Question

Given below are two statements: Statement I: The increasing order of boiling point of hydrogen halides is HCl<HBr<HI<HF. Statement II: The increasing order of melting point of hydrogen halides is HCl<HBr<HF<HI. In the light of the above statements, choose the correct answer from the options given below:

• A. Statement I is true but Statement II is false
• B. Both Statement I and Statement II are false
• C. Both Statement I and Statement II are true
• D. Statement I is false but Statement II is true

Hint:

Boiling points follow intermolecular forces (hydrogen bonding, van der Waals). Melting points depend on crystal lattice packing, so trends may be irregular.

Answer 1

The Correct Option is A

To determine the correctness of the statements, we need to analyze the boiling and melting points of hydrogen halides.

Boiling Points Discussion

• Hydrogen fluoride (HF) exhibits unusually high boiling points compared to other hydrogen halides due to hydrogen bonding. Other hydrogen halides (HCl, HBr, and HI) do not form hydrogen bonds, so their boiling points are governed mainly by van der Waals forces, which increase with the size and molecular weight of the molecule.
• Therefore, the boiling points increase in the order of: HCl < HBr < HI < HF.

This order confirms that Statement I is true.

Melting Points Discussion

• For melting points, HF is also affected by strong hydrogen bonding, but other hydrogen halides mainly see increases in melting points from van der Waals forces.
• The typical trend for melting points is: HCl < HBr < HI < HF due to hydrogen bonding in HF despite the molecular weight of HI.

Hence, Statement II is false, because the order provided in the statement is incorrect.

Conclusion

After analyzing both statements, the correct answer is: Statement I is true but Statement II is false. The boiling point sequence is correct, but the melting point sequence provided in Statement II is incorrect.