E – 6 HEATING AND COOLING CURVES
NGSS: HS: PS1-3 Plan and conduct and investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of the electrical forces between particles. SEP Planning and Carrying Out Investigations. SEP Obtaining, Evaluating, and Communicating Information. SEP
Analyzing and Interpreting Data. SEP Engaging in Argument from Evidence.
From E-5 “Solids on Warming”, we know that some substances melt more easily than others and that the melting points of known substances can be found in the HANDBOOK OF CHEMISTRY AND PHYSICS .
The melting point of a substance is the temperature at which that substance changes from a solid to a liquid.
The boiling point of a substance is the temperature at which that substance changes from a liquid to a gas.
When the substance being investigated is pure, the melting point and the boiling point are single, unchanging temperatures.
Melting and boiling points are two important characteristics or physical properties of pure substance, that is, they can be used to help identify the substance.
In this experiment, you will take quantitative and qualitative observations of a pure, unknown substance as it melts and freezes. You will then use that data to analyze what happens to a substance at its melting/freezing point and to create original graphs using the “Guidelines for Graphing” in your science notebook.
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Purpose:
1.
To define the terms melting, freezing, boiling, condensing, sublimation, and deposition.
2.
To compare and contrast the melting point and the freezing point of a pure substance and to explain how both are shown on a melting point/freezing point graph.
3.
To give the melting and freezing point temperatures of the substance in the E-6 lab.
4.
To contrast the melting and freezing point graphs of a pure and an impure substance.
5.
To list two possible sources of error in the E-6 lab and explain how they would affect the results of the lab.
Materials: labeled test tube with unknown substance and a large thermometer ring stand
400 mL beaker test tube clamp wire gauze ( Day 2 only
Bunsen burner (
)
Day 2 only ) ring ( Day 2 only ) small thermometer ( Day 2 only )
Safety Alerts: Goggles, tie back hair, close fitting sleeves, long pants, closed toed shoes / heat, vapors, eye and skin irritants, glass.
Procedure:
Day 1
1. Obtain a ring stand. Then, place a 400 mL beaker 2/3s full of room temperature tap water on the ring stand base.
2. You and your partner must now decide who will obtain the test tube of unknown substance, stir, and make quantitative (i.e. What is the temperature?) and qualitative (i.e. Is the unknown solid, liquid, or both?) observations and who will watch the time and record the temperature and observations every 30 seconds (Note: If there are three people in your group, you can split this job, and one of you can time and the other can record temperature and observations.).
3. The person who plans to obtain the unknown substance should proceed to the fume hood with a test tube clamp.
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4. In the fume hood on a hot plate, you will find labeled test tubes of unknowns with thermometers. Before lifting your test tube from the hot water bath, tell your partner the letter of your unknown and record it for future reference (You will need the same unknown for Day 2.). Then, clamp the test tube clamp around the submerged test tube of unknown and read the thermometer in the unknown (Note: If the thermometer states that the temperature of the unknown is below 65 degrees Celsius at this time, it is too cold . Look for another unknown. If all unknowns are below 65 degrees Celsius, tell your teacher.).
5. Quickly , remove your test tube from the water bath. This is time “0” and temperature #1. Proceed carefully and quickly to your ring stand. Have your partner help you to clamp the test tube above the beaker of water.
6. Until the temperature drops below 35 degrees Celsius, you must constantly stir the unknown with the thermometer unless it eventually becomes completely frozen in the unknown.
7. In your science notebook, record the time, temperature, and state(s) of matter every 30 seconds (SEE
THE “Sample Data Table” IN THE DATA SECTION ON p.2).
8. After 5 minutes , lower the test tube into the water bath and continue stirring and recording.
9. When the temperature drops below 35 degrees Celsius, you are finished recording data. Return your test tube with thermometer in it to the hot water bath in the fume hood where you found it.
10. Clean up your lab area. Make sure to put away all equipment in its correct place.
Day 2
1. Obtain a ring stand and attach a ring to it approximately 20 cm from the ring stand base. Then, put the wire gauze pad on the ring, and place a 400 mL beaker 2/3s full of room temperature tap water on the pad. Next, put a small thermometer in the tap water. Finally, put a connected, unlit Bunsen Burner on the base of the ring stand underneath the wire gauze pad (If the top of the Bunsen Burner is not approximately 8 cm below the pad, reposition the ring so that it is.).
