In this lesson plan students observe a teacher-directed demonstration that provides a conceptual understanding of latent heat. A reusable handwarmer that uses a solution of sodium acetate is used to illustrate phase changes and the release of energy and latent heat. Educational levels: High school, Middle school, Undergraduate lower division

Bryan Aivazian

English

Digital Library for Earth System Education

TITLE:  OBSERVING LATENT HEAT WITH A PORTABLE HAND WARMER 
    
TOPIC:  Phase changes in matter – latent heat  
 
GRADE LEVEL: 7-12 
 
CONTENT OBJECTIVE & SHORT DESCRIPTION: Students will observe a 
teacher-directed demonstration that provides a conceptual understanding of 
latent heat. 
 
CONTENT STANDARD:   
5 8:Content Standard B Physical Science Standards: Properties and changes of 
properties in matter  
5 8:Content Standard B Physical Science Standards: Transfer of energy  
9 12:Content Standard B Physical Science Standards: Interactions of energy and 
matter  
9 12:Content Standard D Earth and Space Science Standards: Energy in the 
earth system  
 
RESOURCE TYPE: Classroom Demonstration 
 
TIME REQUIRED:  45 minutes 
 
MATERIALS NEEDED:  
· a portable hand warmer (sodium acetate type) 
· heat source (gas flame or electric hot plate) 
· Pyrex beaker 
· ice 
· water 
· thermometer 
 
DIRECTIONS FOR INSTRUCTION/ACTIVITY: 
Introduction 
I have found that a reusable hand warmer can serve as a wonderful aid in 
demonstrating the difficult concept of latent heat.  These pocket hand warmers 
can be found in most sporting good or ski equipment shops, generally at a price 
of about $10 each.  Please be sure, however, to get the kind that uses a solution 
of sodium acetate (see the back of the package when purchasing). 
 
At normal room temperatures and pressures the saturated sodium acetate 
solution exists as a hydrated crystalline solid.  When these packets are heated in 
a microwave oven, or placed in a container of boiling water, heat is added to the 
solid and a phase change from solid to liquid takes place.  However, when the 
temperature drops back down to room temperature the cooled fluid does not 
return to its normal solid-state.  At this point the liquid is called a supersaturated 
fluid.  
 
The liquid within the sealed packets will remain a supercooled fluid until its 
equilibrium is upset.  In the case of the commercial hand warmer unit, this 
process is activated by pressing the metal button that is contained within the 
fluid. While the exact mechanism for initiating this change is not known, various 
representatives of the manufacturer have suggested various plausible 
explanations.  These include: 
 
· that the mechanism is related to that heat that is generated from the 
bending of the metal button,  
· the changing geometry of the surface of the metal desk provides a 
more suitable base on which crystals may grow,  
· or that the release of microscopic fragments of metal that small from 
the bending metal serve as condensation nuclei around which the 
crystals grow. 
 
Regardless of the underlying mechanism, once the button is pushed the liquid 
within the packet immediately undergoes a phase change from liquid to solid and 
in the process releases a great amount of latent heat.  In order to reuse the hand 
warmer the crystal within the packet must be melted down again. This requires 
the addition of heat, which is stored in the material as long as the saturated 
solution of sodium acetate remains in its liquid state. 
 
 
Procedure 
Begin the demonstration by showing students a hand warmer that has not been 
activated (liquid inside).  Press the button inside the package to activate the hand 
warmer. Pass the hand warmer to students to let them directly feel the heat 
release from the package, Ask the students for their ideas as to where the heat 
might come from when the unit was activated. Accept all logical answers, but do 
not offer a "correct" explanation at this point. 
 
Show students a piece of ice and ask the question "what is needed to turn solid 
water (ice) into liquid water?" Accept all logical answers, but summarize that heat 
must be added to turn solid ice into liquid water. 
 
Show students a glass of water and ask the question "what is needed to turn the 
liquid water into water vapor (a gas)?"  Accept all logical answers, but summarize 
that heat must be added to liquid water to turn it into water vapor. 
 
Place a beaker with water over a heat source. Place a thermometer in the water 
and have the students observe that the temperature steadily increases over time.  
Ask the students why the temperature of the water changes over time.  Work 
towards an understanding that as heat is added the temperature will rise. 
 
Place another beaker with ice and water over the same heat source. (The beaker 
should be prepared in advance of the demonstration by placing ice in the water 
to bring the ice and beaker down to freezing temperature.  However, immediately 
before the demonstration, remove most of the ice so that there is only a small 
amount of ice remaining in the water. The more ice in the beaker the longer it 
takes for it to melt and eventually have the water temperature rise.) Place a 
thermometer in the ice water near the surface and stir regularly. Have the 
students observe that the temperature of the ice water does not rise as long as 
there is still ice in the solution. Once the ice has completely melted, the 
temperature will start to rise.  Ask students to the following questions: 
 
· Was heat steadily added to the ice water? (yes) 
· When does the temperature of the water start to rise?  (only after the 
ice had melted) 
· What happened to the heat that was added to the beaker when there 
was still ice in the water?  (Large amounts of heat were needed to 
change 32-degree ice into 32-degree water.  Although the temperature 
did not change, the water underwent a phase change from solid to 
liquid). 
 
 Hold the hand warmer (which is now a solid crystal) up in front of the class. Ask 
the students " what is needed to change the crystalline solid back to liquid". 
Accept all answers, but guide them to the fact that heat must be added. 
 
Now for the tricky part.  Ask the question "what happened to all of the energy that 
went into changing the solid to the liquid".  Students may be tempted to say that 
somehow the energy disappeared as the liquid cooled. However, the important 
concept to develop is that in order for the sodium acetate (or any material, for 
that matter) to stay in liquid form the atoms and molecules in the substance 
needs to stay in a higher state of energy than for atoms and molecules in a solid.  
By their very nature liquids contain much more energy than do solids, and gases 
more energy than liquids. 
 
The Grand Finale - ask the question " what must happen in order for a liquid to 
change to a solid?” The important part of this activity is to have the students 
understand that the heat energy contained within the liquid must be given up in 
order to have the substance turn into a solid.  The liquid sodium acetate in the 
packet gives up its energy very rapidly, and we experience that energy release 
as a significant increase in the temperature of the package.  This "hidden heat", 
otherwise known as latent heat, is present in all changes from one phase to 
another. 
 
EVALUATION:  While there are a number of ways in which to evaluate the 
learning associated with this activity, one technique that is very useful with 
teacher-led demonstrations is for students to write a lab report of the 
demonstration.  I ask them to do four things: 
 
I. What did the teacher use? (a materials list) 
II. What did the teacher do? (a step-by-step list of procedures) 
III.  What did you observe?  (see, hear, feel) 
IV. Offer an explanation (explain in their own words why something 
happened) 
 
LESSON PLAN AUTHOR & CONTACT INFO:  
 
Bryan L. Aivazian 
Science Instructor  
Natrona County School District 
1425 S. Ash 
Casper, Wyoming 82601 
307-235-1536 
bryana@trib.com 
 


Observing Latent Heat with a Portable Hand Warmer

http://www.geosociety.org/educate/LessonPlans/LatentHeat.pdf

Observing Latent Heat with a Portable Hand Warmer

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