Question

You are given a gray substance that melts at \(700^{\circ} \mathrm{C} ;\) the solid is a conductor of electricity and is insoluble in water. Which type of solid (molecular, metallic, covalent-network, or ionic) might this substance be?

Short answer

The given substance is most likely a metallic solid, as its properties of a high melting point at \(700^{\circ}\mathrm{C}\), electrical conductivity in the solid state, and insolubility in water match the characteristics typical of metallic solids.

Step-by-step solution

Analyze the given properties of the substance

The substance has three properties: 1. Melts at \(700^{\circ}\mathrm{C}\): This suggests a high melting point. 2. Conducts electricity in its solid state: This implies the presence of free-moving electrons or charged particles. 3. Insoluble in water: This means that the substance does not dissolve in water.

Compare properties with molecular solids

A molecular solid consists of molecules held together by weak intermolecular forces (van der Waals forces) like dispersion forces and hydrogen bonding. They typically have low melting points, are poor electrical conductors, and can be soluble in water if they are polar. Given that the substance has a high melting point and conducts electricity, it is not likely to be a molecular solid.

Compare properties with metallic solids

Metallic solids consist of metal atoms surrounded by a sea of electrons. The electrons are free to move throughout the material, which makes them good conductors of electricity. Metallic solids also usually have high melting points and are insoluble in water. The substance's properties match those of a metallic solid.

Compare properties with covalent-network solids

Covalent-network solids consist of nonmetal atoms bonded to each other through strong covalent bonds. They have high melting points due to the strong bonds holding the atoms together. However, they are usually poor conductors of electricity as they don't have free-moving electrons or charged particles. This implies that the given substance is not a covalent-network solid.

Compare properties with ionic solids

Ionic solids are composed of positive and negative ions arranged in a crystal lattice. They have high melting points due to strong electrostatic interactions between the ions. They can conduct electricity when dissolved in water or melted, but not in their solid state. They are also usually soluble in water. The given solid conducts electricity in its solid state and is not soluble in water, so it can't be an ionic solid.

Determine the type of solid based on the comparison

Based on the analysis of the substance's properties, it is most likely a metallic solid. Its high melting point, electrical conductivity in the solid state, and insolubility in water are characteristics typical of metallic solids.

Key concepts

Metallic Solids

Metallic solids are an interesting type of solid where metal atoms are tightly packed together in a lattice structure. What makes metallic solids unique is the presence of a "sea of electrons". These electrons are not bound to any specific atom and can move freely throughout the solid.
This movement of electrons is what gives metallic solids their excellent ability to conduct electricity, even in the solid state.

• High Melting Points: Metallic solids are known for their high melting points, which result from the strong forces holding the metal atoms together in the lattice.
• Good Conductors: Because of the free electrons, metallic solids can easily conduct both electricity and heat.
• Insolubility in Water: Most metallic solids do not dissolve in water, making them waterproof in many everyday uses.

Understanding these properties helps explain why a substance with a high melting point, electrical conductivity in its solid state, and insolubility in water is a metallic solid.

Covalent-Network Solids

Covalent-network solids are formed when nonmetal atoms are connected in a continuous network via strong covalent bonds. These types of solids are famous for their robustness and very high melting points, but they do not conduct electricity well.

• Structure: In covalent-network solids, atoms are bonded together in a vast network, where bonds extend throughout the material.
• High Melting Points: Because all atoms are bonded with covalent bonds, it requires a lot of energy to break these bonds, resulting in high melting points.
• Poor Electrical Conductors: Unlike metallic solids, covalent-network solids do not have free-moving electrons, meaning they can't conduct electricity effectively.

The inability to conduct electricity in its solid form differentiates covalent-network solids from metallic solids.

Ionic Solids

Ionic solids are composed of positive and negative ions arranged in a crystal lattice. This structure is due to the electrostatic forces that hold the charged ions in place.

• High Melting Points: The strong attractions between oppositely charged ions give ionic solids high melting points.
• Electrical Conductivity: Ionic solids don't conduct electricity in their solid state because ions are fixed in place. However, when melted or dissolved in water, the ions are free to move, allowing them to conduct electricity.
• Solubility in Water: Ionic compounds often dissolve in water, separating into ions that become surrounded by water molecules.

Since the substance in question conducts electricity as a solid and does not dissolve in water, it does not fit the profile of an ionic solid.

Molecular Solids

Molecular solids are made up of molecules held together by weak forces, such as van der Waals forces or hydrogen bonds. These attractive forces are much weaker compared to the covalent or ionic bonds found in other types of solids.

• Low Melting Points: Molecular solids generally have low melting points due to weak intermolecular attractions.
• Poor Electrical Conductors: In molecular solids, there are no charged particles to carry electrical current, making them poor conductors.
• Solubility Variability: Depending on their polarity, molecular solids might dissolve in solvents that match their polarity.

Given the low melting point and poor conductivity, the properties of molecular solids are very different from those of the substance described in the exercise.