Embedded Files
Title: Determining substances based on physical and chemical properties with help from indicators.
Principle(s) Investigated: Chemical properties, physical properties, chemical reactions, using indicators, finding patterns, creating qualitative data, constructing conclusions of qualitative data, identifying unkown substances.
Standards: MS-PS1-2 : Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occured.
SEP: 1,2,3,4,6,7,8
CCC: 1,2,7
Materials: Reaction Plates, magnifying glasses, small scoops/spoons, baking powder, baking soda, cornstarch, table salt, worksheets (per individual), dropper bottles, water, vinegar, and iodine solution.
Procedure: Students should be reminded of proper safety procedures during labs, especially those that include chemicals/chemical reactions. Students should wear safety goggles, pull back long hair, and be careful not to get any substances on their skin or clothes.
The teacher will introduce the problem students will have to solve in the course of their investigation.
The problem: You and your cousin were making some cookies at home. Your cousin scooped out two substances, but forgot which ingredients they were. You will have to identify the substance (without tasting them) using materials you can find at home.
Students will work individually on the front side of the "White Before Your Eyes" worksheet to Gather Information to recall what the differences are between physical properties and chemical properties. When the students have completed the section section they will then read the the Procedure for the lab in step 3. Students can then individually answer the Review of Procedures section to reinforce important steps in the lab.
The teacher will then break students into their lab groups, usually groups of 3 or 4, and have a member of each group collect the materials they will need for the lab. Each group should collect one magnifying glass, one reaction plate, one container of baking powder, one container of baking soda, one container of cornstarch, one container of table salt, one dropper bottle of water, one dropper bottle of vinegar, one dropper bottle of iodine solution and bring them back to their groups.
Students will place one level scoop (1 gram) of the baking powder in one column of the reaction plate until 4 wells have the substance in it. The group will repeat this process for the other three substances organizing the columns so they match the sequence on the the back of the worksheet. In the first row the groups will leave the substances dry, observing the substances with the magnifying glass and determining whether the substance is crystalline or powdery. In the second row the students will place 10 drops of water into each substance one at a time, recording whether the water dissolved and whether bubbles were produced. 10 drops of Vinegar will be places into each well in the third row one by one to determine if the substance was clear/cloudy, and if bubbles were produced. Finally, in the last row the students will put 5 drops of iodine and mark whether the the substance stayed yellow/brown like the iodine or fi it changed to a purple/black color. Students should be encouraged to write additional information they may find important, such as the size of the grains of the crystalline substances, or whether the bubbles were small fizzing reactions or large bubbling reactions to differentiate them from other stubstance reactions.
Once students have completed their charts they can ask the teacher for the mystery mixture. The teacher will inform the students that the container holds two substances and they will need to conduct the same procedure they did with the other substances to try and solve which two substances are mixed together. After the students gather their data they will move to section 4 of the lab worksheet and write their predictions, along with their justifications as to why they guessed those substances.
Once all groups complete this step, the instructor should have all students wash their reaction trays and return all equipment safely back to the lab trays. The teacher should then post an empty grid of the known substances and have groups provide their data until the grid is complete. If groups are in conflict about an answer, they should engage in conversation until a verdict is reached or until the process is replicated for a satisfactory result. Once the grid is completed, the teacher should have students look for patterns in the data. Afterward, the teacher should pull up an new grid that has each group number, the observations from their mystery powder testings, and the two substances the group believes are in the mystery powder. The class will engage in another discussion about this new data, and each group should explain how they formed their conclusions based on the data they collected.
The teacher will then need to inform the class what the mystery combination is so they can continue their worksheet.
Once the students know the mystery mixture they can complete the fifth section of the worksheet and finish the remaining questions in section 6.
Student prior knowledge: Students should already have a firm understanding of what chemical and physical properties are. This lab will help them apply that knowledge in a chemistry setting. Students should also know safety procedures/protocals, how to measure correctly, how to follow lab procedures, use graphs, analyze/interpret data, and form conclusions.
