EXPERIMENT #

Christina West widmet sich in diesem Beitrag der Rolle von Experimenten bei der Wissensproduktion, insbesondere in transdisziplinären Forschungs- und Lehrkontexten. Nach einer allgemeinen Begriffsbestimmung fokussiert sie auf transdisziplinäre Experimente in engerem Sinne, die insbesondere in den transformativen Wissenschaften und in der Reallaborforschung an Bedeutung gewinnen. Diskutiert werden die hohen Herausforderungen und auch Überforderungen, die sich aus der Ergebnisoffenheit, noch weitgehend fehlenden Evaluationsmöglichkeiten und einem Doppelten Transdisziplinaritätsanspruch nach innen und außen ergeben. Die Anwendbarkeit transdisziplinärer Experimente in der Lehr- und Forschungspraxis wird am Beispiel von »Wissen to Go«, mit dem explorative Gedankenexperimente und Realexperimente im Modus der Integration entwickelt werden, und dem »UrbanUtopiaLAB«, in dem Gedanken- und Realexperimente im Modus der Addition kombiniert werden, demonstriert und veranschaulicht.


Introduction
During this chemistry lab, you will use various pieces of laboratory glassware, e.g., beakers, Erlenmeyer flasks, volumetric flasks, transfer pipettes, micropipettes, burets and graduated cylinders.All of the glassware listed above can measure volume.Why do we have so many pieces of glassware if they all do the same basic job of measuring volumes?Laboratory glassware can be divided into two types based on how they measure volumes.One is manufactured to contain certain volumes, so the volume of liquid in the glassware is well known.Another is made to deliver certain volumes; here the volume of liquid transferred is very well known.
Before using any measuring device in the lab, we should determine how many significant figures the device allows.There is no such thing as a perfect measurement.Even when using expensive lab equipment there is some degree of uncertainty in measurement When using volumetric glassware, the general rule is that you can estimate one more digit past the smallest division on the measuring device.If you look at a 10-mL graduated cylinder, for example, the smallest graduation is a tenth of a milliliter (0.1 mL).That means when you read the volume, you can estimate to the hundredths place (0.01 mL).The bottom of the meniscus is used to determine the volume in a 10-mL graduated cylinder (Figure 1).

Figure 1. Portion of a 10-mL graduated cylinder
However, some glassware such as volumetric flasks and volumetric pipettes only have a single line to indicate volume.This is because they are made to measure just one specific volume.In our chemistry lab, both the 10-mL volumetric pipette and 50-mL volumetric flask are very precisely manufactured and have two significant figures after the decimal point (i.e.10.00 mL and 50.00 mL).A beaker is less precisely manufactured; a 50-mL beaker is assumed to have a maximum of 2 significant figures.
You often need to combine laboratory measurements mathematically, such as subtracting the mass of an empty flask from a full flask to find the mass of liquid it contains.There are two simple calculation rules for significant figures.For multiplication and division, the result will have the same number of significant figures as the number with the fewest significant figures.
For addition and subtraction, the result will have the same number of digits to the right of the decimal point as the measurement with the fewest digits right of the decimal point.Two examples are given below: = 0.511 and

12.1
To help you record and analyze data, let's look at the measurements of the length of a single metal rod using three different measuring devices A, B and C.You may the obtain following experimental data and calculated results as shown in Table 1.The calculated values are explained beneath the The mean (() from multiple trials can be calculated.A calculation example for Device A is given below: The error can be positive or negative, depending on whether the experimental result is greater or less than the true value.
Accuracy is a measure of systematic error -an error caused by something in the procedure or equipment that we may be able to correct.The most accurate measurement was obtained from Device C because bias = 0.00.The lower the bias, the more accurate the measuring device.Therefore, the accuracy increases in order of Devices C > B > A in terms of both systematic error and percent systematic error.

Safety
Glassware is fragile and needs to be handled with care.Hold glassware by the side and/or bottom rather than the top.Check glassware for any chips or cracks before using it and do not use cracked glassware.Inform the instructor immediately if any glassware is broken.

Procedures
You will measure the mass and volume of water using different types of volumetric glassware indicated in Table 2.You will then use the density of water to calculate the volumes delivered.
The theoretical (true) volumes are obtained from the labels on the devices.The error for each volumetric tool can then be calculated.Three trials are required for each piece of glassware so that you can determine the precision of your measurements.1. Obtain approximately 500 mL of de-ionized water in a 500-mL clean beaker.Place a thermometer in the water.Record the temperature after there is no change in temperature for at least ten minutes.This water will be used for all of the experiments 2. Record the density of water at this temperature in the unit of grams per milliliter using the density table attached at the end of this handout.
3. Obtain a 50-mL beaker listed Table 2. Ensure that it is dry and determine its empty mass using an analytical balance.Make sure all of the doors of the analytical balance are closed, as air currents in the lab can affect the balance.Record its mass in a data table (How many significant figures should you keep?).Remove the glassware from the balance.4. From the 500-mL beaker, transfer 50 mL of water to the 50 mL beaker.Use a plastic disposable pipette to add the water drop-wise until the bottom of the meniscus is on the line that corresponds to the volume listed in Table 2.If too much water was added, remove the extra water using the plastic disposable pipette.Wipe down the outside and bottom of the beaker in case any water was spilled during the transfer.5. Using the same balance as before (why?), record the mass of the glassware with the added water.6. Pour the water from the glassware back into the 500-mL beaker and repeat steps 4-5 two more times with the same beaker using the same balance.You can use the empty mass of beaker from step 3 for all three trials.7. Repeat the process with other piece of glassware from Table 2. Do three trials for each volume.8. Create data tables in your lab notebook to record the data and calculations as shown below.Instruction for using the density table above: to find the density of water at 20.8°C, first go to the row in the left hand column at 20, then slide across that row to the column labeled with 0.8.The density of water at 20.8°C is 0.998035 g/mL.

Table . Table 1 .
Measured Lengths of a metal rod using devices A, B, and C Precision is a measure of random error, and may reflect such things as careless technique, poor equipment, or natural variation in how individuals read scales.As indicated by the data in Table1, the precision in the measurements improves from the Devices C to B to A, as indicated by the increasing trend in the S.D. and R.S.D. for Devices C > B > A.Accuracy is a measure of how close the measured value is to the true value.To assess accuracy, you need to know the true or theoretical value from some source.If you are told the length of the metal rod is 1.23 cm as a true value, you can determine the error (systematic error) and relative error.Examples are given below for Device A: 43 + 1.43 + 1.42 12 3 = 1.4267 cm = 1.43 cm How do we describe how "good" this measuring device is?We can use precision and accuracy.Precision is a measure of how close repeated measurements are to each other.The measurement from Device B has a higher degree of precision than that from Device C because the values are clustered closely together.In statistics, precision is evaluated by standard deviation (S.D.) and relative S.D. (R.S.D.).The S.D. can easily be calculated by using Excel, a web site such as https://www.calculator.net/standard-deviation-calculator.html, or a programmable calculator.