In any chemical reaction, bonds in the reactants must be broken (requiring energy input) and new bonds are formed in the products (releasing energy). In an endothermic reaction, the energy needed to break the bonds in the reactants is GREATER than the energy released when new bonds form in the products.
The "deficit" — the extra energy required — is absorbed from the surroundings. This is why the surroundings cool down: the reaction is literally pulling heat energy away from everything around it to power the bond-breaking step. The new bonds formed in the products are weaker (less stable) than the old bonds that were broken, so not enough energy is released to repay what was invested.
A clear example is thermal decomposition: heating calcium carbonate (CaCO₃ → CaO + CO₂) requires continuous heat input because the bonds broken in CaCO₃ require more energy than is released when the new bonds in CaO and CO₂ form. If you stop supplying heat, the reaction stops. This continuous need for energy input is a hallmark of endothermic reactions.
This how it works covers How It Works: Why Energy is Absorbed within Endothermic Reactions for GCSE Chemistry. Revise Endothermic Reactions in Energy Changes for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 3 of 12 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Endothermic Reactions
In an endothermic reaction, energy is:
Explain why a sports cold pack becomes cold when activated.