Record the starting temperature. After mixing, find the highest temperature for an exothermic reaction or the lowest for an endothermic reaction. Temperature change = highest or lowest temperature − starting temperature.
Exothermic: combustion, many oxidation reactions, and neutralisation. Hand warmers and self-heating cans use exothermic changes. Endothermic: thermal decomposition, and citric acid reacting with sodium hydrogencarbonate. Some instant sports injury packs use endothermic changes.
Use the evidence provided. A hand warmer should give a safe temperature for long enough. A cold pack needs a useful cooling effect. Compare temperature, duration, cost, reusability and waste. Do not choose only the largest temperature change: a product could become too hot or too cold. Some packs work by dissolving or crystallisation, so use the information given.
Use the highest or lowest reading. Explain the energy direction, then support a choice with data.
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