Endothermic Reaction
An endothermic reaction absorbs energy from its surroundings, making them cooler; photosynthesis and an instant cold pack are examples.
Definition
An endothermic reaction absorbs energy, usually as heat, from its surroundings, so nearby temperatures drop and the products end up storing more chemical energy than the reactants did. The enthalpy change is positive, $\Delta H > 0$, and photosynthesis, the decomposition of many compounds, and the dissolving of ammonium nitrate are all endothermic. Such reactions are spontaneous only when driven by an entropy increase large enough that $T\Delta S > \Delta H$, which is why endothermic processes often involve producing gases or dissolving solids, and raising the temperature of an endothermic equilibrium shifts it toward products.
Example
An instant cold pack works this way: squeezing it lets the chemicals mix, and the reaction pulls heat from your skin until it feels icy. Photosynthesis stores sunlight as chemical energy, $6CO_2 + 6H_2O + \text{energy} \rightarrow C_6H_{12}O_6 + 6O_2$, and mixing baking soda with vinegar is mildly endothermic too, cooling the cup slightly as it fizzes. Dissolving ammonium nitrate in water has $\Delta H = +25.7\text{ kJ/mol}$ yet happens spontaneously because the entropy gained as ions disperse in water outweighs the enthalpy cost, and a cold pack holding $50\text{ g}$ of $NH_4NO_3$ absorbs about $16\text{ kJ}$, enough to chill it toward $0^\circ\text{C}$.
Key Insight
"Endo" means in, like "enter," so heat goes into an endothermic reaction, and that supply has to keep coming: turn off the light and photosynthesis stops, take away the heat and a decomposition halts. Thermal decomposition of limestone, $CaCO_3 \rightarrow CaO + CO_2$ with $\Delta H = +178\text{ kJ/mol}$, only becomes spontaneous above about $840^\circ\text{C}$, where the entropy term finally wins, which is exactly why cement kilns are run so hot.