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Every chemical reaction either releases energy or absorbs energy. Reactions that release energy (usually as heat) are called exothermic — the surroundings warm up. Reactions that absorb energy from the surroundings are called endothermic — the surroundings cool down. Burning wood is exothermic (you feel heat). A cold pack activating is endothermic (it absorbs heat from your skin). The energy change comes from the difference between the energy needed to break bonds in the reactants and the energy released when new bonds form in the products.
Use two hands-on experiences: mix baking soda and vinegar and feel the container get slightly cooler (endothermic), then discuss or demonstrate a burning candle or hand warmer (exothermic). Measuring temperature before and after each reaction with a thermometer makes the concept quantitative and real.
Chemical reactions do not just rearrange atoms — they also involve energy changes. Every reaction either releases energy to its surroundings or absorbs energy from its surroundings. Understanding which direction the energy flows is one of the most important concepts in chemistry.
Exothermic reactions release energy, usually in the form of heat. The word "exothermic" comes from Greek roots: "exo" means out, and "thermic" refers to heat — so energy flows *out* of the reaction. When you burn wood, natural gas, or a candle, you can feel the heat radiating outward. Burning is one of the most dramatic exothermic processes, but many quieter reactions are exothermic too. Rusting iron releases a small amount of heat (so slowly you do not notice it). Hand warmers use exothermic reactions — the iron powder inside reacts with oxygen, and the released heat warms your hands.
Endothermic reactions absorb energy from their surroundings. "Endo" means in — energy flows *into* the reaction. When you activate an instant cold pack, chemicals inside mix and undergo an endothermic process, pulling heat away from your skin and making the pack feel cold. Mixing baking soda and vinegar is slightly endothermic — if you touch the container, it feels cooler afterward. Photosynthesis is also endothermic: plants absorb light energy from the sun to drive the reaction that turns carbon dioxide and water into glucose and oxygen.
Where does this energy come from, or go to? The answer is chemical bonds. Breaking bonds in the reactants requires energy (it takes effort to pull atoms apart). Forming new bonds in the products releases energy (atoms settling into new bonds give off energy). If the energy released by forming new bonds is greater than the energy needed to break old bonds, the reaction is exothermic — there is leftover energy that escapes as heat. If the energy required to break bonds is greater than the energy released by forming new bonds, the reaction is endothermic — extra energy must be absorbed from the surroundings to make up the difference.
You can often detect the energy change in a reaction simply by monitoring temperature. If the temperature of the surroundings increases, the reaction is exothermic. If the temperature decreases, it is endothermic. This simple temperature check is one of the most practical tools for classifying reactions in the lab.
Understanding energy in reactions matters far beyond the classroom. The engines in cars run on exothermic combustion reactions. Cooking food involves both exothermic and endothermic processes. Your own body runs on exothermic reactions that release energy from food molecules to power your muscles and brain. Energy is at the heart of every chemical change.