Question

You are given a white substance that melts at \(1500^{\circ} \mathrm{C}\). The substance is brittle and soluble in water. Both the molten solid and the solution is a conductor of electricity. Which type of solid (molecular, metallic, covalent-network, or ionic) might this substance be?

Short answer

The given substance might be an ionic solid, as it is brittle, melts at a high temperature (1500°C), is soluble in water, and conducts electricity when melted or in solution, which matches the properties of ionic solids.

Step-by-step solution

Understanding the properties of different types of solids

Molecular solids: These are formed by molecules held together by weak forces like van der Waals forces or hydrogen bonds. They generally have low melting points, are poor conductors of electricity, and can be soluble in water (but not always). Metallic solids: These have a lattice of metal cations surrounded by a sea of delocalized electrons which form a metallic bond. Metallic solids generally have high melting points, are ductile and malleable, conduct electricity in solid and liquid state, but are insoluble in water. Covalent-network solids: These are formed by a network of atoms bonded together by strong covalent bonds. They generally have high melting points, are poor conductors of electricity, are insoluble in water, and are brittle. Ionic solids: These are formed by positively charged cations and negatively charged anions held together by strong electrostatic forces (ionic bonds). They generally have high melting points, are brittle, soluble in water, and conduct electricity when melted or in solution.

Comparing the given properties with the properties of types of solids

1. Melts at 1500°C: This property suggests that the substance could be a metallic, covalent-network, or ionic solid, as all of them have high melting points. 2. Brittle: This property suggests that the substance could be a covalent-network solid or an ionic solid, as both are brittle. 3. Soluble in water: This property suggests that the substance could be a molecular solid or an ionic solid, as both can be soluble in water. 4. Conducts electricity when melted or in solution: This property suggests that the substance could be an ionic solid, as these conduct electricity when melted or in solution.

Determining the possible type of solid based on the given properties

Since the substance is brittle, melts at a high temperature (1500°C), is soluble in water, and conducts electricity when melted or in solution, it matches the properties of an ionic solid. Therefore, the given substance might be an ionic solid.

Key concepts

Properties of Solids

Solids have various properties that define their behavior and applications. These properties include melting points, conductivity, solubility, and structural characteristics. Each type of solid, whether it be molecular, metallic, covalent-network, or ionic, possesses unique properties that distinguish it from others.

• Melting Point: This is the temperature at which a solid turns into a liquid. High melting points are indicative of strong bonds within the solid structure, as seen in ionic and covalent-network solids.
• Brittleness: This is the tendency of a material to break or shatter under stress. Ionic and covalent-network solids often exhibit brittleness due to their rigid, tightly bound structures.
• Solubility: This refers to the ability of a solid to dissolve in a solvent, like water. Ionic and some molecular solids dissolve in water, depending on their polarity and the strength of their ionic or molecular bonds.
• Conductivity: This is a measure of a material's ability to conduct electricity. Ionic solids conduct electricity when melted or dissolved because their ions are free to move.

Understanding these properties helps in identifying the type of solid and its potential uses.

Conductivity in Solutions

Conductivity in solutions is largely dependent on the presence of charged particles. In the context of solids, ionic compounds significantly contribute to conductivity when they are in liquid form or dissolved in liquids.
Ionic solids consist of charged ions. When they dissolve in water or melt, these ions become mobile, allowing them to carry electrical current. This characteristic is crucial in applications like electrochemistry and batteries.

• Ionic Solutions: When ionic solids dissolve, they dissociate into their positive and negative ions. These free ions are what carry the current, making ionic solutions good conductors.
• Molten Ionic Compounds: In the molten state, ionic compounds also conduct electricity as their ions are not fixed and freely move around.

Thus, the ability of a substance to conduct electricity in solution is a key indicator of its ionic nature.

Types of Solids

Solids can be categorized into four primary types: molecular, metallic, covalent-network, and ionic solids. Understanding these categories helps in identifying a solid's properties and potential uses.

• Molecular Solids: These are composed of molecules held together by weak van der Waals forces or hydrogen bonds. They generally manifest low melting points and are poor conductors of electricity.
• Metallic Solids: Characterized by metallic bonds, these solids have high melting points, are ductile, malleable, and can conduct electricity in both solid and molten states due to a "sea of electrons." However, they are typically insoluble in water.
• Covalent-Network Solids: These are giant networks of atoms linked by covalent bonds, resulting in high melting points and a brittle nature. They are usually insoluble in water and do not conduct electricity.
• Ionic Solids: Formed by oppositely charged ions, these solids exhibit high melting points and brittleness. They are usually soluble in water and capable of conducting electricity when molten or in solution.

By understanding these types, one can predict the behavior and applications of different solids.