ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES, AN INTRODUCTORY CHEMISTRY EXPERIMENT (Supplementary Material for Online Publication, JCE ms# 2004-0995) Author:
Margaret J. Steffel The Ohio State University Marion Campus 1465 Mt. Vernon Ave. Marion, Ohio 43302 e-mail
[email protected] phone 614 292-9133 ext. 6261 with voice mail fax 614 292-1181
LAB DOCUMENTATION Chemicals Used
CAS Registry Number
1. cobalt(II) nitrate hexahydrate
10026-22-9
2. sucrose
57-50-1
3. benzoic acid
65-85-0
4. ammonium carbonate
506-87-6
5. copper(II) iodate
13454-89-2
6. nickel(II) carbonate hydrate
3333-67-3
7. potassium nitrate
7757-79-1
8. potassium aluminum sulfate dodecahydrate 9. cobalt(II) chloride hexahydrate
7784-24-9
10. iron(III) oxide
1309-37-1
11. water
7732-18-5
7791-13-1
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Students' Instructions. These follow, as used by students in the first quarter of a three-quarter sequence for science, medicine, and engineering majors. At a more elementary level, it is advisable to give students more guidance by spending quite a bit of time discussing possible observations and their meaning before students do the laboratory work. The pre-laboratory questions enable an instructor to verify that students are using appropriate terms to describe changes and materials. The preliminary report sheet is to guard against students changing answers. Hazards Decomposition of cobalt nitrate produces NO2. The odor of sucrose that is being heated and is decomposing is unpleasant to some people. Vaporization of benzoic acid might put vapor into the hood atmosphere, but typically the vapor deposits on the upper, cooler portion of the test tube. Decomposition of ammonium carbonate produces ammonia. Decomposition of copper(II) iodate produces iodine, which typically deposits near the middle of the test tube, but some might pass into the hood atmosphere. If students are working in a properly ventilated hood, and if they follow proper techniques and carefully point test tubes away from themselves and away from other students with whom they share a hood, these vapors are not hazards. Nickel(II) carbonate decomposes liberating carbon dioxide, not a hazard. Potassium nitrate melts and remains in the bottom of the test tube, not a hazard. It does not decompose at the temperature generated by a microburner. Use of a microburner is essential. Potassium aluminum sulfate dodecahydrate releases water, not a hazard. It does not decompose any further at the temperature generated by a microburner. Cobalt(II) chloride hexahydrate releases water, not a hazard. It does not decompose any further at the temperature generated by a microburner. Iron(III) oxide does not change, not a hazard. Water boils very vigorously in the close environment of a test tube. Some might splash out onto a student. Typically this occurs only if students use more than the specified amount and are not attentive about positioning the tube pointing away from themselves and other students.
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Most Probable Student Errors The students' instructions tell them "Continue heating the substance until there no longer is any apparent change occurring". The most common procedural error is to stop the heating as soon as it is apparent that a change is occurring without continuing the heating until there no longer is any change occurring. The most common interpretive error is to base conclusions on contents of the tube during heating or while material is still hot rather than waiting until the contents are at room temperature. These can lead to the following reporting errors. 1. cobalt(II) nitrate hexahydrate. No yellow gas (NO2). No colorless condensate (water). 2. sucrose. No colorless condensate (water). 3. benzoic acid. White solid remains in the bottom of the test tube. No white deposit. 4. ammonium carbonate. White solid remains in the bottom of the test tube. Because the amount of ammonium carbonate is so small, the amount of water that forms during its decomposition might be so slight that it does not condense, so different descriptions about condensate are acceptable here. The change that students are most likely to identify incorrectly as physical rather chemical is this decomposition of ammonium carbonate. They should recognize this as a chemical change, rather than the vaporization of the solid, because there is no deposit of solid on the cool portion of the tube as there is with benzoic acid. 5. copper(II) iodate. No purple deposit (iodine). 6. nickel(II) carbonate hexahydrate. No colorless condensate (water). 7. potassium nitrate. Not all solid melts. Not all liquid solidifies. 8. potassium aluminum sulfate dodecahydrate. No colorless condensate (water). Even though the remaining solid is very similar in appearance to the original solid, students should recognize that the condensate is a different substance indicating a chemical change. 9. cobalt(II) chloride hexahydrate. A pinkish or lavenderish solid remains rather than a blue solid, completely dehydrated cobalt(II) chloride. No colorless condensate (water). 10. iron(III) oxide. No problems. 11. water. Some liquid remains in the bottom of the test tube.
