question: How do hand warmers with a bit of metal in them work?
comment: In each hand warmer, there’s a ton of a salt called Sodium Acetate dissolved. Water can only dissolve so much of the salt before it’s full, and can’t take any more. But, hot water can dissolve a lot more than room temperature water can. So, they make the solutions in the hand warmers while the water is hot, and then slowly cool them down so that the water is tricked into having more salt dissolved in itself than it normally would be able to at room temperature. When you snap the metal, it creates a disturbance which makes the water “realize” it was “tricked”, and all the extra salt that the water wouldn’t normally dissolve at room temperature is kicked out of the water into solid form. This kicking out of the salt results in a much more stable structure for the salt, its solid form, so all the energy that was being used to keep the unstable dissolved form together at an unusually low temperature is released as the heat that warms your hands.
passages: Battery operated hand warmers use electrically resistive heating devices to convert electrical energy in the battery. Typically hand warmers can heat for up to six hours, with heat outputs from 40-48C. Rechargeable electronic hand warmers can be charged from a mains power supply or from a 5V USB power supply, with up to 500 recharge cycles possible.
  
  Section::::Types.:Charcoal.
   The first commercially-produced hand warmer was created by Japanese inventor Niichi Matoba. Matoba received a patent for applying the principle of an oxidation reaction that produces heat by means of platinum catalysis. He then devoted his time to researching how to make the product suitable for practical use. In 1923, he manufactured a prototype of his device naming it HAKUKIN-kairo (HAKKIN warmer). A version of these original portable hand warmers is still produced in Japan.
  
  Section::::Types.
  
  Section::::Types.:Air activated (iron).
   The most commonly available hand warmers make use of the oxidation of iron to achieve an exothermic reaction:
  
  Section::::Examples of exothermic reactions.
  
  BULLET::::- Combustion reactions of fuels or a substance
  
  BULLET::::- e.g.
  
  BULLET::::- Burning of natural gas: CH4 + 2O2 -> CO2 + 2H2O
  
  BULLET::::- C6H12O6 + 6O2 -> 6CO2 +6H2O
  
  BULLET::::- Neutralization
  
  BULLET::::- The thermite reaction
  
  BULLET::::- Reactions taking place in a self-heating can based on lime aluminium
  
  BULLET::::- Many corrosion reactions such as oxidation of metals
  
  BULLET::::- Most polymerization reactions
  
  BULLET::::- The Haber process of ammonia production
  
  BULLET::::- Respiration
  
  BULLET::::- Decomposition of vegetable matter into compost
   Charcoal hand-warmers provide heat by burning charcoal in a special case. These can last up to 6 hours and become comfortably hot. Charcoal cases for these usually have felt on the outside and have items in it that do not produce heat, but spread the heat such as metal. A charcoal hand warmer can start heating when both ends of charcoal are struck and then extinguished to create a hot charcoal. The smouldering stick is then placed inside the case. The charcoal sticks are available from most outdoor activity shops and are fairly inexpensive.
  
  Section::::See also.
  
  BULLET::::- Instant cold pack
   This process can be scaled up to create a means of domestic heating storage and can produce instant heat.
  
  Section::::Types.:Lighter fuel.
  
  Lighter fuel hand-warmers use lighter fluid (petroleum naptha). These can be reused by simply refuelling. Typical models can generate heat for either half a day or a whole day, depending on conditions.
  
  Section::::Types.:Battery.
   Chemical reaction-based garments have cloth pouches or pockets, where single-use packages of chemical hand warmers can be inserted. The warmers are about the size of a package of cards, and they contain chemicals which produce heat. One traditional hand warmer is a small metal box containing charcoal which, when lit, burns very slowly and produces a mild heat. Disposable heat packs typically contain cellulose, iron, water, activated carbon, vermiculite, and salt. When these packs are exposed to air, an exothermic chemical reaction occurs, which provides several hours of heat. Another chemical heating pad uses a gel that can be activated by twisting the package, thus triggering a chemical reaction. The pack can be put in a pot of boiling water to convert the chemical reaction back and allow it to be reused. While these products are commonly called "hand warmers", they can be placed in boots or, with special garments such as vests, in cloth pockets on the inside of the garment. The warmth produced by these tends to be lowest among the above heating methods.
   Hand warmer
  
  Hand warmers are small (mostly disposable) packets which are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities. Other types of warmers are available to provide soothing heat for muscular or joint aches.
  
  Depending on the type and the source of heat, hand warmers can last 10- 13 hours. (recrystallisation) (platinum catalyst).
  
  Section::::History.
  
  The hand and foot warmer was first patented by Jonathan T. Ellis of New Jersey in 1891, though no evidence exists that it was ever produced.
   This type of hand warmer can be recharged by immersing the hand-warmer in very hot water until the contents are uniform and then allowing it to cool. The release of heat is triggered by flexing a small metal disk in the hand warmer, which generates nucleation centers that initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.
  
  The latent heat of fusion is about 264–289 kJ/kg.
   ColdHeat
  
  ColdHeat was an American company founded to develop and market products using the proprietary graphite-like compound Athalite. The composite material is claimed by the manufacturer to have the unusual ability to conduct large amounts of heat and return to room temperature in a short amount of time.
  
  Section::::Soldering iron.
  
  The first two products were soldering irons powered by alkaline batteries. The manufacturer claims this soldering iron is unique in that its Athalite tip undergoes a temperature change from ambient temperature to approximately and back to ambient within three seconds when the tip is removed from the work. 
   Air-activated hand warmers contain cellulose, iron, water, activated carbon, vermiculite (water reservoir) and salt and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours, although the heat given off rapidly diminishes after 1–2 hours. The oxygen molecules in the air react with iron, forming rust. Salt is often added to catalyze the process.
  
