Many science classes begin with an overview of equipment. Students memorize the names of laboratory glassware and their functions but rarely determine accuracy or relate glassware accuracy to function. In this activity, students employ density calculations to establish the accuracy of several common pieces of laboratory glassware and make a claim relating the accuracy of a piece of glassware to its designated purpose. Accuracy is defined as how close a measurement is to the true or standard value. Students compare accuracy values for different pieces of glassware using percent error calculations and use percent error as evidence for justifying a claim relating glassware accuracy to use.
Students may have seen wet (or liquid) and dry measuring cups in the kitchen and wondered if a wet cup is equal in volume to a dry cup. This quick demonstration is an easy segue into the concept of laboratory glassware accuracy. Obtain dry and liquid kitchen measuring cups. The volume of the dry cup does not matter. Fill the dry cup with water, and then pour the water into the liquid measuring cup. Ask students to observe. Repeat the demonstration with a dry substance—flour, sugar, rice, and small beans are good examples. Ask students to make a claim about the accuracy of the measuring cups.
MS-ETS1-3. Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a newsolution to better meet the criteria for success.
Remind students to be mindful of where they place glassware. Have a broom, dust pan, and a container for broken glassware available. Remind students not to pick up broken glass with their hands.
Group glassware prior to the activity. Make certain the volume of water the volumetric flask holds will fall between the graduations on the other pieces of glassware. No chemical or waste disposal is necessary. Dispose of any broken glass in the appropriate container.
Student answers will vary.
A. Visual Inspection
B. Glassware Accuracy Using Density Calculations
Based on the data collected, rank the 4 pieces of glassware from most accurate to least accurate. Explain your reasoning.
Student answers will vary, but they should notice higher accuracy for the volumetric flask and graduated cylinder.
Using individual data and the density of water table, calculate the actual volume of water in each piece of glassware.
Density = mass/volume
Average the individual actual volumes for each piece of glassware and place the values in the table.
Calculate the percent error for each piece of glassware and record the values.
Based on percent error only, rank the glassware from most accurate to least accurate. Use your data to explain your ranking.
Volumetric flask, graduated cylinder, beaker, flask
The volumetric flask had the lowest percent error (0.3%), so it is the most accurate. The graduated cylinder and beaker were similar in percent error (2.2% and 3.5% respectively). Both were small, so they are fairly accurate. The flask had the highest percent error (17.4%), so it is the least accurate.
How do the 2 methods of determining accuracy compare? Were the results the same or different?
The rankings were the same. The first method provided a less precise ranking of glassware since the volume of water had to be estimated for every piece of glassware but the volumetric flask. The second method was more precise because volume was calculated using density, the temperature of water, and a digital balance.
Based on accuracy, assign each piece of glassware appropriate uses and state evidence for your choice. For example, which pieces should be used for measurement, and which should not be used for measurement?
The volumetric flask is the most accurate, but it only measures 1 volume—in this case, 100mL. If you need to measure other volumes besides those on volumetric flasks, then the graduated cylinder is the best tool. The beaker is pretty accurate, and you can easily stir and heat chemicals in it. I would not use a flask to measure volume given the large percent error. If volume only had to be estimated, a flask would be OK. It is also good for mixing and heating.
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