|The figure below illustrates a demonstration for a spontaneous endothermic reaction. Left: When crystalline solid barium hydroxide octahydrate Ba(OH)2•8H2O and crystalline solid ammonium nitrate HN4NO3 are mixed, a liquid slurry soon forms as the water of hydration are released. Right: The system absorbs heat so quickly that it cools, the beaker becomes covered with frost, and a moistened block of wood freezes to it. This reaction has a positive value for the enthalpy change ΔH. The entropy of the system, supposedly, is increasing (Ѻ) with a positive value for ΔS.|
When the transformation occurs at constant pressure, the enthalpy change in greater than zero: ΔH > 0; at constant volume, the internal energy change is greater than zero: ΔU > 0. If the surroundings do not supply heat, an endothermic transformation leads to a drop in the temperature of the system. 
An example of an endothermic reaction is the heating of calcium carbonate CaCO3 to a temperature above 840°C, where, at which point, the reactant decomposes to form calcium oxide CaO, commonly called quicklime, and carbon dioxide CO2:
CaCO3 → CaO + CO2
The reaction has an enthalpy change ΔH of + 178 kJ / mole.
The term “endothermic” was coined, as most references indicate, by French chemist Marcellin Berthelot, supposedly in 1866, to describe a reaction in which heat was absorbed. 
● Thermal words
1. (a) Perrot, Pierre. (1998). A to Z of Thermodynamics (pg. 83). Oxford University Press.
(b) Endothermic – EtymOnline.com.
2. (a) Look, Dwight C., Sauer, Harry J., and Alexander, Graham I. (1988). Engineering Thermodynamics (pg. 764). Van Nostrand Reinhold.
(b) Muller, Ingo. (2007). A History of Thermodynamics - the Doctrine of Energy and Entropy (pg. 155). New York: Springer.
(c) Endothermic – EtymOnline.com.
● Endothermic – Wikipedia.
● Endothermic reaction – Wikipedia.