Heat Pack - Supersaturation
Author(s): Chris Chui, George Lyle
Science Kits - Teacher's Guide
SED 695B; Fall 2005

Overview: Supersaturation can occur when a saturated solution is cooled below the saturation temperature. If there is no locus where precipitation can start, the solute remains in solution. This principle is used in "instant heat" packs such as the one in this kit. The solution is normally left in a supersaturated state until the pack is needed. When a metal disk is snapped, the solution is shocked and the solute rapidly crystallizes out of solution. The crystal is a lower energy state than the solution, so heat is produced while the crystallization is occurring.

The pack is "recharged" by raising it to a temperature above that at which the solution is saturated. This is easily done by placing it in boiling water. Once the solute is dissolved the pack can be removed and cooled

     

Topics addressed:

  • Supersatuation
  • Crystallization
  • Energy Conservation

California Standard for Chemistry:

Chemical Thermodynamics

7. Energy is exchanged or transformed in all chemical reactions and physical changes of matter. As a basis for understanding this concept:

a.

  Students know how to describe temperature and heat flow in terms of the motion of molecules (or atoms).

b.

  Students know chemical processes can either release (exothermic) or absorb (endothermic) thermal energy.

c.

  Students know energy is released when a material condenses or freezes and absorbed when a material evaporates or melts.
The purpose of this experiment is to determine the temperature achieved by this heat pack while it is operating, as well as to observe the crystallization process.
   
 

Procedure:

Please see illustrated procedure below

 

Questions:

  • What is the temperature achieved by a heat pack when it crystallizes?
  • How long does it maintain this temperature?
This is a heat pack. They come in various sizes. This one is about the size of a hand. The pack is filled with sodium acetate solution. Inside the heat pack behind the "Prism" logo you can see the metal "pop" disk used to initiate crystallization.
The first step is to determine the temperature of the heat pack before it is activated. This is done by laying the heat pack on a glass thermometer with an insulating sheet of styrofoam underneath. Leave it like this for several minutes to get a good temperature reading.
In this example, the initial temperature of the heat pack was 24C.
The "pop disk" is popped to initiate the crystallization. This is easily done by squeezing the disk between the fingers.
The crystallization proceeds rapidly across the pack. It takes about 20 seconds for the crystal to propagate to the opposite end of the pack, but the crystallization process continues for some time.
The pack is laid back onto the thermometer (again, with styrofoam underneath for insulation) to obtain a temperature reading.
After several minutes, the thermometer settles at about 53C.
To reactivate the pack, its temperature must be raised above the saturation temperature of the contents. This is easily done by placing it in boiling water.
As the pack is boiled, the crystal disappears as it dissolves. When the pack is again clear, it is ready to remove.
Here it is, back to its original state!
   

References & Links:

An excellent science fair project on this topic.

A source for these heat packs at $5.00 each.

A science supplier that has these packs at $10.00 each.

The How Stuff Works page on this topic.

MSDS for Sodium Acetate