Chemical and Physical Changes Lab
In this activity (third in a series), students develop observation- and peer argumentation skills. They mix chemicals without any preconceptions of what may occur. This approximates the discovery process that Mendeleev and his precursors experienced discovering properties of the sixty-some elements that provided enough data from which to generalize a periodic law. From standard textbook definitions given by the instructor, they must create their own criteria to distinguish a physical change from a chemical change, but they must draw on observable evidence.
The procedures chosen include some obvious physical or chemical changes, and some which can be interpreted with less certainty, creating another opportunity for students to defend their conclusions in a peer review. It is important that student lab reports are evaluated not on whether or not a physical or chemical change is correctly identified, but by how successfully they defend their conclusions based on the observations they made in lab. A large group criteria is then created, hopefully, leading to some consensus. Finally, a lab performance assessment is used via laserdisc, in which students observe a new experiment, make observations, write and defend their conclusions.
Purpose: To identify chemical and physical changes in matter.
Background: Most science research is done by people working as part of a team. In most fields, individual success is dependent on the group. Your skills as a team member are important to any future job or relationship with other people. Research also shows that students of all ability levels learn better in cooperative groups.
This lab will be a group effort, and each member will be responsible for the learning of the other members. Each member of the group will write a section of the lab, and these sections will be put together to determine the group's grade for the lab report.
Each member of the group will be "in charge" of a section of the lab. This means that she/he will:
Theory: A physical change is a chenge in the appearance of a substance, but not its identity. A chemical change produces one or more new substances.
- prepare a data/observation table in advance to be filled out during the lab,
- execute the lab techniques properly, teaching the other members of the group how to do them,
- and write the conclusion for that section.
Data/Observations: The quantities of chemicals used in this lab are not absolute--this is a qualitative procedure. Do not waste time measuring the exact amount--do not double or triple the amounts, either--a close estimate is good enough.
Have a data chart or observation section prepared before you come into lab, so that it can be filled out as you do the lab, possibly by another member of your group. It will be especially important to notice and describe the appearance of the reagents (another word for chemicals) BEFORE, DURING, and AFTER the experiment. Be sure to write both qualitative and quantitatvie observations.
Conclusions: As a group, you must create a crtiteris which you can use to distinguish between a physical and a chemical change. Use observations from the lab procedures to create this criteria. Then, for each procedure, write a conclusion:
- Was there a chemical or physical change, did both occur, or did neither occur? Discuss what evidence you observed to support this.
- Was there an endothermic or exothermic change or both? Describe your reason for deciding which one of the two occurred. If you cannot decide, then discuss why you cannot decide.
Part 1: Silver nitrate (AgNO3) and Sodium chloride (NaCl)
Caution: Solid silver nitrate is corrosive and is used to cauterize wounds. Even dilute silver nitrate may be corrosive. Silver nitrate produces photochemical stains which darken to a brown, purple, or black color. These stains are permanent, but will wear off as skin is shed naturally through showers, hand washing, etc. You may use gloves when pouring this solution. Spray N' Wash has been somewhat successful in removing stains from clothing if treated quickly.
Technique: Pouring Corrosive Liquids, Filtering
Disposal: Pour the filtrate into the labeled waste container. Silver solutions should not be disposed of in the sewer untreated. Then clean all glassware and the funnel with soapy water. The next day, the filter paper which contains silver chloride should be placed in the labeled waste bag for storage in a secure chemical landfill.
- Obtain 2 small test tubes and transfer them to the reagent table by carrying them in a test tube rack. The test tube you use for silver nitrate must be clean and dry. Measure silver nitrate solution to a depth of 2 cm in one tube using the correct technique for a corrosive liquid. Add sodium chloride solution to a depth of 4-5 cm into the other tube. Return to your lab table.
- After observing the properties of the two liquids, add the sodium chloride to the silver nitrate.
