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

Salt (sodium chloride, NaCl) is an ionic solid, and sugar (sucrose, C12H22O11) is a molecular solid. Molecular solids have lower melting and boiling points, are soft and brittle, and are poor conductors of electricity due to weak intermolecular forces. Ionic solids have higher melting and boiling points, are hard and brittle, and can conduct electricity when dissolved or melted due to strong electrostatic forces between ions. These differences in properties arise from the presence of covalent bonds and intermolecular forces in molecular solids, and ionic bonds in ionic solids.

Step-by-step solution

Molecular and Ionic Solids Overview

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.

Identify Salt and 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.

Comparing Properties: Melting and Boiling Points

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.

Comparing Properties: Hardness and Brittleness

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.

Comparing Properties: Electrical Conductivity

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.

Key concepts

Intermolecular Forces

Intermolecular forces are the various forces of attraction that occur between molecules. These forces are significant when examining molecular solids. In sugar, which is a molecular solid, these forces include London Dispersion forces, dipole-dipole interactions, and hydrogen bonding.

These forces are relatively weak compared to ionic bonds, which leads to several distinct properties of molecular solids:

• Low melting and boiling points, since less energy is needed to separate the molecules.
• Soft and brittle nature due to the ease with which these forces can be disrupted.

This stands in contrast to ionic solids, where strong intermolecular interactions largely contribute to the solid's properties. Understanding the role of these forces is crucial to explaining why substances like sugar behave differently from salts in various conditions.

Electrostatic Forces

Electrostatic forces, also known as ionic bonds, are the forces of attraction between oppositely charged ions. In ionic solids such as salt (sodium chloride), these forces are very strong and impactful.

Electrostatic forces are responsible for:

• High melting and boiling points of ionic solids, as a significant amount of energy is needed to break these bonds.
• The crystal lattice structure, which is highly stable and maintains the integrity of the solid.
• Hardness and the brittle nature of ionic solids when forces cause displacement in the lattice, leading to repulsion between like-charged ions.

These forces explain why ionic solids have different thermodynamic and mechanical properties compared to molecular solids.

Melting and Boiling Points

The melting and boiling points of a substance are determined by the strength of the forces holding the particles together.

Molecular solids like sugar have relatively low melting and boiling points. This is because the intermolecular forces binding the molecules together are weak and require less energy to overcome. As a result, these substances change states at lower temperatures.

In contrast, ionic solids such as salt have high melting and boiling points. A large amount of energy is necessary to overcome the strong electrostatic forces between the ions. This is why salt requires considerably higher temperatures to melt and boil.

Understanding these differences helps to explain why substances may look similar but behave differently when subjected to heat.

Hardness and Brittleness

Hardness and brittleness are noticeable properties that differ between molecular and ionic solids. The way that the particles are held together affects how they react to external forces.

Molecular solids like sugar are often soft because the intermolecular forces are weak. This also makes them brittle; they can easily break when subjected to a force. Their structure doesn't allow for a significant amount of mechanical strength.

Ionic solids, however, are typically hard, a property resulting from strong ionic bonds. Yet, despite their hardness, they are also brittle. When enough force is applied, layers in the lattice can shift, aligning like charges that repel each other and causing the solid to shatter.

This dual characteristic of being both hard and brittle showcases the complex nature of ionic solids.

Electrical Conductivity

Electrical conductivity is a key property that varies between molecular and ionic solids. It depends on the presence of free-moving charge carriers.

Molecular solids, like sugar, typically do not conduct electricity. In the solid state, they lack free electrons or ions to facilitate the flow of electric current.

Ionic solids such as salt, although non-conductive in their solid state due to ions being fixed in a lattice, can conduct electricity when dissolved in water or melted. This is because the ions become free to move, allowing them to carry charge.

These differences in electrical behavior highlight the distinct natures of molecular versus ionic solids based on their respective bonding and structural characteristics.