Question

A solid substance is soft, has low melting point and is poor conductor of electricity. The substance is most likely (1) an ionic solid (2) network solid (3) a metallic solid (4) a molecular solid

Short answer

The substance is most likely a molecular solid.

Step-by-step solution

- Identify the Properties

Analyze the given properties of the solid: soft, low melting point, and poor conductor of electricity. These properties provide clues about the type of solid the substance is.

- Consider Ionic Solids

Ionic solids typically have high melting points and are hard. They are good conductors of electricity in molten state or solution, but not in solid state. Since the given substance is soft and has a low melting point, it is unlikely to be an ionic solid.

- Consider Network Solids

Network solids (like diamond and quartz) have very high melting points and are very hard. They are usually poor conductors of electricity. Given that the substance in question is soft, it is unlikely to be a network solid.

- Consider Metallic Solids

Metallic solids are characterized by high melting points, hardness, and good electrical conductivity in both solid and molten states. Since the substance is described as a poor conductor of electricity, it is unlikely to be a metallic solid.

- Consider Molecular Solids

Molecular solids are usually soft, have low melting points, and are poor conductors of electricity due to the absence of free ions or electrons. This matches the given properties of the substance.

- Conclusion

Based on the properties described (soft, low melting point, poor conductor of electricity), the substance is most likely a molecular solid.

Key concepts

Molecular Solids

Molecular solids are composed of molecules held together by intermolecular forces. These forces include Van der Waals forces, dipole-dipole interactions, and hydrogen bonds. These forces are generally weaker than ionic or covalent bonds, which contributes to the characteristic properties of molecular solids.
Some key properties of molecular solids are:

• Soft texture: They can be easily scratched or deformed due to weak intermolecular forces.
• Low melting points: Typically, they melt at relatively low temperatures because not much energy is needed to overcome the intermolecular forces.
• Poor conductivity: They do not conduct electricity well since they lack free ions or electrons to carry charge.

Examples of molecular solids include ice (frozen water), dry ice (solid CO₂), and numerous organic compounds like sugar and naphthalene.

Types of Solids

Solids can be broadly categorized into four types based on their bonding and structural properties:

• Ionic Solids: Composed of positive and negative ions held together by strong ionic bonds. These solids are hard, have high melting points, and conduct electricity when molten or dissolved in water.
• Network Solids: Also known as covalent solids, where atoms are bonded covalently in a continuous network. They are extremely hard, have high melting points, and are generally poor conductors of electricity. Examples include diamond and silicon dioxide (quartz).
• Metallic Solids: Consist of metal atoms surrounded by a sea of delocalized electrons, which allows them to conduct electricity and heat efficiently. These solids are malleable, ductile, and have a wide range of melting points.
• Molecular Solids: Made up of molecules held together by intermolecular forces, as discussed earlier. They are soft, have low melting points, and are poor conductors of electricity.

Each type of solid has distinctive properties that are determined by the nature of the bonding and structure within the solid.

Properties of Solids

The properties of solids are largely determined by the types of particles they are composed of and the forces holding these particles together. These properties include:

• Hardness: Some solids like diamond are incredibly hard due to strong covalent bonds, while others like molecular solids are soft because of weak intermolecular forces.
• Melting and Boiling Points: Solids with strong bonds, such as ionic and network solids, have high melting and boiling points. Solids with weaker forces, like molecular solids, have lower melting points.
• Electrical Conductivity: Metallic solids are good conductors of electricity due to free-moving electrons. Ionic solids conduct electricity when melted or dissolved, but not in their solid state. Network and molecular solids are generally poor conductors.
• Solubility: Ionic solids are often soluble in water and other polar solvents, while molecular solids vary in solubility depending on their polarity and the solvent.

These properties help identify and classify different types of solids based on empirical observation and practical application.

Conductivity of Solids

Electrical conductivity in solids is greatly influenced by the structure and type of bonding present.

• Ionic Solids: These substances conduct electricity when melted or dissolved because the ions are free to move and carry charge, but they do not conduct in solid form.
• Metallic Solids: Conductivity is high due to the presence of a 'sea of electrons' that allows free movement of charge throughout the solid. This makes metals suitable for electrical wiring and electronic applications.
• Network Solids: Typically poor conductors, since electrons are held tightly in covalent bonds and cannot move freely. Graphite is an exception due to its layered structure, which allows some delocalized electrons to move.
• Molecular Solids: These are generally non-conductive because they lack free ions or electrons. The molecules are held together by weak intermolecular forces that do not support the flow of electrical current.

Understanding the conductivity of different types of solids is crucial for their utilization in various technological and industrial applications.