Topic 1 of 2
Follow the energy transfer
A temperature change is evidence about the surroundings.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Follow the energy transfer
A temperature change is evidence about the surroundings.
Define the system as the reacting substances and the surroundings as what exchanges energy with them, such as the water, container and air. An exothermic process transfers energy to its surroundings, often as heat; the measured mixture temperature usually rises. An endothermic process takes energy from the surroundings, so their temperature usually falls.
| Process | Observation | Energy direction |
|---|---|---|
| Aqueous sodium hydroxide reacts with hydrochloric acid | Temperature rises | Reaction releases energy to the surroundings |
| Ammonium nitrate dissolves in water | Temperature falls | Dissolving process takes energy from the surroundings |
NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l) is a neutralisation. Dissolving ammonium nitrate is a physical process with an energy change; an energy transfer does not by itself prove that a new substance has formed.
Check your understandingA reaction raises the surrounding solution temperature from 22 to 29 degrees C. Is it exothermic or endothermic?Think it through, then reveal the answer
Exothermic: energy has been transferred from the reaction to its surroundings. A warmer thermometer does not mean the reaction system gained energy overall.