Question

A solid is soft and has a low melting point (below \(100^{\circ} \mathrm{C}\) ). The solid, its melt, and an aqueous solution containing the substance are all nonconductors of electricity. Classify the solid.

Short answer

The solid is a molecular solid.

Step-by-step solution

Understand the Characteristics

We are given a solid with specific properties: it is soft, has a low melting point (below 100°C), and neither the solid nor its melt or aqueous solution conducts electricity. Each of these properties can help us determine the type of solid.

Identify Soft Solids

Softness in solids can often indicate that the intermolecular forces holding the material together are relatively weak. Generally, molecular solids or amorphous solids can be soft. Thus, the solid in question is likely to be molecular.

Low Melting Point Analysis

Solids with low melting points typically have weak intermolecular forces, such as Van der Waals forces or hydrogen bonds. Ionic or metallic solids are usually not considered because they often have high melting points. This supports the hypothesis that the solid is molecular.

Analyze Electrical Conductivity

The solid, its melted form, and its aqueous solution are nonconductors of electricity. This rules out ionic and metallic solids, as they typically conduct electricity when melted or dissolved due to the presence of free-moving ions or electrons.

Conclusion - Classifying the Solid

Combining all observations: the solid is soft, has a low melting point, and is a nonconductor in all states. Molecular solids fit all these characteristics. They are composed of molecules held together by weak forces, often exhibit low melting points, and do not conduct electricity.

Key concepts

Understanding Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that exist between molecules. They play a crucial role in determining the physical properties of substances, such as melting point, boiling point, and physical state at room temperature.
For molecular solids, these forces are generally weak compared to other types of solids. For instance, molecular solids are held together by forces such as Van der Waals forces (including London dispersion forces) and hydrogen bonds.

• **Van der Waals Forces:** These are weak forces that arise from temporary dipoles in molecules. They are generally present in nonpolar molecules and contribute to the relative softness and low melting points seen in molecular solids.
• **Hydrogen Bonds:** A stronger form of dipole-dipole interaction where a hydrogen atom bonds to a highly electronegative atom such as nitrogen, oxygen, or fluorine. While stronger than other types of Van der Waals forces, they are still weaker than than the bonds in ionic or metallic solids.

Understanding the type of intermolecular forces present can provide insight into why molecular solids exhibit low melting points and relative nonconductivity.

Exploring Electrical Conductivity

Electrical conductivity in solids is a measure of a material's ability to allow the flow of electric current through it.
Different types of solids exhibit electrical conductivity based on their composition and structure:

• **Molecular Solids:** These solids are poor conductors of electricity because their molecules are neutral and do not have free-flowing electrons or ions to carry an electrical charge. Thus they do not conduct electricity in solid, molten, or aqueous states.
• **Ionic Solids:** In their solid state, ionic solids also do not conduct electricity but can do so when melted or dissolved in water, as their ions become free to move.
• **Metallic Solids:** These are excellent conductors of electricity due to the freely moving electrons within their structure, which allows for the flow of electric current.

Given these characteristics, a solid that is a nonconductor in all states, as described in the problem, is almost certainly a molecular solid.

Low Melting Point Phenomenon in Solids

The melting point of a solid is determined by the strength and type of bonds or forces that hold its components together. Generally, the stronger these forces, the higher the melting point. This takes us to why molecular solids usually have lower melting points:

• **Weak Intermolecular Forces:** As mentioned, molecular solids are held together by weak forces like Van der Waals forces and, in some cases, hydrogen bonds. These forces are relatively easy to overcome, thus requiring less energy for the solid to transition into a liquid state.
• **Comparative Analysis:** Ionic and metallic solids typically possess stronger forces, such as ionic bonds and metallic bonds, which result in much higher melting points compared to molecular solids.

Consequently, a solid with a low melting point, as described in the exercise, is indicative of a molecular solid where weak intermolecular forces are predominant, making it easy to melt at lower temperatures, often below 100°C.