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 gray substance closely resembles a metallic solid due to its high melting point, good electrical conductivity, and insolubility in water. Therefore, the substance is most likely a metallic solid.

Step-by-step solution

Identify the types of solids

Different types of solids exhibit different properties. 1. Molecular Solids: Consist of molecules held together by van der Waals forces or hydrogen bonds. They generally have low melting points, are poor conductors of electricity, and can be soluble in water. 2. Metallic Solids: Composed of metal atoms bonded together through metallic bonding. These solids typically have high melting points, good electrical conductivity, and are insoluble in water. 3. Covalent-Network Solids: Formed by atoms connected through strong covalent bonds in an extended network structure. They usually have high melting points, are poor conductors of electricity, and are insoluble in water. 4. Ionic Solids: Consist of positively and negatively charged ions held together by ionic bonds. Ionic solids tend to have high melting points, conduct electricity only when dissolved in water or in a molten state, and are often soluble in water.

Comparing properties of the solid with different types of solids

Now, let's compare the properties of the given solid with the different types of solids: 1. Molecular Solids: Since the given solid has a high melting point and conducts electricity, it does not match the properties of molecular solids. 2. Metallic Solids: The given solid has properties that resemble metallic solids - high melting point, good electrical conductivity, and insolubility in water. 3. Covalent-Network Solids: The given solid does not match the properties of covalent-network solids, as the substance conducts electricity. 4. Ionic Solids: While the given solid shares high melting point and insolubility in water with ionic solids, it conducts electricity in the solid state, which ionic solids do not.

Conclusion

Based on the comparison of properties, the given gray substance closely resembles a metallic solid. Therefore, the substance is most likely a metallic solid.

Key concepts

Metallic Solids

Understanding the nature of metallic solids is essential for identifying how they behave under various conditions. At a fundamental level, metallic solids consist of closely packed metal atoms. The unique feature of these solids is the presence of a 'sea of electrons'. This is a cloud of delocalized valence electrons that surround the metal cations, which are fixed in a lattice.

The bonding nature of metallic solids leads to several properties which include malleability, ductility, and a pronounced metallic luster. Malleability allows these solids to be hammered into thin sheets, while ductility means they can be stretched into wires. These remarkable properties are due to the ability of metal ions to slide past one another without breaking the metallic bond.

Distinguishing Metallic Solids

When you encounter a substance like the gray material mentioned in the exercise, which has a high melting point and is a good conductor of electricity, it's reasonable to hypothesize that you have a metallic solid in front of you. The high melting point indicates strong metallic bonds, requiring significant energy to break.

Electrical Conductivity

Electrical conductivity is a measure of a material's ability to conduct an electric current. This is an intrinsic property of certain materials which allows them to transport charge with ease. In solids, the movement of electrons or ions account for electrical conductivity.

In metallic solids, it is the sea of electrons free to move that accounts for their high electrical conductivity. When a potential difference is applied across a piece of metal, these electrons flow as an electric current, facilitating a quick transmission of energy.

Importance of Electrical Conductivity

Electrical conductivity is not just an academic concept; it has practical implications in everyday life. From the wiring in your home to the components in your electronics, the principles of conductivity determine the efficiency and safety of these materials. When a student comes across an exercise where a solid conducts electricity, identifying it as metallic based on this property is well-justified.

Melting Points

The melting point of a solid is the temperature at which it changes state from solid to liquid. This physical property is a crucial factor in understanding the bonding and structure of a material. Different types of solids boast a range of melting points based on the forces that hold them together.

Metallic solids commonly have high melting points due to the strong attraction between the metal cations and the sea of electrons. In essence, it takes a considerable amount of thermal energy to disrupt the orderly arrangement of metal ions. The melting point can serve as a clue to the type of bonding present in an unknown substance. For instance, a high melting point suggests strong bonds and, oftentimes, a crystalline structure.

Melting Points in Different Solids

While covalent and ionic solids might rival metals in terms of melting points, it's their other properties like electrical conductivity (or lack thereof) that help distinguish them from metallic solids. In the exercise scenario where the substance has a high melting point but also conducts electricity, this confluence of properties helps narrow down the identity to a metallic solid.