Explanation: The substances that have been chosen for this experiment are intentional. Each substance has similarities and differences with each other substance but no two substances are exactly alike. This makes it just difficult enough for students to determine the mystery mixture without it being too easy. This lab is a stepping stone between introducing chemical vs physical changes, understanding reactants and products in chemical equations, endothermic/exothermic reactions, and the law of conservation of mass. It is also a great opportunity for students to practice following simple procedures in labs using chemicals and safety equipment before they move onto more corrosive substances or chemicals.
The teacher is able to discuss which aspects of the lab are physical changes and which are chemical based on students prior knowledge of the properties of each. Depending on what activities have already been completed in the class, the teacher may also be able to share what is happening chemically between reacting substances at a deeper level than simply identifying a "change of color" or "gas being produced". These conversations should also focus around indicators in chemical reactions.
The greatest purpose of this lab for my class is to draw conclusions from evidence that is not numerical. My classes are familiar with finding patterns in quantitative data , but have little experience doing it from qualitative data. In fact, I stress to my students that even if they do not correctly identify the two mystery substance, as long as their conclusions are supported by their observationial data then they accomplished the purpose of objective of the lab.
As a MODIFICATION for gifted students, do not label the substances provided to them. Instead they will have to observe and conduct tests on each substance and then match their results to a pre filled chart to find out which substance is which before moving onto the mystery mixture.
A MODIFICATION for ELL's or with special needs is to demonstrate the activity to the students and have them write down the information from their observations.
Questions & Answers:
How did you use the data to identify the substances in the mystery mixture?
Many students will compare and contrast data from their substance chart and compare it to their mystery powder observations. The challenge of this includes some reactions preventing the results of a non-reaction from being clearly seen. For example, the baking soda reacts tiwh the iodine solution but the baking soda does not. If these two powders are the mystery mixture, then students need to think critically and use other identifiable means to determine that both substances are not just the ones that react with the iodine because the baking powder reaction will create a result that looks that way even the the baking soda does not react.
What are factors that could have affected your results for any reaction that was not consistent with the correct substance reaction chart?
Not following lab procedures is generally the main culprit for bad data. Students may have put the wrong substances into the wrong wells of the reaction plate. Students may have used the same spoon to scoop the substances which accidentally caused substances to mix when being placed onto the reaction plate. I have also had students use vinegar instead of water accidentally, and vice versa, because they look the same. Not using the magnifying glass to look carefully at each substance to determine if it is crystalline or powdery can be another problem. Finally, not writing enough detailed information can also prohibit students from identifying the mystery powders.
Why do scientists use indicators?
Scientists use indicators to determine whether chemical reactions are occuring or not. They have many different uses including determining pH levels, determining the presence or absence of a molecule, to detect specific reactions, to identify unkown substances and so on.
Applications to Everyday Life:
Using indicators is crucial to discovering the pH of water . The U.S. Geological Survey uses pH to determine environmental factors that may occuring based on water samples. For example, abnormal water pH may affect organisms that live in that water or may indicate an increase of pollution that has reached the water source.
High water pH can be taste bitter and encrust water pipes with deposits. On the opposite end, really low pH can even dissolve or corrode the metal pipes it is passing through. Obviously, these two factors can be extremely important in drinking water being provided to homes.
Chemical and physical indicators can be used to test soil for positve and negative characteristics to determine successful plant growth. They can show what nutrients are in the soil or if there are any contaminants as well.
Videos: The link below shows a very similar experiment packaged a little differently.
https://www.youtube.com/watch?v=XATCq7GQ7wc
Worksheet Link: Collaborative Worksheet
References:
https://www.thoughtco.com/definition-of-indicator-605239
http://www.cfep.uci.edu/cspi/docs/lessons_secondary/Mystery%20Powder.pdf
https://water.usgs.gov/edu/ph.html
http://soilquality.org/indicators.html
inquiryinaction.org
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