How do iron powder heat packs work?

Answer 1

Reusable heat packs are filled with a supersaturated solution of sodium acetate and water. They operate on the principle of "supercooling". Supercooling is the principle of cooling a liquid below the point at which it normally "freezes", or crystallizes into a solid. Many liquids can be supercooled. We're all familiar with the thermal properties of water. Exploring the idea of supercooling water will give us better insight into the concept and how it works in sodium acetate heat packs.

Water typically freezes at 32F (0C). As heat is removed from water, the temperature will decrease to the 32F freezing point. It will remain at 32F until it is completely crystallized into ice, at which point the temperature of the ice begins to fall.

Many of us were first exposed to supercooling in this common high school physics experiment: Fill a very clean glass container with pure, distilled water. Slowly decrease the temperature of the water without physically disturbing the container. It is common to be able to reduce the temperature of the water to well below the 32F (0C) freezing point of water without it actually changing state into ice if we're careful not to provide any nucleation points for crystals to form on. It isn't unusual to be able to reduce the temperature of the water to 20F without it actually freezing. The water in this state is "supercooled" by twelve degrees. That is, it exists as a liquid that is twelve degrees colder than the freezing point.

If we then provide a nucleation point -- a site upon which crystals can begin to form -- the temperature of the water does something truly fascinating. It bounces immediately up to the 32F freezing point until it is all converted to ice. It then begins to cool down below the freezing point. A nucleation point can be formed by dropping a grain of sand into the supercooled water, or by simply tapping the side of the glass container. Sometimes the experiment is concluded by pouring the supercooled water out of the container. It turns instantly to ice as it pours.

The supersaturated sodium acetate used in reusable heat packs has the property of being very stable in a supercooled state. The "freezing" point of the supersaturated sodium acetate mixture is about 140F. It is very happy, however, to exist as a liquid at room temperature or colder if no nucleation sites are provided upon which the crystallization process can begin. Once the sodium acetate solution does begin to freeze, it behave the same way supercooled water does: the temperature jumps immediately to the freezing point until all of solution is converted to solid crystals. Since the freezing point of the sodium acetate solution is 140F, the solution jumps to that temperature until it is crystallized.

Reusable sodium acetate heat packs are usually "activated" by bending a metal disk that is provided inside the plastic pouch containing the solution. When the disk warps, a nucleation site is provided for the sodium acetate crystals to form on. The heat pack immediately jumps in temperature to 140F.

A sodium acetate heat pack can be "reset" by placing it in boiling water until all of the crystals have melted back into a liquid. Even a single remaining crystal will provide a nucleation site for new crystals to form on, so the melting process must be thorough. This cycle can be repeated indefinitely. In practice, reusable heat packs typically last until the plastic pouch becomes damaged.

Answer 2

Heat is given off as a result of a chemical reaction. Hot/cold packs are used by athletes to minimize swelling of injuries such as muscle and joint sprains. They are constructed of a large pouch containing a dry chemical plus an inner pouch of water. The hot/cold pack is activated by breaking the seal on the pouch of water and shaking the pack vigorously. This action mixes the water with the chemical starting the exothermic or endothermic reaction.

Answer 3

Hot packs are used to warm up your hands or other parts of your body. You first warm them up in hot water or the microwave, and then put them in your pockets.

Answer 4

by a chemical reaction called endothermic reactions

Answer 5

Two phenomenons can be implied:

- releasing of thermal energy from an exothermic chemical reaction

- releasing of thermal energy as crystallization energy

Answer 6

The iron (Fe) reacts with oxygen to produce Iron III oxide, commonly known as rust.

The equation is 4Fe + 3O2 --> 2Fe2O3

Answer 7

not answering figuer it out your self