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 because one is a molecular solid and the other is an ionic solid. How do the properties differ and why?

Short answer

Salt, an ionic solid, and sugar, a molecular solid, differ greatly in their properties due to the varying nature of the bonds and forces holding them together. Ionic solids like salt have higher melting and boiling points due to strong electrostatic forces between positive cations and negative anions. On the other hand, molecular solids like sugar have lower melting and boiling points because they are bound by weaker forces like Van der Waals forces, hydrogen bonding, and dipole interactions. Both salt and sugar are soluble in water, but salt is more soluble due to attractive forces between the charged ions and polar water molecules. However, the intermolecular forces between sugar molecules and water molecules are weaker, making sugar less soluble. Salt, in its molten or dissolved state, conducts electricity as its ions are free to move and act as charge carriers, whereas sugar cannot conduct electricity in any form as it doesn't consist of charged particles.

Step-by-step solution

Identify types of solids

Salt (NaCl) is an ionic solid, while sugar (C12H22O11) is a molecular solid.

Compare physical properties

Some physical properties to consider are melting points, boiling points, solubility, and electrical conductivity.

Melting and boiling points

Ionic solids like salt typically have high melting points and boiling points. This is due to the strong electrostatic forces between the positively charged cations and negatively charged anions that make up the crystal lattice. In contrast, molecular solids like sugar have lower melting and boiling points because the forces holding the molecules together (intermolecular forces like Van der Waals forces, hydrogen bonding, and dipole interactions) are weaker compared to the ionic bonds in ionic solids.

Solubility

Both salt and sugar are soluble in water, although their solubilities differ. Salts are more soluble in polar solvents like water since the charged ions in the crystal lattice are attracted to the polar water molecules. In contrast, sugar, although soluble in water, is less soluble than salt because the intermolecular forces between sugar molecules and water molecules are not as strong as the electrostatic forces between ions in salt and water molecules.

Electrical conductivity

Ionic solids like salt are good conductors of electricity when in molten or dissolved state but not in the solid form because their ions are free to move and act as charge carriers. In contrast, molecular solids like sugar do not conduct electricity in any form (solid, liquid or dissolved) because they do not consist of charged particles.

Conclusion

The properties of molecular solids and ionic solids differ significantly due to the nature of the bonds and forces holding them together. Ionic solids such as salt have high melting and boiling points, are more soluble in polar solvents, and can conduct electricity when molten or dissolved, while molecular solids like sugar have lower melting and boiling points, are less soluble in polar solvents and do not conduct electricity in any form. These differences in properties can be attributed to the presence of strong ionic bonds in ionic solids and weaker intermolecular forces in molecular solids.

Key concepts

Ionic solids

Ionic solids are a fascinating type of solid that are made up of ions, which are atoms carrying a charge. In ionic solids, these ions arrange themselves in a repeating pattern known as a crystal lattice. This arrangement allows them to form strong bonds called ionic bonds, which occur due to the attraction between oppositely charged ions. These solids exhibit several distinct characteristics due to their ionic bonding:

• High Melting and Boiling Points: The strong electrostatic forces between the ions in the lattice require large amounts of energy to be overcome, resulting in high melting and boiling points for ionic solids.
• Electrical Conductivity: Ionic solids conduct electricity when melted or dissolved in a solution. This is because the ions become free to move and carry an electric current. In their solid state, however, they do not conduct electricity because the ions are fixed in position.
• Solubility: They are generally soluble in polar solvents like water because the charged ions interact well with polar molecules, allowing them to dissolve.

Molecular solids

Molecular solids are a different class of solids primarily composed of molecules. These solids are held together by intermolecular forces rather than ionic bonds. Intermolecular forces include Van der Waals forces, hydrogen bonds, and dipole-dipole interactions, which are significantly weaker than ionic bonds. Because of this, molecular solids have notable differences in properties:

• Lower Melting and Boiling Points: The weaker intermolecular forces in molecular solids require much less energy to overcome compared to ionic bonds, leading to lower melting and boiling points.
• Solubility: Molecular solids like sugar are generally soluble in polar solvents, though less so than ionic solids. The solubility depends on the ability of the solvent to interact with the molecules through compatible polarity and hydrogen bonding.
• Lack of Electrical Conductivity: Unlike ionic solids, molecular solids do not conduct electricity. This is because they are composed of neutral molecules without free-moving charges.

Physical properties

Physical properties are the characteristics of a material that can be observed or measured without changing its composition. When it comes to distinguishing between solids like salt and sugar, understanding their physical properties can be incredibly helpful. Let's categorize these properties:

• Melting and Boiling Points: High melting and boiling points are found in ionic solids due to strong ionic bonds, whereas molecular solids have lower points due to weaker intermolecular forces.
• Solubility: Ionic solids excel in solubility in polar solvents due to ionic interactions, while molecular solids have varying solubility based on molecular structure and intermolecular forces.
• Electrical Conductivity: Ionic solids conduct electricity when dissolved or melted, thanks to ionic mobility, while molecular solids do not conduct electricity in any state.

Understanding these physical properties not only helps in identifying the type of solid but also in predicting its behavior in various environments and applications.