A cold pack used for athletic injuries contains solid ammonium nitrate and water separated by a thin barrier. When the barrier is broken, the ammonium nitrate dissolves in the water, and the pack becomes noticeably colder.
The process may be represented symbolically as
NH4NO3(s) → NH4+(aq) + NO3−(aq) ΔH = +25 kJ·mol⁻¹
Assume the pack initially contains 50.0 g of NH4NO3 (molar mass = 80.0 g·mol⁻¹) and 100 mL of water at 25 °C. The solution’s specific heat capacity may be taken as 4.18 J·g⁻¹·°C⁻¹ and its mass as 150 g.
(a) Identify whether the dissolution of NH4NO3 is classified as a physical change or a chemical change. Justify your answer with reference to composition.
(b) Calculate the theoretical temperature change, ΔT, of the solution after all the NH4NO3 has dissolved, assuming no heat loss to the surroundings. Show your work.
(c) Explain why the observed temperature change provides evidence supporting your classification in part (a).