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Chemistry · Question of the Day

Chemistry Question of the Day

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Tuesday, August 25, 2026

A disposable hand warmer (iron powder inside a packet) is opened and shaken. After 5 minutes, its temperature rises from 20∘C20^\circ\text{C} to 41∘C41^\circ\text{C} without any external heating. Which choice best describes the energy transfer?

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Question of the Day

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A disposable hand warmer (iron powder inside a packet) is opened and shaken. After 5 minutes, its temperature rises from 20∘C20^\circ\text{C} to 41∘C41^\circ\text{C} without any external heating. Which choice best describes the energy transfer?

  1. Endothermic; energy flows from the surroundings into the system, causing the packet to warm
  2. Exothermic; energy flows from the system to the surroundings, warming the packet and nearby air (correct answer)
  3. Endothermic; energy flows from the system to the surroundings, warming the packet
  4. Neither; temperature change does not indicate energy transfer

Explanation: This question tests your understanding of exothermic reactions (which release energy to surroundings, making them feel hot) and endothermic reactions (which absorb energy from surroundings, making them feel cold). Exothermic and endothermic reactions differ in energy flow direction: EXOTHERMIC reactions release energy—usually as heat—to the surroundings, causing the temperature of the surroundings to increase (the reaction mixture or container feels hot, thermometer reading goes up). Examples include combustion (burning releases heat), hand warmers (iron oxidation releases heat), and acid-base neutralization (mixing acid and base releases heat, warming the solution). ENDOTHERMIC reactions absorb energy from the surroundings, causing the temperature of the surroundings to decrease (reaction mixture feels cold, thermometer reading goes down). Examples include instant cold packs (ammonium nitrate dissolving absorbs heat, cooling the pack), photosynthesis (plants absorb light energy to make glucose), and ice melting (absorbs heat from surroundings, cooling your drink). The key: look at what happens to the surroundings—do they get hotter (exothermic) or colder (endothermic)? The hand warmer's temperature rose from 20°C to 41°C without external heating—the iron oxidation reaction releases energy to the surroundings, making this clearly EXOTHERMIC with energy flowing FROM the system TO the surroundings (warming the packet and nearby air). Choice B correctly identifies this as exothermic: energy flows from the system to the surroundings, which explains the temperature increase and why your hands feel warm when holding the packet. Choice A incorrectly calls it endothermic despite the warming; Choice C mislabels it as endothermic while describing exothermic energy flow; Choice D ignores the clear evidence of energy release. The exothermic vs endothermic identification strategy: (1) Look for temperature change observations: Did temperature increase (solution got warmer, beaker hot to touch)? → EXOTHERMIC (reaction released heat to surroundings). Did temperature decrease (solution got colder, beaker cool to touch)? → ENDOTHERMIC (reaction absorbed heat from surroundings). No thermometer? Use your hand—does it feel warm (exo) or cool (endo)? (2) Look for energy input requirements: Does reaction need continuous heating, light, or electricity to proceed? → likely ENDOTHERMIC (absorbing that energy). Does reaction proceed on its own, producing heat or light? → likely EXOTHERMIC (releasing energy).