In this demonstration, students will observe first-hand the exothermic and endothermic properties of state changes by dipping a finger in water and wax—substances with different freezing and boiling points. They will use the sense of touch to verify whether freezing and boiling are processes that require heat or release heat.
High and Middle School
AP Chemistry Curriculum Framework
This lesson supports the following learning objectives:
- Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
- 5.3 The student can generate explanations or make predictions about the transfer of thermal energy between systems based on this transfer being due to a kinetic energy transfer between systems arising from molecular collisions.
- 5.4 The student is able to use conservation of energy to relate the magnitudes of the energy changes occurring in two or more interacting systems, including identification of the systems, the type (heat versus work), or the direction of energy flow.
- 5.5 The student is able to use conservation of energy to relate the magnitudes of the energy changes when two non-reacting substances are mixed or brought into contact with one another.
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
- Science and Engineering Practices: Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
- Systems and System Models: When investigating or describing a system, the boundaries and initial conditions of the system need to be defined and their inputs and outputs analyzed and described using models.
By the end of this demonstration, students should be able to
- Better understand what happens in terms of energy when a substance evaporates or freezes.
This demonstration supports students’ understanding of
Teacher Preparation: 10 minutes
Lesson: 15–20 minutes
- Hot plate
- Beakers (2 x 250 mL)
- Thermometers (2)
- Paraffin wax
Expected student responses are in italics
- On a hot plate, have two beakers: one with water, one with wax. Heat both beakers until the temperatures of the water/wax reaches approximately 50oC. It is important that both the water and the wax are the same temperature.
- Have students make predictions about what will happen when a volunteer dips their finger into each beaker and then removes it.
- Have at least one student volunteer to dip one finger from each hand into the two beakers at the same time so at least the second knuckle is covered.
- They should share with the class how each of their fingers feel. (They should feel the same temperature, relative to each other because the two solutions are the same temperature.)
- When the volunteer removes his/her fingers from the beakers, they should share with the class how their fingers feel (did they get warmer or cooler)? (The finger in the water should feel cooler, and the finger in the wax should feel warmer. Their finger is above the freezing point of water but below the freezing point of wax. Water at body temperature evaporates readily.)
- Paraffin wax can be bought at most craft stores and online.
- The paraffin we used had a melting point of 55⁰C, so we heated the water to the same temperature.
- It is difficult to completely clean the paraffin from the beaker. You may want to let the paraffin harden in the beaker and save it to use it again.
For the Student
Circle the choice of words in parentheses that you think is correct.
- When wax freezes on your finger, the PE goes (up, down) and the KE goes (up, down). Your finger should get (hot, cold).
- When water evaporates from your finger, the PE goes (up, down) and the KE goes (up, down). Your finger should get (hot, cold).
- Always were goggles when working in the lab.
Compared to each other, what do the fingers feel like when in the beakers?
After removing the fingers from the beakers, how do they feel?
Were your predictions in the prelab correct? Explain.