Question

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?

Short answer

Sugar and salt differ greatly in their properties due to their different structures: sugar is a molecular solid held together by covalent bonds and intermolecular forces, while salt is an ionic solid with a lattice structure of cations and anions held together by strong electrostatic forces. These structural differences result in sugar having low melting and boiling points, and poor electrical conductivity, but being soft and brittle, whereas salt has high melting and boiling points, excellent conductivity in molten or dissolved state, and a hard, brittle structure. Solubility in polar solvents is shared by both types of solids, though the solubility varies based on the strength of the forces that must be overcome to dissolve the substance.

Step-by-step solution

Define Molecular and Ionic Solids

Molecular solids are made up of discrete molecules held together by covalent bonds and intermolecular forces (e.g. hydrogen bonds, van der Waals forces, etc.). An example of a molecular solid is sugar (C12H22O11, sucrose). Ionic solids, on the other hand, consist of a lattice of positively-charged ions (cations) and negatively-charged ions (anions) held together by strong electrostatic forces (ionic bonds). An example of an ionic solid is table salt (NaCl, sodium chloride).

Identify Properties of Molecular Solids

Some key properties of molecular solids include: 1. Low melting and boiling points, due to the relatively weak intermolecular forces that are easily overcome by heating. 2. Poor electrical conductivity, since they don't have free ions or delocalized electrons available to carry a charge. 3. They are often soft and brittle, as the weak intermolecular forces do not provide a robust lattice structure. 4. Most molecular solids are soluble in polar solvents, like water, because of the similar polarity of the molecules.

Identify Properties of Ionic Solids

Key properties of ionic solids include: 1. High melting and boiling points, due to the strong electrostatic forces (ionic bonds) holding the ions together. 2. They are excellent electrical conductors in the molten or dissolved state, as the ions become mobile and are free to carry a charge. 3. They tend to be hard and brittle, as the ionic lattice is rigid and strong. 4. Most ionic solids are soluble in polar solvents, such as water, where the ions can be separated by the polar solvent molecules.

Compare the Properties and Explain the Differences

Despite their similar appearance, sugar (a molecular solid) and salt (an ionic solid) differ greatly in their properties: 1. Melting and boiling points: Sugar has relatively low melting and boiling points while salt has high melting and boiling points. This is due to the weaker intermolecular forces in molecular solids compared to the strong electrostatic forces in ionic solids. 2. Electrical conductivity: Sugar doesn't conduct electricity, while salt is an excellent conductor in the molten or dissolved state. This is because molecular solids don't have free ions or delocalized electrons, whereas ionic solids have mobile ions that can carry a charge when in molten or dissolved state. 3. Hardness and brittleness: Sugar is soft and brittle, while salt is hard and brittle. This difference is due to the weak intermolecular forces in molecular solids that don't provide a robust lattice structure, in contrast to the rigid and strong ionic lattice of ionic solids. 4. Solubility: Both sugar and salt are soluble in polar solvents like water, but the solubility varies based on the strength of the intermolecular forces (for molecular solids) or the strength of the electrostatic forces (for ionic solids) that must be overcome to dissolve the substance. In conclusion, the different properties of sugar (a molecular solid) and salt (an ionic solid) can be attributed to the types of bonding and forces present in each solid. Molecular solids have weak intermolecular forces, while ionic solids have strong electrostatic forces, resulting in differences in melting points, electrical conductivity, and hardness.