Lesson 16-4

Experimental Design

    You will be called upon to design several experiments during the course of this year.  These experiments will differ from your typical laboratory activities in several ways.  Instead of following pre-designed steps, you will need to write procedures for the experiment.  The success of the activity will depend upon the quality of the procedures that you write. 
    Designing the experiment is the first, and most important step, because a poor set of procedures will render the experiment invalid.  Conducting an invalid lab activity is like going fishing without catching any fish, it is not time well spent.   If you don't catch any fish, there is little difference between standing around doing nothing and standing around with a fishing pole.  Similarly, carrying out invalid procedures is a clear waste of time. 
    Another thing to keep in mind as you design your experiment is that it must be replicable.  This means that your design and your report must be so clear that anyone can carry out the experiment exactly as you have.  That is why a clear, concise laboratory report is an essential follow up to each experiment.  This page will offer some advice about designing these experiments, in the format of your typical laboratory report.

Purpose:  The purpose of the activity will be given to you by the instructor.  Without a clear idea of what the purpose of the experiment is, you would be unable to complete the other steps of experimental design.   Keep the purpose foremost in your mind as you design your experiment.  No matter what else occurs, the purpose of the experiment must be achieved.  If you don't understand the purpose as given by the instructor, ask questions until you do.   It is imperative that you understand the purpose before you proceed.

Sample Purpose Statements:

1. "To determine the specific heat of a sample of copper."

Materials:  The materials section of your laboratory report will list all of the materials that are needed to carry out your experimental design.  Select the appropriate equipment to satisfy your needs (see lesson 1-5).   List everything that you intend to use, and be specific.   If your design will call for a beaker, state the size of the beaker that you recommend.  If your design calls for a reagent, state the mass, or size of the sample, required.  The material section serves the same purpose as a shopping list for someone who is going to try a new recipe.  It will allow a person to get everything in order before they begin the procedures.

Sample Material Section:

250 ml beaker, 100 ml graduated cylinder, Bunsen burner, sparker, ring stand, wire gauze, iron ring, calorimeter, two thermometers, tongs, a precut sample of pure copper (5 - 10 grams).

Procedure:  As stated earlier, the procedure section is the most important.  Without proper procedures, the rest of the activity is a waste of time.  Be prepared for people to critique, and look for flaws in, your procedure.  Carrying out flawed procedures can hurt your reputation in the scientific community, or at least in your Chemistry classroom.  I will offer some tips to keep in mind when writing your procedure sections.

1.  Be Specific:  The recipe analogy is a good one.   A cookbook will not have recipes that say things like "put some flour in a bowl and add some water."  Recipes are very specific, because they are designed for others to read and follow.  Your procedure section should be just as specific.   Someone following your procedures to the letter should expect the same results that you observed.

2.  Be Concise:  Don't elaborate on steps that are self-evident.  There is no need to go over the use of a balance in every procedure section.  Massing an object is a procedure unto itself, and you can assume that the reader is familiar with it.  It is sufficient to instruct the reader to "take 5.50 grams of . . ."  How to mass the reagent is up to him or her.

3.  Be Clear:  Sometimes a procedure will call for the use of more than one of a certain vessel or object.  Use letters to indicate which one you are speaking of.  If you need three 250ml beaker, instruct the reader to label them "A", "B", and "C" respectively.

Sample Procedure Section:

1. Mass the sample of copper and record your findings.
2. Assemble your ring stand, iron ring and wire gauze as shown in figure 1-1.
3. Add approximately 200 ml of water into your 250 ml beaker of water, and place it on your wire gauze "stage".
4. Ignite your Bunsen burner with your sparker and adjust the flame to the proper height.
5.  Place the Bunsen burner under the wire gauze and begin heating the water.
6.  Drop your sample of copper into the beaker.
7.  Check to make sure that both of your thermometers are reading the same temperature.
8.  Use one thermometer to monitor the temperature of your hot water.
9.  Use the graduated cylinder to measure out exactly 100.0 ml of water, then pour it   into your calorimeter.
10.  Use your second thermometer to monitor the temperature of the water in the calorimeter.
11.  Let your hot water bath reach and maintain a temperature of approximately 100.0 ^oC for about five minutes.  Then record this temperature as Cu T1.
12.  Record the temperature of the water in the calorimeter as H₂O T1.
13.  Use tongs to quickly and carefully transfer the copper from the hot water bath to the calorimeter.
14.  Monitor the temperature in the calorimeter until the temperature stops rising.   record this final high temperature as both Cu T2 and H₂O T2.
15.  Repeat the experiment as time allows.

Data Section:  When you collect data, you must organize it carefully, or you might not be able to make sense of it at a later date.   Design a data table that will compliment your procedure section.  Your data collection table should have a spot for each piece of data that your procedure calls for.   Headers can be used to help the reader find where a particular piece of data should be recorded.  Your data table should be constructed prior to carrying out the procedures.

Sample Data Section:

Analysis:  The analysis of your data is extremely important.  If you misinterpret your data, you will come up with unfounded conclusion which other readers may catch.  The analysis of your data may include calculations and/or graphing.   When you do calculations, you must show each formula that you use, and each step in your calculation.

Sample Analysis Section: Only a portion is shown (with made up data)

Conclusion:  Each instructor may require a different type of conclusion section.  You may have questions to ask and answer, or you may have an essay to write.  When you design your own experiment, your conclusion should show that you have met the original purpose as described by the instructor.  In labs that deal with any type of a quantitative result, you may be asked to calculate your percent error (See lesson 2-7).  You may also be asked to explain possible sources of error.   Often, your instructor can estimate how well you performed in lab, simply by reading your conclusion essay.  

Sample Conclusion: Only a Portion is Shown:

     ". . . As shown in my percent error calculation, my error was approximately 7 %.  There are many possible sources of error in this experiment, and I will discuss those that I feel are the most likely:

1.  Improper Insulation - The calorimeter that I used for this experiment was made of Styrofoam, which is a good insulator, but, it is far from perfect.  Some heat may have been transferred to the air around the vessel, altering my experimental values for q.

2.  Transferring Copper - When the copper was transferred from the hot water bath to the calorimeter, heat was lost to the surrounding air, altering my values for q.

3.  Transferring Water with Copper - When I transferred the copper from the hot water bath to the calorimeter, a certain amount of water clung to the metal.  This amount of water, although very little, may have thrown off my actual values for the mass of the water.

etc.

That is all of the specific tips that I can think of at this time.  Work with your instructor to find out  other ways of improving your experiments and the reports that are based on them.

Please forward all questions, comments and criticisms to Gregory L. Curran.
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Last Modified February 07, 2008