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Chemistry 121 Dr. Steffel
ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES What happens to something when it is heated? If it is a substance (i. e., just one kind of matter, an element or a compound), it might melt (solid changes to liquid at a specific temperature) or sublime (solid vaporizes from surface over a temperature range) or evaporate (liquid vaporizes from surface over a temperature range) or boil (liquid changes to gas throughout the liquid at a specific temperature) or burn (react with oxygen if heated in air), an exothermic chemical change or react with oxygen from the air but not burn or decompose if it is a compound (break apart into substances simpler in composition) or a combination of these might occur; as examples, the substance might melt and vaporize or evaporate and boil or melt and vaporize and then decompose or react with oxygen or it might not undergo any change at the temperature used. Which of these are changes in identity, changes into different kinds of matter with different properties? These are chemical changes. Which of these are changes in form, such as a change in physical state? These are physical changes. You will work with eleven substances. Each is a compound, a single substance, not a mixture. Heat each compound and observe and describe what happens to it during this heating. Decide whether the compound undergoes a chemical change or a physical change or both during this heating. Your conclusion must be based on the properties of the substance that is present before heating, what you see happening during heating, and the properties of any material that is present after cooling to room temperature. Be as factual as possible about describing only what you actually see. Write down this description in as much detail as possible before you begin, during the heating, and after the heating. If both physical and chemical changes (probably in this order) seem to be occurring, call it chemical, because that determines what is present after all changes are complete. Reach your conclusions while you are doing your work, observing the materials so that the results you report on your preliminary report sheet and the results you report on your final report sheet are the same. #
#
Substances 1-#5 must be heated in a hood. Heat substances #6- 11 at your laboratory desk. Heat each substance in a 15-mL or 20-mL heat-resistant test tube; if the tube does not have the brand name Pyrex or Kimax on it, it is not heat resistant. (If you need to wash test tubes before you begin your work, or during your work, be sure you dry them thoroughly; then let them stand for at least several minutes before you use them.) Hold the tube with a test tube holder. Get just one substance at a time following the instructions on the next page. After you finish with one substance, get some of another substance, and so on. Do all the substances within one group and then do all the substances in the other group; your instructor will assign one of the groups to you for you to do first. Within each group of substances (1-5 and 6-11), you may do them in any order; just be sure to keep track of the numbers. (To help you keep track of the tubes as they cool, you can write the substance's number on the white "spot" on the upper part of the tube; use pencil, not pen, so that you can erase this writing after your use.)
