = Mass/Volume. 1 0 correct 1 correct 50%...100% Over 80%? Y/N carefully. You need to get 4 out of 5 on the entire quiz to earn an 80% or higher (mastery). 1. Ricardo collected the following data in his experiment for the density of an unknown liquid. Volume of Liquid Mass of Empty Graduated Cylinder Mass of Graduated Cylinder + Liquid 9 ml 26 g 39 g Based on his data, what is the mass of his liquid? a. 9 ml b. 3 g c. 13 g d. 26 g e. 39 g 8 th Grade Science Essential Labs Page 15 of 23
16 I can measure an Objective object s volume #3 & Question 0 correct 1 correct 2 #s % Mastery Mastered? Over 80%? mass using appropriate 50%...100% Y/N tools. RETAKE? 2. Vivian finished her experiment early. With the teacher s permission, she decided to conduct her own experiment to determine the density of a rock. Below is a picture of part of her experiment. What is the volume of Vivian s rock? a. 10 ml b. 20 ml c. 30 ml d. 40 ml e. 50 ml 8 th Grade Science Essential Labs Page 16 of 23
17 RETAKE? I can calculate density Objective to identify #4 0 correct 1 correct an Question Over 80%? 3 #s 50%...100% % Mastery Mastered? Y/N unknown substance. I can calculate density by reading a line of best 0 correct 1 correct 2 correct Over 80%? 4,5 fit on a graph of 50%...50%...100% Y/N Mass/Volume. RETAKE? 3. A scientist found a bottle without a label in her lab s chemical closet. The bottle contained a liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below. If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical? a. water b. vinegar c. carbonic acid d. ethanol e. isopropyl alcohol 4. The graph below shows the mass versus volume for three liquids of different densities. Which of the following statement(s) is/are true? 8 th Grade Science Essential Labs Page 17 of 23
18 I. The slope of the line represents the density of the liquids. II. Graph I represents the liquid with the greatest density. III. Graph III is impossible because as the volume of a liquid increases, the mass should also increase. a. I only b. II only c. I and II only d. I and III only e. I, II, and III 5. The graph represents the data collected from a student s experiment to determine the density of an unknown liquid. 8 th Grade Science Essential Labs Page 18 of 23
19 a. Calculate the density of the liquid. Show all work in the space below. b. Your lab partner wanted to use data point A. Would you agree or disagree with your lab partner? Explain why. 8 th Grade Science Essential Labs Page 19 of 23
20 Name Key Date Period Assessment on Lab #1 (Density of an Unknown Liquid) 2. Ricardo collected the following data in his experiment for the density of an unknown liquid. Volume of Liquid Mass of Empty Graduated Cylinder Mass of Graduated Cylinder + Liquid 9 ml 26 g 39 g Based on his data, what is the mass of his liquid? a. 9 ml b. 3 g c. 13 g d. 26 g e. 39 g 3. Vivian finished her experiment early. With the teacher s permission, she decided to conduct her own experiment to determine the density of a rock. Below is a picture of part of her experiment. What is the volume of Vivian s rock? a. 10 ml b. 20 ml c. 30 ml d. 40 ml e. 50 ml 8 th Grade Science Essential Labs Page 20 of 23
21 4. A scientist found a bottle without a label in her lab s chemical closet. The bottle contained a liquid that is cloudy in color and odorless. She measured the mass of an empty graduated cylinder. Then, she measured the mass and volume of the chemical and compared her findings with the data table below. If the mass of the empty graduated cylinder is 45 g, what is the identity of the chemical? f. water g. vinegar h. carbonic acid i. ethanol j. isopropyl alcohol 5. The graph below shows the mass versus volume for three liquids of different densities. Which of the following statement(s) is/are true? 8 th Grade Science Essential Labs Page 21 of 23
22 I. The slope of the line represents the density of the liquids. II. Graph I represents the liquid with the greatest density. III. Graph III is impossible because as the volume of a liquid increases, the mass should also increase. a. I only b. II only c. I and II only d. I and III only e. I, II, and III 6. The graph represents the data collected from a student s experiment to determine the density of an unknown liquid. 8 th Grade Science Essential Labs Page 22 of 23
23 a. Calculate the density of the liquid. Show all work in the space below. 18 g 6 g g Density 2 9 ml 3 ml ml Students, who calculate the density using the slope of the line, should be awarded full credit, while students who use a single point should only be awarded partial credit even though they will get similar values. b. Your lab partner wanted to use data point A. Would you agree or disagree with your lab partner? Explain why. I would disagree because point A is one of the points farthest from the line of best fit. 8 th Grade Science Essential Labs Page 23 of 23
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