Chapter 10
Experiment: Phases of Matter
Purpose: To investigate the melting and freezing behavior of a compound.
Background: If you have ever cooled a glass of water with ice cubes and watched water vapor condense on the outside of the glass, you know water can exist in distinct physical states - the solid state (ice), the liquid state (water) and the gas state (water vapor). In any pure substance, changes of physical state occur at constant, discrete temperatures that are uniquely characteristic for that substance.
In this experiment, you will closely examine what happens when LAURIC ACID (a pure substance) undergoes two different changes of state: from a solid to a liquid (melting) and from a liquid to a solid (freezing). You will be trying to determine if the liquid lauric acid freezes at the same temperature that solid lauric acid begins to melt. And, you will determine what happens to the temperature of the lauric acid between the time freezing or melting begins and the time it is complete. You will be asked to consider what happens to the energy that is put into or removed from the lauric acid system during the solidification or melting process.
Materials:
beakers for hot water bath
thermometer
large test tube (about 1" diameter)
wire mesh
ring stand
gas burner or hot plate
clamp to go on pole on lab bench
timer
triple beam balance
5 grams of LAURIC ACID
Procedure:
GENERAL INSTRUCTIONS - Part of the purpose of this lab is to get you to THINK about the processes which are occurring. USE COMMON SENSE as you set up your apparatus and go through the procedure.
Part A - MELTING (solid to liquid)
1. Set up your apparatus similar (but not necessarily identical) to the graphic below.
You can use a bunsen burner instead of a hot plate.
2. Weigh out 5 grams of lauric acid and carefully put into test tube.
3. Put the thermometer into the solid lauric acid crystals and take a temperature reading. While you can use your thermometer to carefully stir - BE CAREFUL.
4. Make a hot water bath and heat the water to around 60o C. Immerse the test tube with the solid lauric acid into the hot water bath and begin taking temperature readings every 30 seconds. USE DATA TABLE.
5. Continue taking temperature readings until the solid lauric acid has completely melted. (NOTE: You should have a minimum of 8 temperature readings! If you don't, start over.)
6. Continue taking additional temperature readings after it looks like all the lauric acid has melted. Do this for at least 3 more minutes.
Part B - FREEZING (liquid to solid)
1. Use the test tube containing melted lauric acid from part A.
2. Replace the hot water bath with a room temperature water bath. Start taking temperature readings until the melted lauric acid turns into a solid. (NOTE: You should have a minimum of 8 temperature readings! If you don't, start over.) USE DATA TABLE.
3. Continue taking additional temperature readings after it looks like all the lauric acid has solidified. Do this for at least 3 more minutes.
Part C - CLEAN UP
1. Melt the lauric acid again and pour most of it out into the TRASH CAN PROVIDED.
2. Clean out the test tube using the brushes provided up at the front and soapy water. Rinse and dry out the test tube.
3. Dissassemble the apparatus and put everything back up front. But, leave the triple beam balance on the lab table.
Data Table:
Copy/paste this data table into a Microsoft Word document. Click and drag the table so the table takes up most of the whole page. Bring data table to class on the day of the experiment.
| Name: Block: Date: |
|||
|
Phase Changes Data Table for Lauric Acid |
|||
| Part A: Solid to Liquid - MELTING | Part B: Liquid to Solid - FREEZING | ||
| Time (minutes) | Temperature (oC) | Time (minutes) | Temperature (oC) |
| 0 | 0 | ||
| 0.5 | 0.5 | ||
| 1.0 | 1.0 | ||
| 1.5 | 1.5 | ||
| 2.0 | 2.0 | ||
| 2.5 | 2.5 | ||
| 3.0 | 3.0 | ||
| 3.5 | 3.5 | ||
| 4.0 | 4.0 | ||
| 4.5 | 4.5 | ||
| 5.0 | 5.0 | ||
| 5.5 | 5.5 | ||
| 6.0 | 6.0 | ||
| 6.5 | 6.5 | ||
| 7.0 | 7.0 | ||
| 7.5 | 7.5 | ||
| 8.0 | 8.0 | ||
| 8.5 | 8.5 | ||
| 9.0 | 9.0 | ||
| 9.5 | 9.5 | ||
| 10.0 | 10.0 | ||
| 10.5 | 10.5 | ||
| 11.0 | 11.0 | ||
Processing the Data:
Using Microsoft Excel, create a spreadsheet using the data gathered above. Graph it. Be sure to have the TIME on the x-axis and the TEMPERATURE on the y-axis. Be sure to label everything! Use the x-y scatter plot and be sure to create the two curves on the same data table.
Analyzing the Data:
Copy/paste these questions into Microsoft Word. Answer the questions.
1. Does the temperature of the lauric acid vary while it is melting or freezing? Explain in detail.
2. What is the melting point of lauric acid and what is it's freezing point? Are these two points the same?
3. EXPLAIN - in terms of ENERGY CHANGES - the shapes of the curves. Your answer should be complete and thoughtful. Remember, focus on the energy changes!
4. What would happen to the shape of the curve if you used more lauric acid? How would it change? Be specific.
5. EXPLAIN what is going on at the MOLECULAR level as the liquid lauric acid turns into a solid and as the solid lauric acid turns into a liquid. Be sure to include words like kinetic energy , potential kinetic energy, intermolecular forces of attraction, phase changes, molecules, etc.
6. What happens when you introduce an impurity to a pure
substance in terms of changing the melting point?
Lab Report consists of:
In this order and stapled together....
Completed data table.
Excel data table.
Excel graph.
Data analysis questions (word processed).