At the time of writing these lines, there is half a meter of snow outside the window and –10 °C. The sunlight plays beautifully on the frosted trees, but the cold is merciless. If you have to pull off your gloves, your fingers quickly grow cold and stiff, the skin starts to burn. The blood circulation in the hand deteriorates rapidly, and despite the gloves, they no longer help much. It’s better to tuck your hands under your coat or use some kind of hand warmer.
Nowadays, the most popular hand warmers are small plastic pouches filled with some liquid and a metal disc about the size of a coin. When you snap the little disc, it clicks, and the liquid solidifies as if by magic. It turns into a slushy, ice-like substance while heating up to as much as 50 °C. The pouch can be kneaded for a good half an hour, during which it continuously gives off heat; then it hardens completely and cools down. It is perfectly safe: it won’t burn, won’t set anything on fire, produces no soot, requires no electricity, and is reusable. By placing it in a glass of hot water, it can be recharged with heat.
Many people boil it in vigorously bubbling water, but that is a mistake. The water must be at least 90 °C, covering the pouch entirely, but it should not boil! You can heat the water in a microwave or on a stove, but in that case wrap the pouch in a cloth so it doesn’t stick to the pot, or heat it in a water bath! Within 5–10 minutes, the substance becomes completely liquid again. At that point, leave it standing in the hot water to cool down slowly.
What’s inside the pouch?
This strange, ice-like substance is an ordinary compound: sodium acetate, the salt of sodium and acetic acid. It is very cheap and widely used, primarily for adjusting the pH of various solutions. Its aqueous solution is alkaline (a weak base, the saturated solution has a pH of 9.7 at 20 °C). It is non-toxic. In the food industry it is used to preserve canned meat (E262). In small amounts it occurs naturally in almost every fruit and in foods produced by fermentation. Because of its salty-vinegary taste, it is also added to potato chips as a flavor enhancer. It is also used in modern winter de-icing agents for roads, since it is much less harmful to the environment than table salt.
In solid form, it is a white powder. Its beautiful, needle-like crystals contain water of crystallization. What the pouch actually contains is hydrated sodium acetate, namely sodium acetate trihydrate (CH₃COONa·3H₂O). When heated in air above 100 °C, it loses its crystal water, and at 324 °C it decomposes with the release of acetone. It is flammable, and when ignited it can produce toxic vapors and gases.
When the pouch is heated, the water bound in the crystals also leaves, but since it cannot evaporate, it stays trapped inside. The sodium acetate gradually dissolves in the water, forming a solution.
Freezing Needs a Little Disturbance
In the solution, sodium acetate dissociates into sodium and acetate ions. Some of the acetate ions take up hydrogen from the water and turn into acetic acid. In the solution, sodium ions, acetate ions, hydroxide ions, acetic acid, and water molecules all perform their peculiar dance. Everyone knows that solubility depends on temperature—after all, the hotter the tea, the more easily sugar dissolves in it. The same is true for sodium acetate: much more can dissolve in hot water than in cold. When the pouch begins to cool, the amount of substance that can remain dissolved decreases. Logically, some of the ions moving about in the solution should neatly rearrange themselves into solid crystals, just as the textbook says. But in reality, things are not that simple!
In the seemingly calm liquid, at the microscopic level, things are in fact very noisy. Energy fluctuates strongly; particles jiggle chaotically due to thermal motion, constantly colliding. Sometimes they clump together into small groups, but sooner or later these groups fall apart again because of collisions. For crystallization to start, it isn’t enough for a few particles to cluster together; they must also adopt the specific lattice structure of the substance, line up in order, and form a stable crystal nucleus. But creating order requires energy. The energy of a crystal nucleus depends on its size: there is a critical size at which its energy reaches a maximum. Such a nucleus may either dissolve again or continue to grow, but in either case the process involves a release of energy. In other words, a certain amount of energy is needed for a crystal embryo to reach the critical size—large enough to become a crystal seed capable of growing.
In some substances—sodium acetate solution among them—the spontaneous formation of crystal nuclei is difficult. Thermal fluctuations alone cannot provide enough energy for the embryos to overcome the activation barrier. The solution can be supercooled without crystallization beginning. And this is not some fragile, unstable condition! It is stable enough that the solution can be poured, shaken, carried in a pouch or a pocket, without crystallizing. What’s needed is a tiny kick of energy, a little disturbance that triggers crystallization at a single point. When you snap the small steel disc, it makes a sharp click, like the sound of a dog-training clicker. That vibration provides the activation energy. The crystal seeds that appear then grow rapidly, releasing a significant amount of energy as they do, and the pouch warms up.
If the used hand warmer isn’t boiled long enough and solid crystals remain inside, they will serve as nucleation sites, and the material will crystallize again on its own, re-solidifying the hand warmer. That is why the pouch must be thoroughly “cooked.” The water must not boil, because then steam could also form inside the pouch, which may escape through the plastic. The pouch’s contents would lose water, become more concentrated, and overly saturated. Such a hand warmer will crystallize spontaneously without any external trigger, making it unusable. Boiling shortens the lifespan of the hand warmer!
Let’s Make Sodium Acetate!
There’s no need to cut open a hand-warmer pouch if you want to experiment with sodium acetate. It’s easy to produce at home from vinegar and baking soda.
Pour 180 ml of 20% vinegar into a container. Use plain, inexpensive household vinegar—not apple cider vinegar, balsamic vinegar, or the like. Gradually, in small portions, add one packet (50 g) of baking soda (sodium bicarbonate). From the reaction of acetic acid and sodium bicarbonate, sodium acetate and carbonic acid are formed. The carbonic acid immediately decomposes into carbon dioxide and water, causing the solution to fizz vigorously. Be careful not to let it overflow!
The reaction is endothermic, meaning the solution absorbs heat from its surroundings during the process, and the glass cools down. Once the reaction has finished, you are left with a clear, colorless liquid—an aqueous solution of sodium acetate. In about 200 ml of solution, roughly 50 g of sodium acetate is dissolved. This is not yet a saturated solution. To concentrate it, the water must be evaporated.
Place a few quartz pebbles in the glass as boiling chips (to help bubble formation and prevent bumping while boiling), then put it on the stove and boil off the excess water. It gives off a terrible vinegar smell, so work with the window open! When the volume has been reduced to about half, you’ll be left with a slightly yellowish, more viscous solution. Let this cool down, and it’s ready.
If you tap the surface with a toothpick, beautiful needle-shaped crystals will begin to grow. You can also crystallize it on a microscope slide and observe it under a microscope. Poured onto a glass plate, it forms frost-like patterns. If poured into a dish, salt-statue-like columns rise from it. The crystallization is so rapid that it even creeps back up into the glass you’re pouring from.
The crystallized sodium acetate can most easily be melted again in a water bath or microwave. Place the crystals in a heat-resistant glass. Cover it, and set it into a larger container filled with water, then heat it. Be careful with hot liquids while experimenting!
