1. What Is Inside an Instant Cold Pack Before You Squeeze It
An instant cold pack has two parts: a small bag of water sealed inside a larger bag that holds a solid. Poison Control, a U.S. poison-information service, says these solids are typically ammonium nitrate, calcium ammonium nitrate, or urea [3]. Squeezing the pack tears the water bag, the water meets the solid, and the solid dissolves [2].
A Japanese patent for a cold pack gives a sample recipe: 50 grams of ammonium nitrate, 50 grams of urea, and 50 grams of water in the inner bag. According to the patent, after the water bag is broken, the pack's temperature falls quickly to around −5°C (23°F) under the conditions it describes [1]. Real products differ, so treat this as one example rather than a rule. The answer to our question is already on the table: the pack gets cold because the solid dissolves. Next comes the harder part, which is why dissolving would cool anything.
2. Why Dissolving a Solid Can Pull Heat Out of the Water Around It
A solid like ammonium nitrate is made of tiny charged particles called ions, locked into a neat pattern. To dissolve, the ions have to be pulled apart, and that takes energy. Once they are free, water molecules crowd around them, and that step gives some energy back as heat. Whether a pack gets cold or warm depends on which step wins. A simple way to picture it is as a tug-of-war between "pulling apart" and "wrapping in water."
For ammonium nitrate, pulling apart wins. A chemistry textbook gives the heat absorbed when ammonium nitrate dissolves: 25.7 kJ for each mole. The plus sign means heat goes in [4]. One mole of ammonium nitrate weighs about 80 grams, so under standard conditions, dissolving about 80 grams takes in about 25.7 kilojoules. In a real pack that heat has to come from somewhere nearby, mostly the water, so the pack gets colder. How much colder depends on the amounts involved and on how much heat flows in from outside.
The same chemistry text describes a typical commercial pack as about 100 mL of water with 30 g of ammonium nitrate, and says the temperature drops to about 0°C (32°F) [4]. Here is a rough check you can do yourself. 30 g is about 0.375 mole, which absorbs about 9.6 kJ. Water takes about 4.18 joules to warm by one degree per gram, so 100 g of water holds about 418 joules per degree. Divide, and in this simple sum, where only the 100 g of water supplies the heat and none leaks in, the water would cool by about 23°C. This is our own estimate, not a figure from a source, and real temperatures differ because the bag, the solid and your skin also trade heat.
Dissolving does not always cool things. The same text gives a hot pack that uses about 40 g of calcium chloride in about 100 mL of water, which reaches about 90°C (194°F) [4]. Same action, opposite result, because the balance between the two energy steps is different.
3. How a Frozen Gel Pack Cools in a Different Way
A reusable pack works differently: you freeze it first, then it absorbs heat as it melts. Instant packs, by contrast, cool themselves with chemicals and can only be used once [2].
Melting soaks up a lot of heat. Ice absorbs about 333.6 joules per gram as it melts, and its temperature stays at 0°C while it does [5]. Melt 100 g of ice and you absorb about 33 kJ, more than three times the roughly 9.6 kJ from our 30 g of ammonium nitrate. So an ice-based pack can absorb more heat, but it needs a freezer and some planning first. An instant pack gives up some of that capacity in exchange for working anywhere.
| Question | Instant cold pack | Frozen gel pack |
|---|---|---|
| Where does the cooling come from? | A solid dissolving in water | Frozen water melting |
| Need a freezer first? | No | Yes |
| Reusable? | No, one use | Yes, refreeze it |
4. Why Instant Cold Packs Are Used Only Once, and Staying Safe
Once the solid has dissolved, the pack cannot cool again. Getting the solid back out of the water would take special equipment and care, so a used pack is not something to rebuild at home. That reasoning is ours; the sources only say the packs are single-use [2].
The contents are not food. Poison Control says ammonium nitrate is the most toxic of the usual ingredients if swallowed. It advises throwing away any broken or leaking pack, washing skin with soap and water, rinsing eyes with room-temperature water for 15 minutes, and rinsing the mouth and taking a few sips of water if a little is swallowed. If you are worried, in the United States the poison center number is 1-800-222-1222 [3]. Elsewhere, call your local poison center or emergency number.
5. How to Check What Kind of Cold Pack You Have
With an adult, look at the label on a pack you already own. If it lists ammonium nitrate, urea, or similar salts and shows two bags, it is probably the dissolving type. If it is a sealed gel pack that lives in the freezer, it cools by melting. Do not cut a pack open to look inside.
If you want to go further, read the enthalpy-of-solution chapter cited below. With an adult, use the numbers above to estimate how much the water would cool if you changed the amount of water. Notice how much the answer depends on how much water the heat has to be shared with.