It’s Just a Phase
Student Laboratory Kit
Materials Included In Kit
Cetyl alcohol, C16H33OH, 30 g
Lauric acid, C11H23CO2H, 30 g
Stearic acid, C15H33CO2H, 30 g
Styrofoam cups, 30
Test tubes, Pyrex®, large, 20 x 150 mm, 15
Weighing dishes, 15
Additional Materials Required
Computer graphing program (optional)
Hot plate or hot water bath*
Ring stand and clamp
Stopwatch or timer
Test tube clamp or holder
Thermometer (for hot water bath)*
*May be shared.
Use only borosilicate (e.g., Pyrex®) test tubes for this laboratory. Inspect all test tubes and do not use any cracked or chipped test tubes. Warn students to work carefully to avoid scalding themselves with the hot water bath. Cetyl alcohol is slightly toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The organic solids may be disposed of according to Flinn Suggested Disposal Methods #18b or #24a. Alternatively, the solids may be recycled from class to class and also from year to year.
- The laboratory work for this experiment can easily be completed in a typical 50-minute class period. If desired, however, the lab may be simplified by dividing the class into groups of four students, with two “working pairs” in each group. Each working pair would be responsible for only one part of the experiment, either Part A or Part B. The two working pairs would then share the data with each other to complete the Post-Lab Questions. Two groups may share one hot plate or hot water bath.
- Excellent cooling curve data is obtained in Part A with as little as 2–3 g of solid in a medium test tube. We recommend using digital thermometers for best results. In order to get good results in Part B, however, a larger sample size is recommended. When smaller sample sizes were tested in Part B, the temperature of the solid steadily increased when placed in the hot water bath and the heating curve did not exhibit a well-defined “plateau” at the melting point. Three variables—the amount of solid, the temperature of the hot water bath and the amount of water in the hot water bath—will influence the quality of the heating curve data.
- “Phase-change wallboard” has been developed as a passive thermal storage construction material to improve energy efficiency in heating and cooling buildings. Phase-change wallboard contains paraffin wax embedded in gypsum. The paraffin undergoes reversible phase changes when heated or cooled, absorbing or releasing large amounts of heat and maintaining a constant temperature in the process. Consider how a phase-change material (PCM) might be used in buildings in a mild-climate area. During the day, when the outside temperature increases, the solid PCM melts and absorbs (stores) the excess heat energy. This cools the building and reduces the need for external cooling (air conditioning). The reverse occurs at night—when the outside temperature decreases, the liquid PCM solidifies and releases its “stored” heat energy.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
HS-PS3.A: Definitions of Energy
HS-PS3.B: Conservation of Energy and Energy Transfer
Cause and effect
Systems and system models
Energy and matter
Stability and change
HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
HS-PS3-1. Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
HS-PS3-2. Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motion of particles (objects) and energy associated with the relative position of particles (objects).
Answers to Prelab Questions
- The kinetic-molecular theory (KMT) describes how close together the molecules are in a solid, liquid and a gas; their relative motion; and the attractive forces between the molecules. Use the KMT to explain the following properties of liquids and solids:
- A liquid flows and takes the shape of its container.
The molecules in a liquid remain close together, but move freely and randomly.
- Solids are generally incompressible.
The molecules in a solid are physically as close together as possible (tightly packed).
- Liquids have a definite volume.
The molecules in a liquid are close together and there are attractive forces between the molecules.
- A solid absorbs heat from its surroundings as it melts.
Relatively strong attractive forces exist between the molecules in the solid state—these forces must be (partially) broken when the solid changes to a liquid.
- The following graph shows heating curve data for ice, water, and steam as heat energy is added to the system at a constant rate. (a) In what regions of the curve (A–E) is the average kinetic energy of the molecules increasing? (b) In what region of the curve are ice and water present together? (c) What happens to the heat energy that is absorbed by the molecules in this region of the curve?
- The average kinetic energy of the molecules increases as the temperature increases in regions A, C and E.
- Ice and water are present together at the melting point (freezing point) of water (0 °C, region B).
- The heat energy absorbed by the ice molecules is used to break the attractive forces between the molecules in the crystal lattice. Note: The average kinetic energy of the molecules remains constant. The heat energy increases the potential energy of the molecules. At the same temperature, molecules in the liquid state have a higher potential energy than molecules in the solid state.
