Demonstration

Summary

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.

Grade Level

Middle and high school

Objectives

By the end of this lesson, students should be able to

• Better understand what happens in terms of energy when a substance evaporates or freezes.

Chemistry Topics

This lesson supports students’ understanding of

• Energy
• Exothermic/endothermic

Time

Teacher Preparation: 10 minutes
Lesson: 15–20 minutes

Materials

• Hot plate
• Beakers (2 x 250 mL)
• Thermometers (2)
  
• Water
• Paraffin wax

Safety

• Always wear safety goggles when working in the lab.
• The teacher should use a thermometer when heating both the water and the wax to ensure the temperature of each remains safe and does not exceed 50⁰C.
• Paraffin wax SDS

Procedure

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 50^oC.  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.)

Teacher Notes

• 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.

Downloads

• Teacher Guide doc
• Teacher Guide pdf
• Student Activity doc
• Student Activity pdf

Submitted by
Courtney Lantz
Boise State
Boise, Idaho

Thanks to
PASCO

For the Student

Lesson

Prelab Questions

Circle the choice of words in parentheses that you think is correct.

1. When wax freezes on your finger, the PE goes (up, down) and the KE goes (up, down). Your finger should get (hot, cold).
2. When water evaporates from your finger, the PE goes (up, down) and the KE goes (up, down). Your finger should get (hot, cold).

Safety

Always were goggles when working in the lab.

Results

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?

          Water                                                 Wax

 

Conclusion

Were your predictions in the prelab correct? Explain.

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