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To get a sample of a material if it is liquid, use the medicine-dropper bottle top to put about five to ten drops of the liquid into your test tube. To get a sample of a material if it is a solid, pour a small amount of it (about the volume of one to two grains of rice) into the beaker that is provided with it (and has the same number on it) and then pour the solid from the beaker into your test tube. Use your microburner (not your larger burner) to heat a test tube. Have the test tube holder positioned at or above the white "spot" on the tube so the holder is high enough that it, and you, will not be heated by the burner flame. Be sure the test tube is pointing away from you and from other people when you heat it. When you are working in the hood, be sure you keep the hood window down between you and the test tube. Do not put your head into the hood. Do not have your notebook in the hood. Continue heating the substance until there no longer is any apparent change occurring. Describe the original appearance of the substance and what you see during heating. After you finish heating a test tube, put it in a test tube rack to cool; look at it after it is cool (room temperature) and describe the physical state, color, and location of any remaining material. Don't touch the tube for several minutes after heating it. For some substances, liquid or solid will form on the upper, cooler part of the test tube. Here is information to help you decide whether this is the result of a physical or chemical change. When a substance changes from one physical state to another, its color remains about the same, perhaps exactly the same, perhaps somewhat darker or lighter, but not a completely different color. A white solid might melt to a colorless liquid that vaporizes to a colorless gas that condenses to a colorless liquid that changes to a white solid as it cools; these are physical changes. If you heat a liquid or solid, and a liquid or solid of a different color condenses or deposits on the cooler part of the tube, it is a different substance; a chemical change occurred. When you describe material that is toward the top of the tube, use the following terminology. A condensate is liquid that formed by condensation, change of gas (vapor) to liquid. A deposit is solid that formed by deposition, change of gas (vapor) to solid. These terms give more information than just "liquid" or "solid" because they include both a substance's physical state and how it formed. The terms condensate and deposit are not appropriate for material that remains in the bottom of the test tube. And just to keep things interesting, there is one substance (only one) that does not undergo a chemical or physical change! Be sure to base your conclusions not only on what you see during the heating, but also on a comparison of the initial and final materials at room temperature. In your notebook, record as much detail as possible about what you observe before you heat a substance, what you observe while you are heating it, and what remains in the test tube (top and bottom) after the tube and its contents have completely cooled. On the Preliminary Report Sheet, record only the small amount of information that is requested. Do this after you have completed and recorded all your work. You must turn in this preliminary report sheet before you leave the laboratory to receive credit for your final report. On the final Report Sheet, use your detailed record to decide what information is most relevant. Do not give information other than what is requested. For example, do not describe texture (particle size) of solids; this is not an intensive property of a substance and it might seem to change slightly during heating even if there is not any other chemical or physical change occurring. Be sure you put your descriptions of final materials in the proper "box"--upper or lower box to describe material in the upper or lower part of the test tube. Each response should have only a few words, such as "colorless condensate", "dark red deposit", or "green solid".
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This list tells you how to dispose of each tube and its contents after you complete your work with it. TUBE #
INSTRUCTIONS FOR DISPOSAL
1.
While the tube is still in the hood fill it with tap water from the faucet in the hood. Wash this water down the sink drain in the hood with the water still running. Then discard the tube and its contents into the "broken glass" container.
2.
Discard the tube and its contents into the "broken glass" container.
3.
The remaining material is slightly water soluble. Scrub the tube with a brush and lots of hot water.
4.
Return the tube to your desk drawer.
5.
Put the tube into the labeled test tube rack that is on the instructor's desk for proper disposal later.*
6.
Discard the contents of the tube into the regular waste basket. The remaining small amount of material is not water soluble, but you should be able to brush all of it out of the test tube. Scrub the tube vigorously with a brush and water.
7.
The remaining material is water soluble. Wash out the tube with water and a brush.
8.
The remaining material is water soluble. Wash out the tube with water and a brush.
9.
The remaining material is water soluble. Wash out the tube with water and a brush.
10.
Discard the contents of the tube into the regular waste basket. Any remaining small amount of material is not water soluble, but you should be able to brush all of it out of the test tube; scrub the tube vigorously with water and a brush.
11.
Return the tube to your desk drawer.
*Note
added for instructor users' benefit. Iodine remains in the test tube. This is not considered hazardous waste and can be allowed to vaporize in a properly ventilated hood.
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ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES PRE-LABORATORY REPORT SHEET Must be turned in before you begin your laboratory work
Name
Date (Date you do the lab work)
For each of these brief descriptions of a change of a substance when it is heated describe what you would see happening while it is being heated. Then describe what you would see in the test tube after heating and after the contents of the tube have cooled to room temperature. Describe color and physical state of material that would be in the bottom of the tube. Describe color and physical state of material that would be toward the top of the tube; use the terminology that is described on page 2 of your instructions. State whether the change is chemical or physical. A blue liquid boils when heated.
