Exothermic and Endothermic Reactions

Exothermic reactions are those reactions which proceed with the evolution of heat. For example, when a fuel like cooking gas or coal is burnt in air, heat is evolved besides light. Many chemical reactions lead to release of energy (heat) to the surroundings.

Endothermic reactions are those reactions which proceed with the absorption of heat from the surroundings. For example, add a small amount of solid ammonium chloride in a test tube half-filled with water. Shake and feel the test tube. It will feel cold.