How Do You Get Heat from a Supercooled Solution? Explore the Chemistry Within Hand Warmers
Abstract
You're at the high school football game and it's getting pretty chilly as the sun goes down. You're determined to keep cheering for your team, but your hands are freezing—have you ever tried hand warmers? The chemistry within these little packets is pretty cool. Hand warmers provide a unique and fun way to study the chemistry of crystal formation and heat generation. By pressing a button in a pouch, which contains a supercooled solution, you start a rapid exothermic (heat-producing) crystallization. In this science fair project, you will determine how the starting temperature affects hand warmer chemistry.Summary
David B. Whyte, PhD, Science Buddies
Parts of the procedure were inspired by the following NASA project:
- Donald, J. (n.d.). Rapid Crystallization. Retrieved May 6, 2009.
- StyrofoamTM is a registered trademark of The Dow Chemical Company.
Objective
The objective of this chemistry science fair project is to determine how the starting temperature affects crystal growth and heat generation in a supersaturated sodium acetate solution—the solution used in hand warmers.
Introduction
Hand warmers are pretty cool devices—they are plastic pouches that contain a clear solution of sodium acetate and a small metal disk. When the disk is pushed, the solution crystallizes and the pouch warms up to 58 degrees Celsius (130 degrees Fahrenheit). It stays warm for 20–30 minutes. The hand warmer can be reused many times by simply putting it in hot water. The crystals dissolve when the temperature rises above 58°C, and they stay in the solution as the hand warmer cools to room temperature. Because the solution stays liquid at room temperature, which is below the temperature at which crystals would normally form, it is called a supercooled solution.
The disk is slightly curved, so that when it is pushed or flexed, it snaps. The snap of the disk initiates crystallization by producing a small amount of solid sodium acetate trihydrate that functions as a nucleation center for further crystal growth. The crystals form because the sodium acetate solution is supersaturated. A supersaturated solution is a solution that contains more of the solute than the liquid would ordinarily dissolve. You can see the crystals form as a wave of white that flows from the disk to the edges of the hand warmer, as shown in the video below.
The crystallization process is exothermic, meaning that heat is emitted. Chemical energy that was stored in the solution is converted into heat energy. In thermochemistry, the amount of energy released by a chemical substance during an exothermic change of state from liquid to solid (or solid to liquid) is called the latent heat of fusion. The latent heat of fusion for the formation of the crystal sodium acetate trihydrate in the hand warmer is approximately 264–289 joules/gram (J/g). Joules (J) are units of energy. The value for the latent heat of fusion tells us that for every gram (g) of crystal formed, about 264 J of energy are released.
In this chemistry science fair project, you will investigate how the starting temperature affects the growth of sodium acetate trihydrate crystals. You will also track how the starting temperature affects the temperature vs. time profile associated with crystallization.
Terms and Concepts
- Sodium acetate
- Supercooled solution
- Sodium acetate trihydrate
- Nucleation center
- Supersaturated
- Solute
- Exothermic
- Thermochemistry
- Change of state
- Latent heat of fusion
- Metastable
Questions
- Why don't the sodium acetate molecules in the supersaturated room-temperature solution precipitate to form crystals? Hint: What is created in the nucleation center by the snap of the metal disk?
- What is the trihydrate in sodium acetate trihydrate crystals?
- What does it mean to refer to the supersaturated solution of sodium acetate as metastable?
Bibliography
- HowStuffWorks, Inc. (n.d.). How do sodium-acetate heat pads work?. Retrieved May 4, 2009.
- Anne Marie Helmenstine, Ph.D., About Chemistry (2009). Exothermic reactions. Retrieved November 25, 2009.
- Wikipedia Contributors (2009). Crystallization. Retrieved November 25, 2009.
Materials and Equipment
- Pot and a lid
- Water
- Heat pack hand warmers (1 package); available from sporting goods stores or from online suppliers such as Amazon.com. A single hand warmer is sufficient, but the procedure will go faster with two or three hand warmers. The hand warmers should be the type that have a metal disc that you snap to activate them — this type of hand warmer, also called the crystallization-type, uses a supersaturated solution and is usually reuseable.
- Tea kettle
- Coolers, any size (2)
- Infrared thermometer, such as the Mastercool IR thermometer, model # 52224-A, available from www.amazon.com
- StyrofoamTM or paper plates or trays (3)
- Lab notebook
- Stopwatch or other timer
- Optional: Camera or video camera
- Optional: Ruler
- Small piece of cloth, about 30 cm (12 inches) square
- Ice
- Tongs
- Helper
- Graph paper
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Experimental Procedure
Important Notes Before You Begin:
- In this science fair project, you will observe how the starting temperature affects the rate of crystal growth, the size of the crystals, and the temperature in the hand warmer after activation.
- The procedure calls for varying the starting temperature of the hand warmer. The starting temperatures will be 0°C, 20°C, and 40°C. To get the hand warmers to these temperatures, you will place the hand warmers in water baths. The water bath at 0°C will be just ice water. The other water baths will be made by adding hot water to room temperature water until the proper temperature is obtained.