- Next, transfer the contents of the test tube to a filter setup, using tapwater to rinse out the test tube. After filtering, remove the filter paper with tongs or tweezers, and save it on a paper towel labeled with your names. There will be a tray in which to save the papers overnight, during which they will be exposed to sunlight or fluorescent light. Dispose of the filtrate as indicated below.
- The next day, observe any change in the product left on the filter paper.
Part 2: Sodium Polyacrylate and Water
Technique: Measuring a Solid
Disposal: Dispose of the mixture in a plastic-lined wastebasket. Clean beaker with soap and tapwater.
- Measure one small scoop of sodium polyacrylate into a beaker (100, 150, or 250 mL). Return to the lab table.
- Add 10 mL of water. Record observations. Invert the beaker over the sink, record observations. Repeat process until beaker is full, or you are bored with the results. You may touch the product with your hands.
Part 3: Stearic Acid ( CH3(CH2)16COOH )
Technique: Measuring a Solid, Heating a Test Tube--waterbath
Disposal: When the tube is at room temperature or colder, discard it in the glass disposal box. The chemical does not dissolve in water which makes the glassware clenaup difficult . This chemical is not hazardous and can be disposed of in ordinary garbage, but the glass tube is separated from burnable waste, such as paper towels.
- Scoop enough stearic acid into a micro test tube to fill it to a depth of 1-2 cm.
- Half-fill a 100 or 150 mL beaker with tapwater and place it on a hotplate as a water bath. Place the test tube in the water bath--making sure it does not tip over. Note any changes that occur during heating--heat until water boils or there are no more changes in the appearance of the stearic acid.
- Also prepare a 150mL cold water bath. Use a crucible tongs to remove the tube from the hot water and place it into the cold water bath. Record any changes that occur during cooling.
Part 4: Heating Metals
Technique: Heating a Solid, Lighting a Burner
- Platinum: Examine the platinum wire (attached to glass rod). Observe the color and luster of the metal. Hold the wire in the high temperature flame of your burner for 10 seconds. Allow the wire to cool and re-examine it. Disposal: Platinum is expensive and reusable, return to labeled beaker.
- Copper: Examine the copper strip. Heat the strip in the cool outer portion of the burner flame for 15 seconds or until a coating forms. Be careful not to melt the strip. Place the strip on a glass plate or watch glass and use tongs to hold it and another pair of tongs or tweezers to scrape off some of the coating produced by heating. Record your observations. Disposal: Place copper scrap in the Cu waste container. Copper may be disposed of in a landfill; large amounts of the coating are toxic.
- Magnesium: Examine the magnesium ribbon. Obtain a Mg waste can and light a burner.
Caution: DO NOT LOOK DIRECTLY AT THE MAGNESIUM WHILE IT IS BURNING. EYE DAMAGE TO THE RETINA RESULTS FROM UV LIGHT.
With tongs, hold the ribbon in the hottest part of the flame (tip of inner blue cone) to ignite it. Then hold it inside the waste can to complete the burning. Disposal: Magnesium ribbon and ash are left in the waste can. The ash can be safely disposed of in a regular landfill. The ribbon is too reactive for an ordinary landfill and the ribbon scraps will either be heated to ash in a crucible or reacted with acid.
UV goggles may be worn to reduce exposure to ultraviolet light--see instructor.
Part 5: Liver and Hydrogen Peroxide (H2O2)
Caution: Concentrated hydrogen peroxide is very hazardous. It will bleach skin and clothes and is very corrosive to eyes.
Technique: Pouring Corrosive Liquids, Lighting a Burner, Splint Test for Gases
Disposal: Remove the piece of liver with the tongs and return it to the waste beaker--DO NOT put it in the wastebasket or sink. Then add tapwater to the hydrogen peroxide carefully, so as not to splash. Empty the beaker into the sink. Wash and brush beaker with soapy water. Rinse with tapwater.
- Obtain a small piece of liver with crucible tongs, and place it into a 100 or 150 mL beaker.
- Pour 10-15 mL of 6% hydrogen peroxide (Clairoxide) into a graduated cylinder. Add to the beaker.
