Which one of the following halogen acids has the lowest melting point ?

Options

A. HF
B. HCI
C. HBr
D. HI

Answer

To determine which halogen acid has the lowest melting point, we need to analyze the melting points of the common halogen acids: HF (hydrofluoric acid), HCl (hydrochloric acid), HBr (hydrobromic acid), and HI (hydroiodic acid).

### Step-by-Step Solution:

1. **Understanding Halogen Acids**: The halogen acids in question are HF, HCl, HBr, and HI. Each of these acids is formed by the combination of hydrogen with a halogen element.

2. **Factors Affecting Melting Point**: The melting point of these acids can be influenced by several factors, including:
- Molecular mass
- Intermolecular forces (such as Van der Waals forces and hydrogen bonding)
- Lattice enthalpy

3. **Analyzing Intermolecular Forces**:
- HF has strong hydrogen bonding due to the presence of highly electronegative fluorine, which leads to a higher melting point.
- HCl, HBr, and HI primarily exhibit Van der Waals forces. The strength of these forces increases with molecular mass.

4. **Comparing Melting Points**:
- HF has the highest melting point due to hydrogen bonding.
- HI has the highest molecular mass and thus has stronger Van der Waals forces compared to HCl and HBr.
- HBr has a higher melting point than HCl due to its larger molecular mass.
- Therefore, the order of melting points from highest to lowest is: HI > HF > HBr > HCl.

5. **Conclusion**: Based on the analysis, HCl has the lowest melting point among the halogen acids.

### Final Answer:
HCl (hydrochloric acid) has the lowest melting point among the halogen acids.

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