2. At this point, you and your partners should decide if you wish to keep the same roles as in Day 1 or switch roles.
3. Proceed to the fume hood with a test tube clamp. Clamp the test tube clamp around the submerged test tube of unknown with large thermometer and lift your test tube from the water bath ( Remember: You will need the same unknown that you used on Day 1.).
4. Return to your ring stand and clamp the test tube of frozen unknown onto the ring stand and lower it into the water ( Make sure not to get any water into the test tube! Water will make the sample impure!
).
5. Remove the Bunsen Burner from under the ring stand, light it , adjust it to an adequate height blue flame without an inner cone, and then replace it under the ring and pad. This is time “0” and temperature #1
(Make sure you are recording the temperature from the large thermometer in the test tube NOT the small thermometer!).
6. Until the temperature rises above 65 degrees, you must constantly stir the unknown with the thermometer as soon as it becomes unfrozen from the unknown (Test it occasionally to see when it becomes loose.
You may be able to move it up and down before you can move it from side to side.).
7. In your science notebook, record the time, temperature, and state(s) of matter every 30 seconds.
8. Once a minute, check the temperature on the small thermometer in the water. DO NOT write this temperature down, however, if you notice this temperature rising very quickly, make your flame smaller or yellow. If the temperature of the water bath gets above 55 degrees, turn off your Bunsen Burner until all of the substance has melted and then, relight the Bunsen Burner.
9. When the temperature of the unknown substance rises above 65 degrees, you are finished recording data.
Return your test tube with thermometer in it to the water bath in the fume hood where you found it.
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10. Clean up your lab area. Make sure to put away all equipment in its correct place.
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Data: time (minutes)
0
0.5
1.0
1.5
. . .
20
Sample Data Table temperature (degrees C) state of matter (solid, liquid, or both)
Graphing:
Make a neat, full page , two line graph of the heating and cooling of the unknown substance (You may wish to use two different colored pens, one for the cooling line and one for the heating line.). Make sure to refer to the
“Guidelines for Graphing” ( Remember: Put the independent variable , that you controlled , on the horizontal axis and the dependent variable , that was free to vary , on the vertical axis . Also, make sure to use even scales that start at zero. Finally, make sure to circle your data points and include a title box !)
Analysis:
1. What portion of the graph represents your unknown substance as all solid? Label it “SOLID”.
2. What portion of the graph represents your unknown substance as all liquid? Label it “LIQUID”.
3. Is any portion of your graph both solid and liquid? If so, label it “SOLID/LIQUID”.
4. Does your graph have a flat plateau area? If so, what was happening at that portion of the graph? If not, why not?
5. What are the melting and freezing points of your unknown substance?
6. How are the melting and freezing point related?
7. Was the substance in the test tube a pure substance? How do you know?
Critical Thinking Questions:
1. Why was it important to use the same test tube both days?
2. Why was it important to stir the mixture constantly?
3. What might happen to the results if you got water into the test tube on Day 2?
4. Which variable (i.e. time to cool/heat from solid to liquid or vise versa; melting/freezing point temperature; or both) would change on your graph if you: a. b. c. increased or decreased the amount of unknown used a different unknown substance used a candle or blow torch instead of a Bunsen Burner d. heated the substance from solid to gas or gas to solid
5. Sketch (No more than 5 cm 2 .) the graphs resulting from: a. decreasing the amount of your unknown b. using a different unknown substance with a higher melting point c. using a candle instead of a Bunsen Burner to heat your unknown d. heating your unknown all the way from solid to a gas
6. What can you presume about a substance with a melting range of 61-66 degrees C? From your E-5 lab, what might that substance be? What might occur during the formation of this substance to give it a melting range instead of a melting point ?
7. What do the terms boiling and condensation mean, and how are boiling point and condensation point related (Hint: Use a textbook!)?
8. What do the terms sublimation and deposition mean (Hint: Use a textbook!)?
Conclusion:
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