Hot Hands, Instant Crystals, and Super Cool Solutions

As the weather gets colder, fingers get frosty. Most chemical hand warmers are single use and take a while to warm up. But it's easy to make an instant, reusable hot pack with two simple ingredients: baking soda and vinegar. Follow the recipe below to make your own at home!

Materials:
  • 1 cup white vinegar (acetic acid C2O2H4)
  • 5 level teaspoons of baking soda (sodium bicarbonate NaHCO3
OR
  • 1 cup and 2 tablespoons white vinegar (acetic acid C2O2H4)
  • 3 level teaspoons of washing soda (sodium carbonate Na2CO3)
PLUS
  • One small, clean glass jar with lid (baby food jars work well, or spice bottles)
  • A small resealable plastic bag or another small jar
  • Frying pan
  • Spoon
  • Plastic spatula 
materials

To Do:
  • Measure the baking soda or washing soda into the frying pan. Add a small amount of vinegar. What happens?
pouring vinegar into frying pan
  • Stir the solution until the bubbles disappear.
stir the solution 
  • With an adult's help, warm the pan over medium heat. Continue to stir the solution. 
  • Repeat steps 2 and 3 until the solution starts to turn into a white powder around the edges and there is less than 2 tablespoons of solution left in the pan. This is now a super saturated solution!
crystals form along the edge of the solution
  • Carefully pour this solution into the jar. Caution! The solution is very hot! Set it aside until it is at room temperature and then put the lid on the jar until you are ready to use it.
the super saturated solution
  • Do not wash the frying pan yet! Allow it to cool completely and wait for all the liquid to evaporate. What is left in the pan?
  • Using a plastic spatula, scrape off the white powder (this is sodium acetate) and store it in a resealable plastic bag or another small jar. Label it "sodium acetate powder". This powder will be used as seed crystals.
  • Once the liquid in the jar is at room temperature, feel the bottom of the jar. Is it hot or cool?
  • Use a spoon to scoop up only a few grains of the sodium acetate powder. Open the jar and drop them in. What do you see?
drop a seed crystal into your solution
  • Feel the bottom of the jar. What do you notice?
the solution will crystalize and begin to turn solid
  • To reset the sodium acetate solution, pour water into a pan about 1 inch deep. Heat the water over medium-high heat. Once the water is hot (but not boiling) set the jar in the water. The now solid sodium acetate will melt. Once it has melted completely, remove the jar from the hot water bath and set it aside to cool. When it it completely cool, replace the cap until you're ready to use it again!
  • Wash the pan, spoon, and spatula thoroughly.

Troubleshooting:
  • If the super saturated solution does not make crystals immediately or at all, or slowly grows long, thin crystals in the solution after you add the seed crystal, then there is too much water in the solution. You can either leave the container in a sunny place with the lid off to allow some to evaporate, or heat the jar in the water bath for an extended amount of time to get rid of excess water.
  • If the solid solution won't melt completely or it forms crystals when it cools after melting, there isn't enough water in the solution. Add a few drops of water to the jar and repeat the melting process.
  • If you run out of seed crystals, put a drop or two of solution onto a smooth surface (like a plate, tile or jar lid) and wait for the water to evaporate and leave behind white crystals.
  • This solution can be used over and over again!

A Closer Look

Both baking soda (sodium bicarbonate NaHCO3) and washing soda (sodium carbonate Na2CO3) are bases. Vinegar (acetic acid C2O2H4) is an acid. When either baking soda or washing soda is mixed with vinegar, they react to form liquid water (H2O) carbon dioxide gas (CO2) which is the source of the bubbles, and disolved sodium acetate (NaC2O2H3). Heat helps to speed up this reaction and causes some of the liquid water to turn into steam. As more water evaporates into steam, it leaves behind the sodium acetate.

Eventually there is so much more sodium acetate left behind than water that it begins to turn solid. when you add heat, you're giving energy to the molecules of sodium acetate. The moving molecules can't latch on to each other to make the organized structures that make up crystals. Adding a few grains of sodium acetate powder provides a little organization to the liquid, and the molescules are able to line up to form the crystals.

this commercial hand warmer uses a plate to induce crystallization

As the solution crystallizes, the molecules slow down and give off that extra energy as heat. Because sodium acetate is so good at this, it is used in some kinds of commercial instant hand warmers. We use a few on them here in OMSI's Chemistry Lab!

Commercial sodium acetate hand warmers 

These commercial hand warmers use a metal plate within the enclosed solution. When bent or rubbed the plate will irritate the solution and cause crystallization to occur. Instant warm hands! These can be melted back down to the original solution and used again and again.



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5 Comments

What would happen if you cooled the hand warmers before activating them?

Hi Kaleigh! Thanks for asking! This is Elizabeth, Senior Educator in OMSI’s Chemistry Lab, here with an answer to your question. I decided to try cooling some sodium acetate hand warmers to find out. I placed two commercial hand warmers, like those snowmen pictured above, and two prepared hand warmers, like the ones created in the activity, in our 40°F/5°C refrigerator overnight. I also put one of each into our 32°F/0°C freezer for about four hours. The refrigerated commercial hand warmers didn't seem very different from ones at room temperature—they both solidified at the same speed and felt equally warm. The prepared bottles were a different story: they solidified much more slowly than their room temperature counterparts, and only reached about 90°F/32°C, while the room temperature bottles reached closer to 110°F/43°C. The results from the commercial and prepared hand warmers from the freezer were a little different. The commercial hand warmer solidified at the same rate as the room temperature one, but the heat given off was only a little warmer than room temperature. The prepared hand warmer solidified slower than the room temperature one, and also felt cooler than room temperature; its final temperature was close to 50°F/10°C. (For reference, our labs are usually in the 60-70°F/15-21°C range.) My hypothesis is that the cooled hand warmers from both the freezer and the refrigerator didn't warm up as much as the room temperature hand warmers because the heat had to overcome the cold. Since they were starting at a lower temperature, they didn't rise to as warm of a temperature.

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Hi, I was wondering if you knew it the metal plates that are found inside the commercial hand warmers could be brought? If not could you use anything else? Thanks, Zoe

Hi Zoe! Thanks for asking! This is Elizabeth, Senior Educator in OMSI’s Chemistry Lab, back with an answer to your question. Unfortunately, I do not know where to purchase the metal discs inside the commercial hand warmers. However, there are other substances that could be used to initiate the solidification of the sodium acetate. All sodium acetate needs to begin crystallizing is a nucleation site; that is, a place where the crystals begin to grow. (Think of the classic Mentos and soda eruption: the Mentos candies have the perfect surface to be a nucleation site for the rapid formation of bubbles in the soda.) In the activity above this is a few grains of sodium acetate crystals; in commercial hand warmers, the ridges and grooves on the metal disc do the job. Other items that will start the process include grains of sand and scratching the inside of the container holding the liquid sodium acetate. Both of those introduce fresh, dry, rough surfaces that are perfect for starting the first crystals growing. However, both of these will have to be repeated after the sodium acetate is melted to start the crystals growing again (some sand still sitting in the bottom or an old scratch won’t work). This would be a great experiment to try at home. What other things can you add or do to the container of liquid sodium acetate that causes it to solidify? I’m interested in hearing what you or others find out!

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