Unknown Compound Identification

Unknown Compound Identification

Activity Overview:

During this activity students work in groups to answer the question, “What is the identity of the unknown compound.” Each group is given an unknown compound and then conducts different tests to determine the identity of the ionic compound. To find the compound’s identity, students conduct flame tests, silver nitrate tests, and reactions with acids and bases. After completing and recording their data, students write up a conclusion identifying their unknown based on the results of their tests. Depending on time constraints, the lesson should concluded with review of expected results, sources of error, or applications to real world science.

Standards/ Benchmarks

CH.1The student will investigate and understand that experiments in which variables are measured, analyzed, and evaluated produce observations and verifiable data. Key concepts include

a)designated laboratory techniques;

b)safe use of chemicals and equipment;

CH.2The student will investigate and understand that the placement of elements on the periodic table is a function of their atomic structure. The periodic table is a tool used for the investigations of
h) chemical and physical properties;

g) electron configurations, valence electrons, and oxidation numbers;

CH.3The student will investigate and understand how conservation of energy and matter is expressed in chemical formulas and balanced equations. Key concepts include

d) bonding types

Aspects of Inquiry: This lesson can be structured as guided or structured inquiry. In the structured inquiry investigation students are given the research questions and procedures for each test as well as an overall procedure. In the guided inquiry investigation, students are given the research question and the procedures for each test, but they develop their own procedure for the order in which each tests is completed and they complete their own research to determine the meaning of each test’s the results.

Research Question: What the identity of this compound?
Data: Results of flame test, silver nitrate test, pH of solution, and reaction with acid and base

Below, each of these options are described.

Materials

Sodium Hydroxide solution~0.4 M
Hydrochloric Acid Solution ~0.4 M
Nitric Acid 0.4 M
Silver Nitrate solution 0.1 M
Barium Chloride 0.1M
Unknown substances labeled A, B, C
containing one of the following anions and cations:
Anions: CO3, Cl-, NO3,SO4, C2H3O2
Cations (Ca+2, Mg, Na, NH4, K)
Bunsen burner
Wire loops (for flame tests)
pH paper

Structured Inquiry Procedure (See Unknown Compound Identification Lab Procedure – Structured Inquiry Handout)

Introduction: (25-30 minutes the day before, 20-25 minutes for directions + procedure the day of)

1)Prior to the day of the lab students receive instruction on the different tests they will be performing. Break up the tests into test for anions and cations. For each of the tests, the teacher demonstrates how to properly conduct the test while demonstrating appropriate lab safety.

2)On the day of the lab, introduce the scenario:

4-5 “unknown” compounds were found in the lab. While we have a list of compounds that these unknowns might be, we need to conduct tests to determine their identity, so they can be disposed of properly.

3)The teacher then provides the test procedures to the class and split them into groups of 4-5 students.

4)Students are given five minutes to read the procedure and discuss in groups. Once students have read the procedure the teacher asks questions of the class to check their understanding of the procedure.

Activity (30-35 minutes)

1)Students work at their lab station and collect all the necessary equipment.

2)Students in each group are assigned different roles (data recorder, project director, safety manager, material manager).

3)Students follow the procedure on the Unknown Compound – Structured Inquiry handout. (Note: Procedures, Safety guidelines, and results are indicated on this handout.)

4)Students record their results.

5)Data analysis: Students synthesize the results of each test to determine the identity of the cation and anion, and therefore the identity of the entire compound.

Guided Inquiry Procedure:(See Unknown Compound Identification Lab Procedure – Guided Inquiry Handout)

Introduction: (~60 minutes)

1)Prior to creating a procedure students receive instruction on different tests to be performed. Break up the tests into test for anions and cations. (See Unknown Compound Identification Lab Procedure – Guided Inquiry Handout)

2)The teacher demonstrates how to properly conduct the test while discussing appropriate lab safety.

3)The teacher introduces the scenario:

4)Split students into groups of 4-5.

5)After students split into groups,provide a list of possible unknowns, useful tests to perform, and available materials to the students. (See Unknown Compound Lab Identification Procedure- Guided Inquiry Handout.) The unknowns should all be soluble ionic solids. Students need the handout outlining the different tests that can be used to determine an unknown.

6)Students have 50 minutes to create a procedure that they will use in lab the next day. The teacher will not give students the procedure but will ask facilitation questions such as:

-How will you be performing flame/silver nitrate/etc test? (Get students to outline how to perform a flame test or AgNO3 test – if they can’t develop their own procedure the teacher can guid e them to the references below)

-What would you expect the results to be of flame/silver nitrate/etc test?

-What do you mean by a “positive” test? What will you observe?

-If you get a certain result (i.e. ppt for silver nitrate test) what test would you do next?

-What materials will you need?

Note: It may also be useful to have students create a flow chart for what they will do based on the results of the different tests. Students should be sure to know which parts of the procedure are testing for cation, and which are testing for an anion.

7)At the end of the day students will turn in their procedure and flow chart to the teacher.

Activity (60-80 minutes)

1)Flow charts and procedures are returned to students with feedback from the teacher. The feedback should be focused not on creating the “right” procedure, but on areas where students are not specific enough (e.g. a procedure could lead to a false negative or positive, or where the procedure does not.) (It may also be necessary to explicitly tell students that the first step of the procedure should be to create a solution of their unknown, to save some of their unknown, and to make sure they are using clean glassware.)

2)Give students10-15 minutes to refine or change their procedures before they start the lab.

