JEE Main 2023 · 30 January · evening · Single correct · +4 / −1
Boric acid is solid, whereas BF₃ is gas at room temperature because of :
- (a) Strong van der Waal's interaction in Boric acid
- (b) Strong covalent bond in BF₃
- (c) Strong hydrogen bond in Boric acid
- (d) Strong ionic bond in Boric acid
Answer and solution
Answer (c) Strong hydrogen bond in Boric acid
Solution
Chemistry ma’am · 4 steps
Look at what holds the molecules to each other: B(OH)3 has O–H groups, BF3 has none.
- Two different molecules. Boric acid is B(OH)₃, with three O–H groups on boron. BF₃ has fluorine on boron but no hydrogen at all.
- Boric acid layers. The H of one O–H group is attracted to the O of a neighbouring molecule. Each B(OH)₃ unit is hydrogen bonded to its neighbours, which builds flat layers and holds the solid together.
- BF3 molecules. BF₃ is a small, trigonal planar, non-polar molecule with no H to give. Only weak van der Waals forces act between its molecules, so it is a gas at room temperature.
- So. Strong intermolecular hydrogen bonding in boric acid makes it a solid. The answer is (c).
The idea to keep. Physical state depends on forces between molecules, not on bonds inside them. Spot H attached to O, N or F and expect hydrogen bonding, a higher melting point, often a solid.
Answer from the NTA final answer key. Solution by PrepWiser.