Hello There!
A Heat Pack is An Example Of A Endothermic Process.
An Endothermic Reaction Is A Reaction That Requires Energy To Move Forward.
The SI unit for measuring distance is the
meter.
I attached a table of SI measurements for you :)
Hope this helped!
Answer to this is Radioactive isotopes.
Isotopes are the species of the same element having different atomic masses that means the number of protons remains the same but number of neutrons do differ. For example
and
are the two isotopes of Hydrogen (
).
Radioactive isotopes are the isotopes which release some kind of energy in the form of alpha particles, beta particles or gamma radiation. Examples of each of the decay processes are :
Alpha Decay: In this decay one alpha particle having atomic mass 4 and atomic number 2 or we can say a He molecule will come out. 
Beta Decay: In this decay a
particle is emitted increasing the atomic number of the reactant by 1 unit.

Gamma Radiation: In this type of reaction only radiation is emitted out which does not change the original molecule.

The blueprint process
The best known is a process using ammonium ferric citrate and potassium . The paper is impregnated with a solution of ammonium ferric citr