A student dissolves ammonium nitrate in water in a polystyrene cup. The temperature falls from 21.0 ∘C21.0\,^\circ\mathrm{C}21.0∘C to 12.5 ∘C12.5\,^\circ\mathrm{C}12.5∘C within one minute, then rises slowly back to 21.0 ∘C21.0\,^\circ\mathrm{C}21.0∘C over the next 20 minutes.
Which statement explains the slow rise in temperature?
The dissolving becomes exothermic once all of the solid has dissolved
Heat energy is transferred from the warmer surroundings into the cold solution
Bonds re-form in the solution, giving back the energy that was taken in
The endothermic change reverses, so solid ammonium nitrate forms again
94 exam-style questions on Edexcel GCSE Chemistry 8.2 Heat energy changes in chemical reactions, covering 8.2.1 Exothermic and endothermic changes, 8.2.2 Bond breaking, bond making and overall energy change, 8.2.3 Calculating energy change from bond energies, and 8.2.4 Activation energy and reaction profiles. Each one has a worked solution and a mark scheme showing where the marks go.