A researcher investigates four transformations of solid substances at 1 atm pressure.
1. Heating ammonium chloride results in the formation of a colorless gas mixture that recombines on a cold surface to yield the original white solid.
2. Crushing crystalline quartz (SiO₂) into a fine powder.
3. Strongly heating solid calcium carbonate produces a white solid residue and an odorless gas that turns limewater cloudy.
4. Gently warming solid iodine results in the gradual disappearance of the solid and the appearance of violet vapor in the vessel.
For transformations 1 and 4 the researcher notes that individual molecules present in the solid are also present in the gas phase. For transformation 3 the gas produced is a different chemical species from the solid.
(a) For EACH transformation, indicate whether it is best described as a physical change or a chemical change. Provide brief reasoning in each case.
(b) The researcher proposes using the magnitude of the enthalpy change, ∣ΔH∣, as a quantitative proxy for the extent of chemical bond disruption. Outline a procedure the researcher could use to test this proposal using calorimetric measurements for the four transformations.
(c) Predict which transformation would have the largest ∣ΔH∣ value per mole of solid and justify your prediction with reference to bond energies and phase changes.