3)Once a group is clear on their procedure they will check it with then teacher, then go to their lab station and collect all the necessary equipment.

4)Give each group 5g of their unknown, marked A, B, C, D, or E, once they are ready to start the lab.

5)Students then conduct their procedure with oversight from the teacher. All students should conduct some kind of flame test, a silver nitrate test, and a reaction with acid and barium chloride. The order that they do these in is not as important as long as they are using new samples of their unknown to do so.

6)Once students complete their procedure they try and determine their unknown. If there results lead them to a nonexistent unknown, or the data are conflicting, they may re-do parts of their procedure.

Debrief (for both structured and guided inquiry) (15-20 minutes)
Once a group believes they have identified their unknown, they write a thorough explanation of what data led them to their conclusion on the identity of their unknown.
If students data does not lead them to identify their unknown, the teacher may give feedback, or have students redo part of their data. However students are primarily be assessed on the reasoning behind their explanations, instead of the correct answer.
Following this, the teacher briefly presents what each of the unknowns (A, B, C, D, or E) were, the properties of that solid, and what the result of each test should have looked like.
If there is extra time, the teacher may discuss possible sources of error with the class, applications to real life lab safety, and how their process reflects nature of science tenets.
Possible discussion questions:
How might this reflect what a scientist might encounter in the lab?Expected answer: Scientists could have unknown compounds in the lab, scientists could need to determine the properties of an unknown, scientists might need to test the properties or reactivities of a new compound. Also the uncertain nature of the identification reflects the uncertainty scientists encounter in expirements -> need for multiple trials and multiple tests for same result
How certain are you of your groups identification of the unknown?
EA: Depending on a groups data they may be very uncertain-> pretty certain of their identification. Groups that are uncertain could conduct additional tests to compare their unknown versus a known solution that contains their suspected anion or cation
What could you do to be more certain? EA: Conduct additional test, compare unknown solutions to other known solutions, test pure water (control) for each of the tests, conduct research of their suspected unknown online and compare it’s appearance and physical and chemical properties, conduct other anion and cation tests
Can we ever know for certain what the unknown is?EA: No, based on our tests we can never be totally certain only get a better idea of the identity of our unknown. More accurate tests (mass spec, gas chromatography, other quantitative analysis) would probably be necessary to be 99% percent certain of the unknown. Even then we cannot truly know for certain.
How does this reflect science as a social activity? EA: Students had to work together in groups to identify their unknown. Different students may have had different ideas on how to design/conduct procedure which may or may not have led to different results. Additionally other students in the class could evaluate students results and give them feedback on ways to improve their procedure or results
Would this experiment be more effective if multiple groups were testing the same solid? EA: Probably. More students conducting the same test would lead to more trials, making it more likely that we get the “correct”
results for each test. However even if multiple groups tested the same unknown there is still the possibility of unknown variables changing students results. Example (large possibility of contamination of glassware, or from the use of tap water, especially for Na+ and Cl- ions. This may lead all students to misidentify their unknown as containing Na+ or Cl-. Additionally for the cation test many times other compounds on the wire may burn yellow and give a false positive identification for a cation.
How might we control sources of error?EA: Have multiple students conduct tests on same unknown. Be extremely diligent in cleaning glassware before and after the experiment. Conduct multiple trials within the same group. Use a control (conduct all tests with water used for solution, before using unknown solution or conduct all tests compared against known solutions).

Modifications

-Give all groups the same unknown. Groups will then all conduct the same tests. Each group would then put their results on a large chart on the board, and from there the class could decide the identity of their unknown compound.

-Students could write balanced chemical equations that took place for any positive test results.

-Give students post-lab questions to check their understanding. These questions could include application – have them identify an unknown based on provided data, identify sources of error in their tests, or improve on a given test procedure.

-This activity could also be modified based on the unknowns you use. If you want students to be able to compare molecular and ionic compounds you could add in a few possible molecular compounds (sugar, phosphorous, sulfur, charcoal (carbon)) to the unknowns. Students would then also want to conduct conductivity and solubility tests (to determine if the compound is ionic or molecular).

Assessment Strategies

Structured inquiry:

-Students’ explanations and the evidence they use for their unknown. The teacher previously determined what each of the unknowns are, so it will be easy to check a students work, but the main point of the activity is to have students create a real scientific argument.

-Additional assessment depends on where this activity takes place in the greater chemistry curriculum. It may be useful to design a pre-lab that links the knowledge students are learning in class (How to write/identify ionic compounds) with the lab. If students had already learned about writing or predicting reactions students could be asked to write equations for the reactions that took place in the lab. It would also be possible to have each student write their own write-up or explanation for the lab, or how each part of the procedure (flame test, reaction with barium chloride, silver nitrate, and acid) helped identify the unknown.

Guided Inquiry:

-Assessments similar to the structured inquiry lab is also possible within the guided inquiry lab

-Designing the procedure is also an assessment. During this point the teacher should ask groups questions to build understanding, and also as a formative assessment of students knowledge. When students turn in their procedures to receive feedback the teacher should focus on problem areas, or to address large errors in the procedure.

-After students complete their investigation students could be asked to comment on sources of error and how they might refine their procedure in the future as an individual writing assignment, or small group or full class discussion.

Sources:
University of Virginia: Original Identifying an Unknown Compound
Lab Safety + Set procedure: Royal Society of Chemistry (
Lab Roles and Descriptions: