A common prank on college campuses is to switch the salt and sugar on dining hall tables, which is usually easy because the substances look so much alike. Yet, despite the similarity in their appearance, these two substances differ greatly in their properties, since one is a molecular solid and the other is an ionic solid. How do the properties differ and why?

Molecular solids are made up of covalently bonded molecules held together by intermolecular forces like London Dispersion forces, dipole-dipole interactions, and hydrogen bonding. These substances typically have low melting and boiling points and are often soft and brittle. On the other hand, ionic solids are formed when positively charged ions (cations) and negatively charged ions (anions) are held together by electrostatic forces, known as ionic bonds. These substances usually have high melting and boiling points and are typically hard and brittle. Now, let's identify which one is salt and which one is sugar.

Salt (sodium chloride, NaCl) is an ionic solid, as it is composed of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). They are held together by ionic bonds. Sugar (sucrose, C12H22O11) is a molecular solid, consisting of carbohydrate molecules that are covalently bonded and held together by intermolecular forces such as hydrogen bonding. With this information, we can now explore how their properties differ and the reasons behind these differences.

Molecular solids, like sugar, have lower melting and boiling points because the intermolecular forces holding their molecules together are weaker compared to the ionic bonds in ionic solids like salt. This means that less energy is required to overcome these forces and change the state of the molecular solid. Ionic solids, like salt, have high melting and boiling points due to the strong electrostatic force between their oppositely charged ions. A significant amount of energy is needed to overcome these forces to change the state of the ionic solid.

Molecular solids, like sugar, are often soft and brittle as the intermolecular forces holding their molecules together can be easily broken when subjected to a mechanical force. In contrast, ionic solids, like salt, are hard and brittle because of the strong electrostatic forces between their ions. However, when subjected to a mechanical force, these solids can break, causing the repulsion between ions with the same charge and separation along the crystal planes.

Molecular solids, like sugar, are poor conductors of electricity because, in their solid state, they do not have any free electrons or ions available to facilitate the flow of electricity. Ionic solids, like salt, do not conduct electricity in their solid state either because their ions are locked in position within the lattice structure. However, when dissolved in water or melted, the ions become mobile, and the substance can conduct electricity. In conclusion, although salt and sugar may look similar in appearance, their properties differ significantly because one (sugar) is a molecular solid, held together by weak intermolecular forces, and the other (salt) is an ionic solid, bound by strong electrostatic forces.