Here's why:

* NaCl (Sodium Chloride) is an ionic compound. The strong electrostatic forces between the positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) create a very strong crystal lattice structure. To boil NaCl, you need to overcome these strong ionic bonds, requiring a lot of energy and resulting in a high boiling point (1465 °C).

* SCl2 (Sulfur Dichloride) is a covalent compound. The bonds between sulfur and chlorine are weaker than the ionic bonds in NaCl. SCl2 molecules also have weaker intermolecular forces (dipole-dipole interactions) compared to the strong ionic forces in NaCl. This means less energy is needed to separate the SCl2 molecules, leading to a lower boiling point (-78 °C).

In summary: The type of bonding and the strength of intermolecular forces play a crucial role in determining a substance's boiling point. Ionic compounds like NaCl have much higher boiling points than covalent compounds like SCl2.