exothermic and endothermic
/ ek-soh-THUR-mik / en-doh-THUR-mik /
These two words simply label which way the heat flows. An exothermic process releases heat into its surroundings — it warms the room, the flask, your hand. A burning match, setting concrete, a hand-warmer: all give energy out. An endothermic process is the opposite; it pulls heat in from its surroundings and leaves them cooler. An instant cold pack and melting ice both drink heat from whatever is near.
In the language of enthalpy, exothermic means a negative enthalpy change (ΔH < 0): the products sit lower in energy than the reactants, and the surplus leaves as heat. Endothermic means a positive enthalpy change (ΔH > 0): the products sit higher, so the process must borrow energy from the surroundings to climb there. The arrows of heat and the sign of ΔH always agree.
The everyday clue is temperature. Touch the container: if it feels hot, heat is coming out and the reaction is exothermic; if it feels cold, heat is going in and it is endothermic. The energy isn't lost or made in either case — the first law still balances; it has merely flowed across the boundary in one direction or the other.
Snap a chemical hand-warmer and it grows pleasantly hot — exothermic. Tear open an instant cold pack for a sprained ankle and it turns icy in seconds — endothermic.
Hot to the touch = exothermic; cold to the touch = endothermic.
Exothermic does not mean spontaneous, nor endothermic mean impossible. Plenty of endothermic changes happen happily — ice melts on a warm day, and salt dissolves while chilling the water. Whether a process actually proceeds depends on entropy too, captured by free energy, not on heat flow alone.