Can the same chemical substance have two different packing groups at different concentrations?

Yes — concentration and purity shift the packing group for many substances. Acetic acid above 80% concentration is UN 2789, Packing Group I. Below 80%, it is UN 2790, which falls at Packing Group II or III depending on the specific concentration. The same applies to sulphuric acid, hydrochloric acid, and other corrosives: as concentration drops, the rate of tissue destruction changes and the packing group follows. Shippers must verify the packing group against the actual concentration of their substance in ADR Table A — the base chemical name alone is not sufficient for correct classification.