Snap a glow stick and it warms slightly. Dissolve a cold pack's crystals and the pouch turns icy. Both are chemical changes — but one releases energy and the other soaks it up.
Two directions of energy
The surroundings simply means everything around the reaction — the beaker, the air, the thermometer. Energy can flow either way:
- Exothermic — the reaction releases energy to the surroundings, so the temperature rises and the beaker feels hot. Examples: burning (combustion) and neutralisation.
- Endothermic — the reaction takes in energy from the surroundings, so the temperature falls and the beaker feels cold. Examples: thermal decomposition and some dissolving.
You spot the difference in the lab with a thermometer: measure the temperature before the reaction and again during it. A rise means the reaction gave out heat (exothermic); a drop means it absorbed heat (endothermic). Instant-cold sports packs use an endothermic reaction on purpose, while hand-warmers use an exothermic one.
An analogy to keep them straight
Think of the reaction as a person and the surroundings as their bank account. An exothermic reaction pays out — energy exits to the surroundings, which get richer (hotter). An endothermic reaction withdraws — it pulls energy in from the surroundings, leaving them poorer (colder). The prefix tells you the direction: exo = exit, endo = into.
Comparing the two
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy | Released | Taken in |
| Surroundings | Get hotter | Get colder |
| Beaker feels | Hot | Cold |
| Examples | Combustion, neutralisation | Thermal decomposition, some dissolving |
Worked reasoning
Ask: "is heat flowing out to me, or is the reaction stealing heat from its surroundings?"
If it feels warm, heat is coming out of the reaction into the beaker and your hand — that is energy released.
Answer: exothermic. (If it felt cold, the reaction would be drawing heat in — endothermic.)
Memory hook: Exo = exit heat → hot. Endo = heat enters the reaction → cold outside.