Lesson 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)
I can explain exothermic processes.Syllabus
Syllabus K326 / K328, 9(a). Explain exothermic processes
I can explain endothermic processes.Syllabus
Syllabus K326 / K328, 9(b). Explain endothermic processes
Follow the energy transfer
A temperature change is evidence about the surroundings.
Make a guess. You are not marked.
A solid dissolves in water and the beaker feels cold to touch. What happened to energy?
Show the answer
The dissolving took in energy from the water and your hand
The process absorbed energy from its surroundings, so the surroundings cooled. That is an endothermic change.
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 understanding
A reaction raises the temperature of the surrounding solution from 22 degrees C to 29 degrees C. Is it exothermic or endothermic?
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Exothermic, because energy moved from the reacting chemicals to the solution
The surroundings (the water and thermometer) got warmer, so they gained energy from the reaction. The reacting system lost energy, so delta H is negative.