An athlete cracks open an instant cold pack and, within seconds, it turns icy cold, even though nothing inside it was ever put in a freezer. Squeezing it breaks an inner pouch of water, which mixes with a solid chemical (commonly ammonium nitrate) already inside the pack. What is happening chemically to make the pack feel cold?
- The chemical dissolving in the water is an endothermic process, absorbing heat energy from its surroundings, including the water and the athlete's hand, as the bonds holding the solid together break apart
- The chemical reacts with the water to produce a gas that expands and cools rapidly, similar to how a fizzy drink cools when opened
- The plastic pouch itself was pre-chilled before use, and breaking it just releases that stored cold into the pack
- The water freezes instantly on contact with the chemical, and it is the resulting ice that makes the pack feel cold
Why A? And why not the others?
Correct answer: A. The chemical dissolving in the water is an endothermic process, absorbing heat energy from its surroundings, including the water and the athlete's hand, as the bonds holding the solid together break apart
When ammonium nitrate dissolves in water, breaking apart the ionic bonds holding its solid crystal together takes in more energy than is released when the resulting ions become surrounded by water molecules, so the overall process is endothermic - it absorbs heat from its surroundings, including the remaining water and anything touching the pack, which is why the pack feels noticeably colder within seconds. The option describing an expanding, cooling gas is wrong because no gas is produced in this process at all; the ammonium nitrate simply dissolves into dissolved ions within the liquid water, unlike a fizzy drink's cooling, which comes from dissolved carbon dioxide gas escaping. The option describing a pre-chilled pouch is wrong because the pack works reliably and repeatably from room temperature every time it's activated, which wouldn't be possible if it depended on prior refrigeration. The option describing the water freezing is wrong because the pack only gets cold, dropping to a chilly but unfrozen temperature well above zero Celsius - the water does not actually turn to ice, it simply loses heat energy to the dissolving process.
Source: BBC Bitesize GCSE Chemistry: Energy changes -- exothermic and endothermic reactions