Final answer:
Based on the properties of halogens and their trends in the periodic table, ClF₅ is likely a gas, BrF₅ a liquid, and IF₅ a solid at 0°C, due to their increasing molecular size and corresponding intermolecular forces.
Step-by-step explanation:
To determine which of ClF₅, BrF₅, and IF₅ is a gas, a liquid, and a solid at 0°C, we can use the trends observed in the physical properties of halogens as they appear in the periodic table. As we move down the group from fluorine to iodine, the physical state of the halogens at room temperature changes from gases to liquids to solids. This trend is due to increasing molecular size, which results in stronger intermolecular forces, such as van der Waals forces, and consequently higher boiling points.
Based on these properties, we can determine that as the lightest and smallest molecule, ClF₅ would likely be a gas at 0°C; BrF₅ is more massive than ClF₅, and would typically be a liquid at this temperature; and since IF₅ is the heaviest with the strongest intermolecular forces, it would most likely be a solid. Thus, the correct answer is: (a) ClF₅ - gas, BrF₅ - liquid, IF₅ - solid.