A red solid melts when heated.
A green solid vaporizes when heated.
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ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES PRE-LABORATORY REPORT SHEET Must be turned in before you begin your laboratory work
Name
Sample Answers
Date (Date you do the lab work)
For each of these brief descriptions of a change of a substance when it is heated describe what you would see happening while it is being heated. Then describe what you would see in the test tube after heating and after the contents of the tube have cooled to room temperature. Describe color and physical state of material that would be in the bottom of the tube. Describe color and physical state of material that would be toward the top of the tube; use the terminology that is described on page 2 of your instructions. State whether the change is chemical or physical. A blue liquid boils when heated. During heating bubbles will form in the liquid and a blue vapor will form. The amount of liquid will gradually decrease and eventually it will all vaporize. When the tube is cool there will be nothing in the bottom of the tube and there will be a blue condensate at the top of the tube. This is a physical change.
A red solid melts when heated. During heating the solid will change to liquid. When the tube is cool there will be a red solid in the bottom of the tube and there will be nothing at the top of the tube. This is a physical change.
A green solid vaporizes when heated. During heating green vapor will form. The amount of solid will gradually decrease and eventually it will all vaporize. When the tube is cool there will be nothing in the bottom of the tube and there will be a green deposit at the top of the tube. This is a physical change.
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ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES PRELIMINARY REPORT SHEET Must be turned in before you leave the laboratory to receive credit for your report Name
Date
Section For each initial substance, briefly state its color (in one to three words) and physical state. (Remember that "clear" is not a color.) Write C, P, or N to state whether it underwent a chemical change, a physical change, or no change. number
color
physical state
change?
1 2 3 4 5 6 7 8 9 10 11
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ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES REPORT SHEET Name
Date
Section For each initial substance, state only its color and physical state. In a very concise manner, state what you observed happening during heating. State the color and physical state of any material that remained after cooling to room temperature; use the upper space to describe a condensate or deposit (use these terms, not liquid or solid) on the upper part of the tube and the lower space to describe material in the bottom of the tube. Put an answer in each part; put "nothing" if there was no material in that part of the tube. In the ? column, state the kind of change that occurred: C, P, or N. #
initial
during heating
cooled
1
2
3
4
1
?
#
initial
during heating
cooled
5
6
7
8
9
10
11
2
?
ENDOTHERMIC CHEMICAL AND PHYSICAL CHANGES REPORT SHEET Name
Sample Report Sheet
Date
Section For each initial substance, state only its color and physical state. In a very concise manner, state what you observed happening during heating. State the color and physical state of any material that remained after cooling to room temperature; use the upper space to describe a condensate or deposit (use these terms, not liquid or solid) on the upper part of the tube and the lower space to describe material in the bottom of the tube. Put an answer in each part; put "nothing" if there was no material in that part of the tube. In the ? column, state the kind of change that occurred: C, P, or N. #
initial
1
brownish solid
liquifies, bubbles, darkens, yellow gas
white solid
darkens, liquifies, bubbles, white vapor
white solid
liquifies, white vapor
white solid
gradually decreases in volume and disappears
2
3
4
during heating
cooled colorless condensate
? C
black solid colorless condensate
C
brown-black "gooey" solid white deposit
P
nothing very slight colorless condensate nothing
1
C
# 5
6
7
8
9
initial
during heating
light bluesolid green
bubbles, darkens, purple vapor
light green solid
darkens, white vapor
white solid
liquefies
white solid
liquifies, bubbles, resolidifies
lavender -pink solid solid
darkens, bubbles
cooled purple deposit
? C
black solid colorless condensate
C
black solid nothing
P
white solid colorless condensate
C
white solid colorless condensate
C
blue solid
10 brick red solid
becomes more ruby red
11 colorless solid
a very vigorous rolling boil
nothing
N
brick red solid colorless condensate nothing
2
P