* The cooling and heating curve trials may be stopped when the temperature reaches 30 °C and 55 °C, respectively.
Answers to Questions
- Prepare a graph of temperature on the y-axis versus time on the x-axis. Plot the data from Parts A and B as two series of points, using different color pencils or different shapes to mark the points for Part A versus Part B. Draw a smooth (continuous) curve through the plotted points for each series of data.
- Label the following regions (A–C) on the cooling curve: A, only liquid is present; B, liquid and solid are present together; C, only solid is present.
- What happens to the temperature of a pure substance while it is freezing or melting? Estimate the freezing point and the melting point of the organic solid from the cooling curve and the heating curve, respectively. Does the freezing point/melting point depend on the direction in which the phase change takes place?
The temperature of a pure substance should remain constant at the freezing point or melting point as long as both phases are present. The freezing point of the melted solid is estimated from the “flat” region of the cooling curve—44.0 °C. The heating curve did not “level off” at a particular temperature as did the cooling curve. However, the rate at which the temperature increased slowed down noticeably in the 43–44 °C temperature range, giving an estimate of 43.5 °C for the melting point. The slight difference in the estimated freezing point/melting point is not significant (within experimental error). The freezing point/melting point does not depend on the direction in which the phase change takes place.
- Use the following information to identify the unknown organic solid.
Cetyl alcohol, C16H33OH, mp 54–56 °C
Lauric acid, C11H23CO2H, mp 43–44 °C
Stearic acid, C15H33CO2H, mp 67–69°C
The unknown solid had a melting point of 43–44 °C—lauric acid.
- Which set of data (the cooling curve or the heating curve) provided a more accurate or a more precise estimate of the melting point? Which variables in the design of the experiment might account for any difference in the results?
The literature melting point of lauric acid is 43–44 °C. The heating curve data gave a more accurate estimate of the melting point, but the cooling curve data gave a more precise value. The cooling curve was qualitatively easier to interpret because there was a true temperature plateau at the freezing point. The amount of lauric acid was the same in both parts of the experiment. Several variables, however, were different in Part A versus Part B. The hot water bath contained more water than the cold water bath and was therefore a greater heat “reservoir.” The solid sample in Part B could not be stirred, making heat transfer less efficient. The cold water bath was insulated in a coffee cup, the hot water bath was not. Note: This is a great discussion question—students usually grasp at straws when it comes to analyzing experimental error. Ask students to consider how these variables would affect the rate at which heat is transferred into or out of the system, and how the temperature versus time data would be affected as a result.
- Circle the correct choices: Freezing is an exothermic process—the liquid releases heat to its surroundings. At the freezing point, the average potential energy of the molecules decreases and the liquid solidifies.
Note: Question 6 may be difficult for students to grasp. Energy is released during freezing but the temperature does not change. Average kinetic energy is dependent on temperature so it does not change. The potential energy must decrease and this is due to the formation of intermolecular forces holding the solid together. Molecules in the liquid state have a higher potential energy than molecules in the solid state.
- Answer increases, decreases or no change to predict how doubling the amount of solid would change the results in Part B:
- The rate at which the temperature of the solid increases. ___Decreases___
- The temperature at which the solid melts. ___No change___
- The amount of heat absorbed by the sample as it melts. ___Increases___
- (Optional) Lauric and stearic acid are fatty acids (components of fats and oils) that are used to make soap. What factor might explain the regular increase in the melting point of the following fatty acids as the number of carbon atoms increases?
Lauric acid, CH3(CH2)10CO2H, mp 43.2 °C
Myristic acid, CH3(CH2)12CO2H, mp 54.0 °C
Palmitic acid, CH3(CH2)14CO2H, mp 61.8 °C
Stearic acid, CH3(CH2)16CO2H, mp 68.8 °C
In general, the greater the attractive forces between the molecules in a molecular solid, the higher its melting point. This implies that the strength of the attractive forces increases as the number of carbon atoms increases. Note: The properties of the fatty acids are dominated by the long nonpolar hydrocarbon “tail.” The principal attractive forces acting between nonpolar molecules are London dispersion forces. The strength of London dispersion forces increases as the size of the molecules increases.
This kit was adapted from Solids and Liquids, Flinn ChemTopic™ Labs, Vol. 11, Cesa, I., Editor; Flinn Scientific Inc.: Batavia, IL (2005).