Preparing the Setup
-
Heat water to boiling in the pot with the lid. Keep it boiling as you start your experiment.
- This pot of hot water will be used to melt the crystals and to regenerate the hand warmer.
-
Heat water in the tea kettle to boiling, then remove it from the heat.
- This water will be used to raise the temperature in the water baths.
- Add room-temperature tap water to one of the coolers so that the water level will cover a hand warmer, but do not place a hand warmer in there yet.
- Add hot water from the tea kettle to the water in the cooler until the water temperature is 20°C.
- Place a hand warmer in the 20°C water bath.
- Allow a few minutes for the hand warmer to come to the same temperature as the water in the cooler.
- Adjust the temperature with hot or cold water to keep the water bath at 20°.
Observing Crystallization Growth
Note: In the following steps, be ready with the timer and the infrared thermometer to observe and record the changes that occur after crystallization is initiated. Have your helper assist you in taking readings and writing the data.
- Remove the hand warmer from the 20° water bath.
- Record the starting temperature and the time in your lab notebook.
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Place the hand warmer on the Styrofoam tray.
- This insulates the hand warmer so that heat lost to the surface is minimized.
- You can also use an upside-down paper plate.
- Snap the metal disk in the hand warmer and start the timer. Start the timer at the same time as you snap the disk.
-
Time how long it takes for the entire hand warmer to crystallize.
- Watch the hand warmer carefully as the crystals form.
- Use your judgment about when the entire contents of the hand warmer have crystallized.
- Write down your criteria for determining that the entire contents of the hand warmer have crystallized.
-
Record the temperature of the hand warmer every minute for 15 minutes.
- Feel free to change the time or duration of the measurements if you choose, but be sure to use the same time/duration for each trial. For example, take the temperature every 15 seconds for the first minute and every minute for 30 minutes thereafter.
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Observe the crystals that are formed. Write all observations in your lab notebook.
- Record their shape.
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Record the maximum length of the crystals.
- It will be difficult to obtain precise measurements of the crystals' length in the hand warmer since they break easily and grow into each other. Just do your best to estimate.
- As an option, photograph or film the hand warmer during crystallization and analyze the images later. Include a ruler in the pictures so you know the scale.
- Sketch the crystals in your lab notebook, or use photographs.
- To reactivate the hand warmer, wrap it in a piece of cloth and place it in the pot of boiling water for several minutes.
- Use tongs to remove the hand warmer from the water.
- Repeat steps 3–7 of Preparing the Setup and steps 1–9 of this section two more times.
Observing the Effect of a Cold Starting Temperature on Crystallization Growth
- Add ice and water to one of the coolers, enough that it will cover a hand warmer.
- Place the reactivated hand warmer in the ice water.
- Allow a few minutes for the hand warmer to cool to the same temperature as the ice water.
- Remove the hand warmer from the ice water.
- Record the starting temperature and the time in your lab notebook.
- Snap the metal disk in the hand warmer.
- Start the timer.
- Observe the crystal growth and temperature change, as you did in the previous section.
- Reactivate the hand warmer and repeat steps 1–8 of this section two more times.
Observing the Effect of a Hot Starting Temperature on Crystallization Growth
- Add enough room-temperature tap water to one of the coolers so that the water level will cover a hand warmer. Do not place the hand warmer inside yet.
- Add hot water from the kettle to the water in the cooler until the water temperature is 40°C.
- Place a reactivated hand warmer in the 40°C water bath.
- Allow a few minutes for the hand warmer to come to the same temperature as the water in the cooler.
- Remove the hand warmer from the water.
- Record the starting temperature and the time in your lab notebook.
- Snap the metal disk in the hand warmer.
- Start the timer.
- Observe the crystal growth and temperature change, as you did in the previous sections.
- Reactivate the hand warmer and repeat steps 1–8 of this section two more times.
Analyzing Your Results
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For the time it took to complete crystallization:
- Graph the Starting Temperature on the x-axis vs. the Time to Complete Crystallization on the y-axis.
-
For the temperature vs. time data:
- Graph the Time on the x-axis and the Temperature of the Hand Warmer on the y-axis.
-
For the length of the crystals:
- Graph the Starting Temperature on the x-axis and the Maximum Length of the Observed Crystals on the y-axis.
- Discuss your results. For example, what was the maximum temperature reached for each starting temperature, and how long did it take to reach it? Does the temperature remain constant for any length of time for each of the hand warmers? Why? Which hand warmer had the longest crystals?
Ask an Expert
Variations
- Repeat the procedure above with additional starting temperatures, such as: minus 10°C (use a freezer), 30°C, and 50°C.
- Set up a calorimetry apparatus to measure the amount of energy produced by the hand warmer. Calculate the mass of the sodium acetate trihydrate, based on the results.
- The mechanism by which the metal disk initiates crystallization is not well characterized. Make a hypothesis about what happens when the disk is snapped and devise a procedure to test it.
- What is the highest temperature at which you can form crystals in the hand warmer?
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