Step 1 : Understanding the Question:
The question asks to identify which of the listed interhalogen compounds exists in a state other than gas at standard room temperature ($25^\circ$C). Interhalogen compounds are formed when different halogen atoms combine. Their physical states at a given temperature depend heavily on their molecular mass and the strength of the intermolecular London dispersion forces. As we move down the halogen group or increase the number of atoms in the molecule, these forces strengthen, leading to higher boiling and melting points.
Step 2 : Key Formulas and approach:
We evaluate the physical state based on the composition of the molecules. The trend in the periodic table shows that as atoms become larger and have more electrons (higher polarizability), the intermolecular attractions become stronger. Small, light interhalogens are gases, while heavier ones (especially those containing iodine) are typically liquids or solids.
Our approach involves:
1. Checking the boiling points and common physical properties of each option at $25^\circ$C.
2. Identifying the heaviest or most polarizable molecule among the choices.
Step 3 : Detailed Explanation:
ClF (Chlorine monofluoride) is a very small molecule with a boiling point around $-100^\circ$C, meaning it is a gas at room temperature.
BrF (Bromine monofluoride) is a relatively light diatomic molecule that is gaseous at standard conditions, although it tends to be unstable.
ClF$_3$ (Chlorine trifluoride) has a boiling point of $11.7^\circ$C. Since $25^\circ$C is significantly higher than its boiling point, it exists as a gas.
IF$_3$ (Iodine trifluoride) contains the iodine atom, which is the largest and heaviest of the common halogens. The large electron cloud of iodine makes the molecule highly polarizable.
The resulting strong London dispersion forces make IF$_3$ a solid (a yellow powder) at room temperature.
Consequently, IF$_3$ is the only compound in the list that is not a gas at $25^\circ$C.
Step 4 : Final Answer:
The compound IF$_3$ is not in the gaseous phase at $25^\circ$C; it is a solid.