Chemistry XXI Unit 1 How do we analyze a system? Searching for Differences

Chemistry XXI Unit 1 of your General Chemistry course, both in lecture and lab, will be focused on helping you understand how to go about separating and identifying the chemical substances present in systems of interest. Unit 1: Overall Goal 1.How do we identify an unknown substance? 2.How do we separate substances? 3.How do we use models to differentiate substances?

Chemistry XXI Lab Session 2 How do we identify an unknown substance?

Chemistry XXI Initial Ideas In your own words:  What are the central goals or purposes of the experiment you will be doing today?  What knowledge and skills will help you be successful in your work today?  What do you expect to learn by doing this experiment?

Chemistry XXI Chemical thinking and practices are very useful in determining the identity and analyzing the properties of chemical substances. Detection: Does the sample contain substance X? Identification: What is the identity of the substance in this sample? Central Goals and Questions Separation: How can the substances present in the sample be separated? Quantitation: How much of substance X is in the sample? The Motives These are the types of questions we want to answer.

Chemistry XXI Imagine that you are employed by a recycling firm. You have been assigned to devise a quick and cheap method for identifying different types of plastics. The Challenge What beginning questions do you need to answer to complete your job? Given these unknown samples of plastics:

Chemistry XXI The Resources  Pure samples of known plastics.  Balances (TC 4).  Vernier calipers.  Glassware. These are the resources you have available to complete your task:

Chemistry XXI Initial Decisions Given the resources available, discuss in your groups how you propose to answer the central question for this lab: What is the identity of the unknown plastics? What differentiating characteristics would you use? What will you measure? How will you measure it? What would you do with these measurements? How will you stay safe?

Chemistry XXI Trial Tests Let′s explore! As a class, decide which groups will be determining the value of the property that differentiates the different reference plastics. What should each group do? How can you ensure reproducibility of measurements? How can you determine what technique is best? You have 30 minutes Available resources:  Pure samples of known plastics.  Balances (TC 4).  Vernier calipers.  Glassware.

Chemistry XXI  The tools you use needs to measure the property that you want (length, volume, etc.)  The tools need to measure in the range you need (accuracy).  The tool needs to meet the precision you need (tenths, hundreths, etc.)  The measurements need to be repeated.  The measurements need to be reported taking into account the precision of your tools. To Keep in Mind

Chemistry XXI Initial results Report your results for the initial tests on the board. Plastic PETE HDPE V LDPE PP PS

Chemistry XXI Your Reflections  Discuss the variation in the data and the way the data are reported. How do you explain the differences?  Why did you chose to report the numerical data as you did? Let′s think! Did you use the right measuring tools? Did you use them properly? Did you report your results properly?

Chemistry XXI Design and apply a procedure to determine the identity of the four samples of unknown plastics provided.  Determine the precision of all of the instruments that you use.  Remember that you need to do repeated measurements. You have 40 minutes Based on what you have learned: Face Your Real Challenge

Chemistry XXI Plastic PETE HDPE V LDPE PP PS Claims and Evidence Use your data to calculate the densities of the six plastic samples. Discuss how can you plan to report the value of the density taking into account the precision of each of your measurements.

Chemistry XXI Final Claims and Evidence Sample1234 Density (g/cm 3 ) Identity Report your claims about for the identity of each plastic on the board. Present and discuss the evidence that you have to support your claims.

Chemistry XXI  Discuss how the results obtained by the different groups compare with each other? Which are most credible? Which are least? What is your evidence?  What were the strengths and weaknesses of the methods you used to identify plastics?  How could you avoid or reduce the difficulties that you encountered? Final Reflections Let′s think!

Chemistry XXI Final Ideas In your own words:  What were the central goals or purposes of the experiment you did today?  What knowledge and skills would have helped you be more successful in your work today?  What did you learn by doing the experiment today?

Chemistry XXI  Beginning questions (2 p): What questions guided your explorations?  Safety Considerations (2 p): What did you do to stay safe in the lab?  Procedures and Tests (2 p): What experiments did you do to answer your questions?  Data, calculations, and representations (6 p): What observations did you make? What data did you collect? What calculations and representations helped you make sense of the data? Your Report

Chemistry XXI  Claims (2 p): What can you claim to answer your questions?  Evidence and Analysis (6 p): How did you interpret your results to support your claims?  Reflections and additional questions (10 p): What did you learn? What do you not completely understand? How have your ideas changed as a result of this lab? What new questions do you have? How would you improve what you did? Your Report

Chemistry XXI  Post-Laboratory Questions: 1.Is it practical to use density to identify plastics? Explain. 2.Propose another way to identify plastics making use of a property different from density. 3.Of the measurements you made, which were more precise; the mass measurements or the volume measurements? Why? 4.In your own words, what was the main purpose of this lab experiment. Your Report

Chemistry XXI Our Next Lab How do we separate substances?

Chemistry XXI The Challenge Imagine that you work for a clothing company interested in using natural pigments to dye fibers. Your task is to devise the most efficient method to separate the pigments in a colored natural product (such as green leaves, a vegetable, or a fruit). Write your ideas in your notebook before you come to lab. What would you propose to do? What questions do you need to answer to complete your task?