Laboratory Experiment pubs.acs.org/jchemeduc
An Inquiry-Based Density Laboratory for Teaching Experimental Error Stephen G. Prilliman* Department of Chemistry, Oklahoma City University, Oklahoma City, Oklahoma 73106, United States S Supporting Information *
ABSTRACT: An inquiry-based laboratory exercise is described in which introductory chemistry students measure the density of water five times using either a beaker, a graduated cylinder, or a volumetric pipet. Students are also assigned to use one of two analytical balances, one of which is purposefully miscalibrated by 5%. Each group collects data using only their assigned glassware and balance. After the experiment, the complete classes’ data is compiled. The variability in the precision of the glassware and the different calibrations of the balances leads to observations of both random and systematic error in the class data. These data provide a starting point for teaching the types of error commonly encountered in introductory chemistry labs.
KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Calibration, Laboratory Equipment/Apparatus, Quantitative Analysis, Student-Centered Learning
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The lab is appropriate for high school or college introductory chemistry courses and can be implemented in a variety of ways as time allows, ranging from a 50 min high school period to a three-hour university lab.
igh school and college introductory chemistry courses typically represent students’ first encounter with experimental error, and students often struggle with the topic throughout the course. Many suggestions for teaching this subject have been made in this Journal.1−9 However, only a few of these2,3 utilize research-based methods that take into account the current understanding of the nature of teaching and learning in science10−12 and the role of the laboratory in science education.13,14 Inquiry11in which students derive an understanding starting from datahelps students construct ideas and concepts.13,15 The data acquired may also help confront previously held misconceptions.16 Calls for more widespread use of inquiry are ongoing.17−19 Inquiry will become the basis of the Advanced Placement Chemistry curriculum starting in 2013−2014,20 creating an even greater demand for the development and dissemination of inquiry-based laboratories. This paper describes an inquiry-based laboratory experiment in which students observe experimental error and analyze those observations through guided-inquiry questions.21 Students determine the density of 10 mL of water five times using glassware of varying precision and one of two balances, one of which is purposefully miscalibrated. When data from the entire class is compiled, both systematic and random error are observed. Follow-up questions guide students to an understanding of the inherent limits of measuring a quantity of water. © 2012 American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL DESIGN Students enter the lab with no prior instruction in the subject and are given no background information, only instructions to measure the density of 10 mL of distilled or deionized water (see the Supporting Information). Each student or group is assigned one piece of glassware: a beaker (low precision), a graduated cylinder (moderate precision), or volumetric pipet (high precision). Students are also assigned to one of two analytical balances, one of which is deliberately miscalibrated prior the laboratory. With the Fischer XL-300 analytical balances in our laboratory, the miscalibration is performed by placing 210 g of calibration masses on the balance and calibrating this mass to 200 g making this balance systematically low by 5%. The possible combinations of equipment assignments are summarized in Table 1. It is important before students begin to provide instruction on the correct use of their assigned glassware to include the need to read at the meniscus and at eye level. Ensuring the Published: August 10, 2012 1305
dx.doi.org/10.1021/ed2006339 | J. Chem. Educ. 2012, 89, 1305−1307
Journal of Chemical Education
Laboratory Experiment
practice using the volumetric pipets. Follow-up questions make the point that random error is observable with multiple trials but systematic error cannot be detected solely by repeating an experiment. In Part III students are introduced to the calculation of standard deviation using either Microsoft Excel or a TI-84 graphing calculator. The follow-up questions guide students to an understanding of the meaning of standard deviation as well as additional practice.
Table 1. Combinations of Equipment Assignments and Resulting Observed Precision and Accuracy Glassware 50 mL beaker 50 mL graduate cylinder 10 mL volumetric pipet
Balance 1 (Calibrated)
Balance 2 (Miscalibrated, −5%)
Low precision; Low accuracy Moderate precision; High accuracy High Precision; High accuracy
Low precision; Low accuracy Moderate precision; Low accuracy High precision; Low accuracy
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HAZARDS The use of glassware means that protective eyewear is required at all times. Pipet bulbs with end guards (such as Flinn Scientific AP7082 or AP1884) are recommended to prevent injuries that can occur when pipets break as they are inserted into bulbs.
correct use of glassware (especially the pipets) is critical for obtaining data of the expected quality. After the experiment, students calculate and post their density values and means on a whiteboard in the room. Typical student data is presented in Table 2. Similar data has been obtained with high school and college students over the past five years.
