Ionic compounds have higher melting points than molecular ones

Steps:

• Identify the bonding type for each substance: ammonia and methane are nonpolar molecular, water is polar molecular with hydrogen bonding, sodium chloride is ionic.
• Recall that ionic bonds require high energy to break due to strong electrostatic forces between ions.
• Compare approximate melting points: ammonia (-78°C), methane (-183°C), water (0°C), sodium chloride (801°C).
• Select the substance with the highest melting point based on bonding strength.

Why B is correct:

• Sodium chloride is an ionic compound, so its melting point is high per the lattice energy formula, where energy ∝ (q₁q₂)/r, with strong ion attractions.

Why the others are wrong:

• A: Ammonia has weak intermolecular forces as a molecular compound.
• C: Methane has only weak London dispersion forces.
• D: Water's hydrogen bonding is weaker than ionic bonds.

Final answer: B