Problem 32 - Chemistry An Atoms First Approach

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?

Step-by-Step Solution

Answer

In summary, the properties of molecular solids (e.g., sugar) and ionic solids (e.g., salt) differ due to the types of bonding present. Molecular solids have weaker intermolecular forces, resulting in lower melting points, poor conductivity, and varying solubilities. On the other hand, ionic solids have strong ionic bonds, leading to high melting points, electrical conductivity when dissolved or melted, and high solubilities in polar solvents, such as water.

Step 1: Define Molecular Solids

Molecular solids are made up of atoms or molecules held together by intermolecular forces, such as dispersion forces, dipole-dipole forces, and hydrogen bonding. These forces are generally weaker than the ionic or covalent bonds found in other solid types. One prime example of a molecular solid is sugar.

Step 2: Define Ionic Solids

Ionic solids consist of positive and negative ions held together by strong ionic bonds, which are electrostatic attractions between ions of opposite charges. These bonds result in crystalline structures with high melting points, electrical conductivity, and solubility in water. One example of an ionic solid is table salt, or sodium chloride (NaCl).

Step 3: Compare Properties: Melting Points

Due to the strong ionic bonds in ionic solids, they usually have high melting and boiling points, as a lot of energy is required to break those bonds. On the other hand, molecular solids have relatively weak intermolecular forces, resulting in lower melting and boiling points compared to ionic solids. For example, sugar (a molecular solid) has a melting point of around 186°C, while salt (an ionic solid) has a melting point of approximately 801°C.

Step 4: Compare Properties: Conductivity

Ionic solids, such as salt, are good conductors of electricity when dissolved in water or melted because their ions can move freely and carry electric charges. On the contrary, molecular solids like sugar do not have free ions and hence are poor conductors of electricity, both in solid form and when dissolved in water.

Step 5: Compare Properties: Solubility

Ionic solids generally have high solubilities in polar solvents like water because the electrostatic interactions between the ions and polar solvent molecules help break the ionic bonds and dissolve the solid. Molecular solids have varying solubilities, mainly depending on the types of intermolecular forces present. Sugar is highly soluble in water due to hydrogen bonding between the sugar molecules and water.

Step 6: Conclusion

In summary, molecular solids (like sugar) and ionic solids (like salt) differ in their properties because of the different types of bonding present. Molecular solids have weaker intermolecular forces and therefore have lower melting points, poor conductivity, and varying solubilities. In contrast, ionic solids have strong ionic bonds, leading to high melting points, electrical conductivity when dissolved or melted, and high solubilities in polar solvents like water.