Freezing Point of an Unknown Substance
Students will observe the freezing behavior of an unknown substance, measure the temperature at timed intervals as the substance is cooled, and determine experimentally its freezing point, which is used to identify the substance from a list of freezing points of known materials.
- 1 Student Handout: Freezing Point of an Unknown Substance (per student, group, class)
- 1 Student Reference Sheet: Data Table and Molecule Diagram (per group)
- 1 Temperature probe or thermometer (per group)
- 1 Large test tube (per group)
- 2 Test tube clamps (per group)
- 1 Timer (one per group)
- 1 75oC hot water bath (per group)
- 1 400 ml beaker for hot water bath (per group)
- 1 Bunsen burner with ring stand and wire mesh cover or hot plate to heat water (per group)
- Lauric acid (C12H24O2), 25g or enough to fill test tube ¼ full (per group)
- Student Handouts can be printed individually for student use, as a reusable class set, or assigned online.
- The unknown substance is lauric acid, a non-toxic white powder, and a safe-to-handle compound often used in laboratory investigations of melting/freezing point. Lauric acid is a solid at room temperature, but melts easily in boiling water. When heated, lauric acid may smell like bay oil. The substance is a fatty acid and often used in soaps.
- To save lab time, pre-melt lauric acid by preparing a hot water bath for groups’ test tubes and heat to a temperature of no more than 75oC.
- Use test tube clamps to hold test tube.
- Print class sets of the Student Handout: Freezing Point of an Unknown Substance and group sets of the Student Reference Sheet: Data Table and Molecule Diagram.
- Gather safety goggles, lab aprons, and heat resistant gloves for students at lab stations.
Appropriate grouping/differentiated inquiry can be provided with the following scaffolding suggestions:
- Group students who need more guided practice together and spend more time with them to develop their procedure and let the other groups work more independently.
- Group students with mixed needs and have them work together to develop a procedure. Monitor all groups equally.
- Describe what is occurring energetically to water particles as they are heated from 90oC to 110oC.
- At 90-99oC the heat applied to the system is giving energy to the particles causing them to move faster. Once the temperature reaches boiling point, at 100oC, the energy is used to break intermolecular attractions to allow the liquid water to break away into a vapor. From 101-110oC, the particles are gaining energy to move faster again within the gas phase.
Procedue and Facilitation
- Building on the concepts acquired in Task 1, students will determine the identity of an unknown liquid that starts at 75oC by letting it cool. Then students determine the freezing point and compare to a table of standard melting/freezing points of various substances.
- Give the prompt below and allow students to answer the first question in a whole class discussion:
- “Look at the table of standard melting points provided. Why do you think the table identifies the temperature listed as either the melting point or the freezing point?”
- The point at which a substance melts when being warmed is the same point that a substance freezes when being cooled.
- Students can decide on roles: timer, holder, recorder, etc.
- Read the temperature of the melted substance using the thermometer inside the test tube. Record that temperature at time 0 second.Remove the test tube with the test tube clamp and read the thermometer every 30 seconds. Stir the substance gently with the thermometer in between readings. Continue until the substance becomes solid and maintains the same temperature for 10 minutes.
- Let students know that when this substance becomes a solid at its freezing point, it will cling to the thermometer and test tube. Consequently, students must not pull out the thermometer, but instead carry the test tube with the tongs to replace it in the hot water bath to melt the substance. The thermometer can then be removed safely.
- Be watchful of the reheated water bath so that the test tube with lauric acid does not go over 75oC.
- For the temperature probe, a digital metal probe works best. If a glass thermometer is used, provide for a safe constant mixing of the ice water, e.g. magnetic stirrer. Some temperature probes may require a data interface in order to transfer information to a graphing calculator or computer, which also may require updating certain operating systems. Please check before the lesson in order to use as much available time for the lesson as possible.
- In your lab groups, create a data table that includes temperature vs. time.
- Measure the initial temperature of your sample while still in the hot water bath and record at time 0 minute.
- Begin the timer as soon as the sample is retrieved from the hot water bath.
- Place the test tube in a test tube holder and continue to measure the temperature in one-minute increments until the temperature is no longer changing.
- Use the table of standard melting/freezing points for various substances at your station to determine the unknown substance. Look at the table of standard melting/freezing points provided. Why do you think the table identifies the temperature listed as either the melting point or the freezing point?
- Because the substance when cooled will solidify around the thermometer, do not pull out the thermometer. When your group has completed your work, carry the test tube by the test tube holder, replace it in the hot water bath to re-melt the substance and remove the thermometer safely.
- Caution: Do not reheat the test tube with the substance at any temperature over 75oC or harmful fumes may form.
- Prepare a cooling curve graph with temperature vs. time as the substance cooled. Identify the melting/freezing point and other observations on the graph. Include labels for your x- and y axes, intervals, title, and legend.
- Complete a scientific explanation (CER) for this prompt: Look at the table of standard melting points provided. Why do you think the table identifies the temperature listed as either the melting point or the freezing point? Provide a claim, evidence, reasoning, and rebuttal.
Student Handout: Freezing Point of an Unknown Substance
Student Reference Sheet: Data Table and Molecule Diagram
Sample Student Responses and Answer Key
CER Answer Key:
The unknown substance, based on its freezing point, is lauric acid.
- As the substance changed from liquid to solid, the temperature varied less than one degree.
- The graph created from the time and temperature data plateaued at 42oC.
When a substance freezes or melts, it does so at a unique temperature that remains constant until most of the substance has undergone the change in state. The data and graph show this to occur at 42oC which corresponds to lauric acid in the standards table.
Writing a Scientific Explanation Rubric Key