A self-heating can is an enhancement of the common food can. Self-heating cans have dual chambers, one surrounding the other.
In one version, the inner chamber holds the food or drink, and the outer chamber houses chemicals that undergo an exothermic reaction when combined. When the user wants to heat the contents of the can, a ring on the can is pulled to break the barrier separating the chemicals in the outer chamber. In another type, the chemicals are in the inner chamber and the beverage surrounds it in the outer chamber. To heat the contents of the can, the user pushes on the bottom of the can to break the barrier separating the water from the chemicals. This design has the advantages of being more efficient (less heat is lost to the surrounding air) as well as reducing excessive heating of the product's exterior, causing possible discomfort to the user. In either case, after the heat from the reaction has been absorbed by the food, the user can enjoy a hot meal or drink.
Self-heating cans offer benefits to campers and people without access to a microwave oven, stove or camp-fire, but the technology is not yet common. This is because self-heating cans are considerably more expensive than the conventional type, they have problems with uneven heating of their contents, and the heater takes up considerable package space.
The source of the heat for the self-heated can is an exothermic reaction that the user initiates by pressing on the bottom of the can. The can is manufactured as three containers. A container for the beverage surrounds a container of the heating agent separated from a container of water by a thin breakable membrane. When the user pushes on the bottom of the can, a rod pierces the membrane, allowing the water and heating agent to mix. The resulting reaction releases heat thus warms the beverage surrounding it.
The heating agent and responsible reaction vary from product to product. Calcium oxide is used in the following reaction:
- CaO(s)+ H2O(l) → Ca(OH)2(s)
- CuSO4(s) + Zn(s) → ZnSO4(s) + Cu(s)
[In simply] It consists of 2 chambers and lid: 1 for solid component; 1 for liquid. All process is exotermical chemical reaction. When you pierce the lid, liquid component mixed with the solid one and starts a reaction. The most common chemicals for the self heating can is Al + Mg + Zn or CaO (solid component), and HCl or H2O as liquid. (edited by Ransike Randeni, from LKA)
See also 
- An article on the concept
- Single-serving coffee can heats itself
- Will Self-Heating Coffee Be Cool?
- BBC News Online - Hot drink can tested in Midlands
- An article with photos on disassembly