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DISCUSSION The experiment is designed to provide students with direct evidence for the existence and nature of experimental error. Students need such direct experience because of misconceptions about error stemming from the language used22 and assumptions based on limited experience, for example, that repeated measurements should yield the same result.23 Although misconceptions are persistent and difficult to overcome, the experience of this laboratory is designed to help students reach the cognitive dissonance associated with conceptual change.24,25 Although the choice of an inquiry lab is not unique2,3 nor is the use of density as an observable,4,26−28 this lab has the advantage of being quick and simple to execute (Part I can be conducted in a single 50 min high school class period) and consistently generates a large, robust data set that demonstrates both random and systematic error. The experiment can also be used as a starting point for a discussion of significant figures27,28 and is often referred to throughout the year as error is encountered in other lab experiments. The lab utilizes the framework of a process-oriented guided inquiry learning (POGIL) laboratory,29 which informs a number of design choices. First, the lab is used to introduce the subject, not reinforce concepts introduced through lecture. Second, distributed data collection is employed, allowing students to quickly collect and analyze a large number of data points. A class of 24 students working in pairs can generate 12 × 5 = 60 data points in about 40 min. This large quantity of data makes it possible for students to observe trends and make generalizations about the nature of error and its relationship to the instruments used. Third, the lab uses relatively easy to follow procedures to ensure that the data is of sufficiently high quality to achieve the desired learning objective. Given that a 10 mL volumetric pipet has a tolerance of ±0.02 mL (±0.2%),30 even novice students can produce much more precise data using the pipets than can students using the other glassware.
Table 2. Typical Student Data of the Observed Density of Water Density/(g/mL) Glassware 50 mL beaker
50 mL graduated cylinder
10 mL pipet
Balance 1 (Calibrated)
Balance 2 (Miscalibrated, −5%)
0.7924 0.8870 0.8474 0.9158 0.9455 mean = 0.8776 0.8955 0.8666 0.86665 0.8613 0.8627 mean = 0.8627 0.9920 0.9917 0.9910 0.9907 0.9897 mean = 0.9910
0.7000 0.8421 0.9137 0.8645 0.8570 mean = 0.8356 0.8617 0.8833 0.9020 0.8907 0.8966 mean = 0.8779 0.9460 0.9466 0.9432 0.9463 0.9292 mean = 0.9423
Once all of the data is posted, a brief (∼10 min) class discussion is held. Students are asked which data set is “best”. Students generally notice that the pipet data have the best internal agreement, but one of two sets gives a mean closer to the accepted value of 1.0 g/mL. At this time, the miscalibration of the balance is revealed and the vocabulary of error is introduced. To complete this portion of the lab, students work with their lab partner on a series of follow-up questions. The questions are in the form of a Process-Oriented Guided Inquiry Learning (POGIL) activity.21 The activity guides the students to examine the class data and introduces the terms precision, accuracy, random error, and systematic error. Students apply the terms to the class data as well as an additional set of model data. This portion of the lab (labeled “Part I” in the student handout) constitutes the core of the lab, but it may be followed by two optional activities as time allows. In Part II, students measure the density of a 7% saltwater solution (with a density of about 1.05 g/mL), providing all students the opportunity to
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CONCLUSION A simple inquiry-based density activity has been used to introduce students to experimental error. This laboratory provides students with an observable basis for learning and understanding the nature of experimental error in the laboratory. Such an introductory experience is important for students who will encounter these types of error throughout their laboratory experiences. 1306
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Laboratory Experiment
(29) Lamba, R. S.; Creegan, F. J. The POGIL (Discovery) Laboratory. In Process-Oriented Guided Inquiry Learning; Moog, R. S., Spencer, J. N., Eds.; ACS Symposium Series 994; American Chemical Society: Washington, DC, 2008; pp 186−199. (30) Fritz, J. S.; Schenk, G. H. Quantitative Analytical Chemistry; Allyn and Bacon: Boston, MA., 1979; p 541.
ASSOCIATED CONTENT
S Supporting Information *
Student instructions; teacher preparation notes; expected answers to follow-up questions. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author wishes to thank his students at Harding Charter Preparatory High School and Oklahoma City University who helped test and refine the laboratory.
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