- Test any gas produced with a glowing splint. To prepare the splint, hold it in the tongs in a burner flame, then blow it out, leaving a glowing ember at the tip of the splint.
Part 6: Citric Acid (H3C6H5O7 ) and Baking Soda (NaHCO3)
Technique: Measuring a Solid, Lighting a Burner, Splint Test for Gases
Disposal: Empty contents of bag into the sink, flush with water. Discard plastic bag in wastebasket.
- Measure one scoop of citric acid and one scoop of baking soda into a Ziploc bag, and observe any reactions. Return to your lab table.
- Pour about 25 mL of tapwater into the bag and quickly seal it. Hold the bag to observe any temperature change.
- Test any gas produced with a burning splint. Open the bag and quickly test the gas before it all escapes and without igniting the plastic bag.
Lab Report Check
Give your observation/conclusion sheet to one of your lab partners for them to evaluate and sign--then staple this sheet to it and turn it in before the end of the class period.
Do not sign the lab sheet until all are completed:
Signature of evaluator___________________________________________________
- Name of person responsible for lab conclusion and number of experiment (1-6)
- Observations labeled before/during/after and quantitative/qualitative
- At least one quantitative observation
- Observations include all these items: color, odor, temperature, clear or cloudy (for liquids only)
- Observations tell if heat, light, or electricity were added during the experiment
- Conclusion uses the term physical or chemical change (may be both). Observations are used to back up an opinion.
- Conclusion uses the term endothermic or exothermic (may be both). Observations are used to back up an opinion.
- Check observations to see if more than one change occurred. There is a conclusion written for each change.
These may not be the only right answers--REMEMBER, we grade on how well your conclusions fit your observations and the evidence you use to back up your conclusion.
- SILVER NITRATE AND SODIUM CHLORIDE
- chemical change--precipitate forms
no evidence of energy change
chemical change--color change from white to grey-violet
- SODIUM POLYACRYLATE AND WATER
- physical change--gel forms when water added
no evidence of energy change
- STEARIC ACID
- physical change--solid melts
endothermic--requires input of heat to melt
physical change--liquid freezes
(exothermic)-- but no evidence of energy change
- HEATING METALS:
- no change or physical change--color change only in flame, returns to original state
either endothermic--only glows in burner flame
or exothermic--gives off light when in flame
- chemical change--tarnish forms, new compound
endothermic--requires addition of heat
- chemical change--new compound (ash) forms
exothermic--emits its own heat and light after removed from burner
- LIVER AND HYDROGEN PEROXIDE
- chemical change--oxygen gas produced
may be exothermic--temperature increase often observed
- CITRIC ACID AND BAKING SODA
- chemical change--carbon dioxide gas produced
Lab Assessment: Physical and Chemical Changes
This assessment uses Laserdiscs from Optical Data:
(Q#1) "Cosmic Chemistry," Side 1, Chapter 2 Time 1:16
"Physical Sciences," Side 2, Chapter 22 Time 0:22 [turn off volume]
(Q#3) "Cosmic Chemistry," Side 3, Chapter 19 Time 2:06 [turn off volume, use only until close-up of ignition]
1) Tulip in Liquid Nitrogen (Temp. 175o C)
Teacher's Instructions for Students: "Is this a physical or chemical change? Discuss your choice using an observation. If you are undecided, give evidence for both choices."
2) Identify each as physical change (P) or chemical change (C)
_____ a) milk souring
_____ b) iron rusting
_____ c) coal burning
_____ d) plants decaying
_____ e) ice melting
3) Ignition of Ammonium Dichromate.
Teacher's Instructions for Students: "Record observations, both wquantitatve and qualitative, of the chemical before, during, and after the change. Record at least two quantitative observations."
-----"Is this a physical or chemical change? Discuss your choice using an observation. If you are undecided, give evidence for both choices."
-----"Is this an endothermic or exothermic process? Discuss your choice. If you are undecided, explain why."
Link to overview of "Mendeleev Puzzle" curriculum module.