  Section::::Types.:Supersaturated solution (crystallisation-type).
   In May 1994 Vollrath acquired a line of food warmers and accessories with the purchase of Idea/Medalie Division, Rogers, Minnesota. Production of the warmers was moved to Vollrath's Kewaunee, Wisconsin plant.
   The direct contact method of heat sealing utilizes a constantly heated die or sealing bar to apply heat to a specific contact area or path to seal or weld the thermoplastics together. Heat sealing is used for many applications, including heat seal connectors, thermally activated adhesives, film media, plastic ports or foil sealing.
  
  Section::::Heat sealing process.:Applications for the heat sealing process.
  
  Heat seal connectors are used to join LCDs to PCBs in many consumer electronics, as well as in medical and telecommunication devices.
   Disposable chemical pads employ a one-time exothermic chemical reaction. One type, frequently used for hand warmers, is triggered by unwrapping an air-tight packet containing slightly moist iron powder and salt or catalysts which rusts over a period of hours after being exposed to oxygen in the air. Another type contains separate compartments within the pad; when the user squeezes the pad, a barrier ruptures and the compartments mix, producing heat such as the enthalpy change of solution of calcium chloride dissolving.
   The company has developed two versions of the Embrace infant warmer: Embrace Nest, and Embrace Care. The infant warmers are composed of three components: an infant-sized sleeping bag or baby interface, a pouch of phase change material (PCM), and a heater. The pouch, when warmed in the heater and placed into a compartment of the sleeping bag, maintains the World Health Organisation recommended temperature of 37 °C for a period of up to 8 hours. The product was designed to complement skin-to-skin care.
   Candle warmer
  
  A candle warmer is an electric warmer that melts a candle or scented wax to release its scent. The candle warmer shown is intended to be used with jar candles or candles in cups, not with taper candles or candles without containers large enough to accommodate all the melted wax. Some candle warmers have a built-in bowl in which the candle is placed.
   Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs. "(See: Copper in heat exchangers)."
  
  Section::::Types.:Plate heat exchangers.
   A fluid warmer is a medical device used in healthcare facilities for warming fluids, crystalloid, colloid, or blood product, prior to being administered (intravenously or by other parenteral routes) to body temperature levels in order to prevent hypothermia in physically traumatized or surgical patients. Infusion Fluid Warmers are FDA regulated medical devices, product code LGZ. They are unclassified devices with special considerations and require 510(k) clearance to be legally marketed in the United States. There are two primary categories of fluid warmers- those that warm fluids before use, typically warming cabinets, and those that actively warm fluids while being administered, in-line warming.
   BULLET::::- Impulse heat sealers- have heating elements (one or two) of Nichrome placed between a resilient synthetic rubber and a release surface of film or fabric. The heating elements are not continuously heated; heat is generated only when current flows. When the materials are placed in the heat sealer, they are held in place by pressure. An electric current heats the heating element for a specified time to create the required temperature. the Jaws hold the material in place after the heat is stopped, sometimes with cooling water: this allows the material to fuse before stress can be applied.
   The first description of melt electrospinning was by Charles Norton in a patent approved in 1936. After this first discovery, it wasn't until 1981 that melt electrospinning was described as part of a three-paper series. A meeting abstract on melt electrospinning in a vacuum was published by Reneker and Rangkupan 20 years later in 2001. Since this scientific publication in 2001, there have been regular articles on melt electrospinning, including reviews on the subject. In 2011, melt electrospinning combined with a translating collector was with proposed as a new class of 3D printing.
  
  Section::::Principles.
   Section::::Patents.
  
  The patent for this product can be found on the list of google patents. There, it lists a clear description of each part integral to the function and process used to create squeeze cast metals. For example, it describes what temperatures and pressures are required, what function each step serves, and how it relates to the whole process.
   Section::::History.
  
  The portable ice chest was invented by Richard C. Laramy of Joliet, Illinois. On February 24, 1951, Laramy filed an application with the United States Patent Office for a portable ice chest (Serial No. 212,573). The patent (#2,663,157) was issued December 22, 1953. 
  
  The Coleman Company popularized the cooler with its initial offering of a galvanized cooler in 1954. Three years later, Coleman developed a process to make a plastic liner for coolers and jugs. 
  
  Section::::Thermoelectric coolers.
   Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs.
   The final part of the assembly is the thermocouple itself, whose wires are insulated from one another and from the material of the assembly by a tube made of mullite.
  
  Section::::Components.:Capsules.
  
  The sample capsule must contain the sample and prevent reaction between the sample and the other materials of the sample assembly and not, itself, react with the sample. It must also be weak so as not to interfere with pressure transmission during the run. For this purpose the materials most used are: Au, Pt, AgPd alloys, Ni and graphite.
   Section::::Types of glass-to-metal hermetic seals.
  
  When the glass and the metal being hermetically sealed have the same coefficient of thermal expansion, a "matched seal" derives its strength from bond between the glass and the metal's oxide. This type of glass-to-metal hermetic seal is the weaker of the two types and is generally used for low-intensity applications such as in light bulb bases.
   The Arctic Silver company was founded by Nevin House in 1999. Headquartered in Visalia, California, USA, House's original product was a custom modified version of HP's Turbocooler heatsink design from servers and workstations. That experience led to research in thermal compounds, and their material composition. House experimented with silver based solutions, and success with improved formulas led to a change in business focus, to thermal interface compounds.
  