Question

You are given a white substance that melts at \(100^{\circ} \mathrm{C}\) . The substance is soluble in water. Neither the solid nor 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 a molecular solid, as it has a relatively low melting point (100°C), is soluble in water, and does not conduct electricity in either solid or solution forms.

Step-by-step solution

Analyze Molecular Solids

Molecular solids are composed of molecules held together by weak van der Waals forces or hydrogen bonds. They tend to have relatively low melting points, and they do not conduct electricity. When dissolved in water, they form non-conductive solutions. Considering all these properties, the substance could be a molecular solid.

Analyze Metallic Solids

Metallic solids are composed of metal atoms held together by metallic bonds. These solids have high melting points and are good conductors of electricity, both in solid form and when dissolved (forming ions in a solution). Considering the given properties, the substance cannot be a metallic solid because it does not conduct electricity.

Analyze Covalent-Network Solids

Covalent-network solids are composed of atoms connected by strong covalent bonds in a large network. They usually have high melting points and are not soluble in water. Also, they do not conduct electricity. Given these properties, our substance cannot be a covalent-network solid because it is soluble in water.

Analyze Ionic Solids

Ionic solids are composed of oppositely charged ions held together by strong ionic bonds. They have high melting points and can conduct electricity when dissolved in water (forming ions in a solution). However, they do not conduct electricity in their solid form. In this case, our substance cannot be an ionic solid because neither the solid nor the solution conducts electricity.

Determine the Type of Solid

Comparing the properties of the given substance with those of the various types of solids, we can conclude that the substance might be a molecular solid, as it fits the properties of having a relatively low melting point, being soluble in water, and not conducting electricity in either solid or solution forms.

Key concepts

Conductivity

When discussing the conductivity of solids, it’s important to understand what allows a material to conduct electricity. Conductivity in any substance depends on the presence of mobile charge carriers such as electrons or ions. In the case of molecular solids, these contain molecules held together by soft forces like van der Waals or hydrogen bonds. Hence, they lack free-moving charge carriers.

• In solid form, molecular solids do not conduct electricity because their electrons are localized within the molecules and cannot move freely.
• Even when dissolved in water, they continue not to conduct because they do not form ions, differentiating them from ionic solids.

Understanding why molecular solids do not conduct electricity helps identify them among other types of solids. This is particularly useful when distinguishing them from metallic and ionic solids, both of which show distinct conductive properties when in specific states.

Solubility

Solubility is a key factor determining how a substance interacts with solvents like water. For molecular solids, their solubility is primarily determined by the intermolecular forces present in the substance compared to those in the solvent.

• Molecular solids are usually soluble in polar solvents like water because they can engage in dipole interactions with polar solvent molecules, helping them dissolve.
• Solubility does not necessarily confer conductivity, as molecular solids dissolve as whole molecules without ionization.

Aiming for water solubility while remaining non-conductive, such as in the given problem scenario, suggests traits common to molecular solids and distinct from covalent-network solids, which are typically insoluble.

Melting Point

Understanding melting points helps us assess the strength of interactions within a solid. Molecular solids generally have lower melting points because only weak van der Waals forces or hydrogen bonds need to be overcome to transition to a liquid state.

• The given melting point of approximately 100°C indicates relatively weak forces are at play compared to higher-melting ionic or covalent-network solids.
• However, it is higher than typical for very weak molecular forces indicating perhaps stronger hydrogen bonding.

Using melting points as diagnostic tools can effectively help in determining the type of solid, reaffirming the likelihood of a substance being a molecular solid based on its low melting point, along with its solubility profile and lack of conductivity.