Last printed 9/25/2001 9:46 PM Page 1 of 3
Chemical or Physical Change Lab
Teacher Manuel Chapter 2 Exp 2A
Objective: SWBAT
1. distinguish the differences between physical and chemical change.
2. calculate % accuracy and % error.
Prelab Discussion:
1.) Equipment
2.) Burner Use
3.) Safety
Lab Setup:
Station 1: Magnesium Sulfate Pentahydrate, Inorg #2, (label compound A). Need burner, striker, ring stand, ring clamp, clay triangle, scoopula, and crucible.
Station 3: Lead Nitrate, Inorg #3 3.3g/100 mL .1M (compound B) and Potassium Iodide, Inorg #2 1.6g/100mL .1M (compound C). Need 50 mL beaker.
Station 5: Copper II Sulfate Pentahydrate, Inorg#2 (Compound D). Need burner, striker, ring stand, ring clamp, clay triangle, scoopula, and crucible.
Station 9: Sugar, misc. (Compound E). Need burner, striker, ring stand, ring clamp, clay triangle, scoopula, and crucible.
Station 11: Sodium Bicarbonate, Inorg #4 (unknown F) and vinegar, Acid Cabinet (unknown G). Need 50 mL beaker.
Station 13: Water (Compound H). Need burner, striker, ring stand, ring clamp, dropper bottle, clay triangle, and crucible
Other Possible Materials:
A Heated materials
1) Naphthalene (mothballs; they burn with a thick, black smoke. Only burn these in the hood!)
2) Alcohol (burns)
B Mixed materials:
1) Silver Nitrate and hydrochloric acid. (white silver chloride precipitate)
2) Copper metal and nitric acid. (NOx is produced, making a brown toxic cloud....only do in the hood).
Background:
Chemical change is any change that results in the formation of new chemical substances. At the molecular level, chemical change involves making or breaking of bonds between atoms. These changes are chemical:
Ø iron rusting (iron oxide forms)
Ø gasoline burning (water vapor and carbon dioxide form)
Ø eggs cooking (fluid protein molecules uncoil and cross link to form a network)
Ø bread rising (yeast converts carbohydrates into carbon dioxide gas)
Ø milk souring (sour-tasting lactic acid is produced)
Ø sun tanning (vitamin D and melanin is produced)
Physical change rearranges molecules but doesn't affect their internal structures. Some examples of physical change are:
Ø whipping egg whites (air is forced into the fluid, but no new substance is produced)
Ø magnetizing a compass needle (there is a realignment of groups of iron atoms, but no real change within the iron atoms themselves).
Ø boiling water (water molecules are forced away from each other when the liquid changes to vapor, but the molecules are still H2O.
Ø dissolving sugar in water (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged).
Ø dicing potatoes (cutting usually separates molecules without changing them).
Classification of real processes can be tricky. Complex changes can be broken down into many simpler steps. Some of the steps are chemical and some may be physical, so the overall process can't cleanly be placed in either category. For example, boiling coffee involves chemical change (the delicate molecules that give coffee its flavor react with air and become new, bitter-tasting substances) and physical change (the water in the coffee is going from liquid to gaseous form). Use what you have learned to determine if the following stations involve chemical or physical changes.
Accuracy and Error calculations are based upon the difference between experimental values and theoretical values. Experimental Value: Experimenter’s results, the outcome of the experiment. Theoretical Value: Value that should have been found (expected outcome).
Procedure:
General: Students may start at any station. After each station is completed go to the next higher numbered station. After station #13 go to station #1. Continue this until you have done all six stations. At all stations clean glassware before starting.
Station 1: In this station, get a small amount of the unknown A compound and place it in a crucible. Heat on clay triangle until a change takes place. Record your observations in the data table. Do not touch crucible after heating. Use crucible tongs to move and dump remaining chemical in trash.
Magnesium Sulfate Heptahydrate (Epsom Salts): The crystals jump around!
Station 3: In this station, add one dropper of compound B into a 50 mL beaker followed by one dropper of compound C. Swirl and record observations in the data table.
Lead Nitrate and Potassium Iodide forms a yellow lead iodide precipitate.
Station 5: In this station take a little bit of the unknown D and place it in a crucible. Heat the crucible until a change takes place. Record observations in the data table. Use crucible tongs to move crucible and dump chemical in trash.
Copper II Sulfate Pentahydrate, they change color from blue to white (hydrous to anhydrous).
Station 9: In this station heat one small scoopful of the unknown E in a crucible until you see a change take place. Record observations in the data table. Do not clean the crucible after heating on this station.
Sugar smells like caramel when heated.
Station 11: In this station add about 1 g of compound F into a 50 mL beaker followed by one dropper of compound G. Record observations in the data table.
Sodium bicarbonate (Compound C) and dilute acetic acid. The product is carbon dioxide.
Station 13: In this station, add 10 drops of the unknown H to a crucible and heat over a Bunsen burner. Record observations in the data table.
Water boiling
Data Table 1:
Station / Observation Before / Observation During / Observation After / Student Data(P or C) / Validation Data
(P or C) /
1
3
5
9
11
13
Data Table 2:
Number of Correct Student Responses / ResponsesNumber of Incorrect Student Responses / Responses
Total # of Possible Responses / Responses
Percent Accuracy / %
% Error / %
Data Analysis:
To introduce the ideas of Accuracy and Error in this lab use the number of times you were correct and number you were right for the six stations.
1. Count the number of correct student responses, number of incorrect student responses, and total number of possible responses and place in data table 2.
2. Accuracy is when you get experimental results (experimental value) close to the accepted standard (theoretical value). What was your percent accuracy?
% Accuracy = Experimental Value X 100% **Equation normally used**
Theoretical Value
% Accuracy = Number of correct Station Observations X 100% **Use this equation**
Number of possible Station Observations
3. Error is when the experiment results (experimental value) is different than the accepted standard (theoretical value). What was your percent error?
% Error = Experimental Value – Theoretical Value X 100% = **Equation normally used**
Theoretical Value
% Error = Number of possible Observations – Number of correct observations X 100%
Number of possible observations
Conclusion: (Use Science Rubric)
1. Determining the whether it is a chemical or physical change can be difficult. Use the corrected experimental data to describe and distinguish between the types of changes that are physical and the types that are chemical. Is there a pattern that can apply to these changes?
C:\My Files 3\School\Chemistry\Chap 2 Matter and Energy\Chemical or Physical Change Lab.doc Ó2001 Tim Bass