Problem 50 - Chemistry The Molecular Science

Question

What type of solid exhibits each of these sets of properties? (a) Melts below \(100{ }^{\circ} \mathrm{C}\) and is insoluble in water (b) Conducts electricity only when melted (c) Insoluble in water and conducts electricity (d) Noncrystalline and melts over a wide temperature range

Step-by-Step Solution

Answer

(a) Molecular, (b) Ionic, (c) Metallic, (d) Amorphous.

Step 1: Identifying Solid Type (a)

The solid that melts below 100°C and is insoluble in water is typically a molecular solid. These solids are often made of nonpolar molecules, like certain organic compounds, which have low melting points and do not dissolve well in water.

Step 2: Identifying Solid Type (b)

A solid that conducts electricity only when melted is an ionic solid. In the solid state, ionic compounds have fixed ions, preventing electrical conductivity. When melted, the ions can move freely, allowing electricity to flow.

Step 3: Identifying Solid Type (c)

Solids that are insoluble in water and conduct electricity are usually metallic solids. Metals have a lattice of positive ions surrounded by a 'sea of electrons' that facilitates conductivity, and their lattice structure makes them generally insoluble in water.

Step 4: Identifying Solid Type (d)

A noncrystalline solid that melts over a wide temperature range is an amorphous solid. These materials, like glass, do not have a well-defined melting point because they lack an ordered crystal structure.

Key Concepts

Molecular Solids

Molecular solids are fascinating because they are primarily held together by weak intermolecular forces such as Van der Waals forces, hydrogen bonds, or dipole-dipole interactions.
This unique molecular arrangement results in a lower strength bond compared to other types of solids. Because of these weak forces, molecular solids often have relatively low melting points, sometimes below 100°C, as seen in the case of many organic compounds.
Molecular solids tend to be insoluble in water, especially if they are composed of nonpolar molecules. This is because polar and nonpolar substances typically do not mix ( "like dissolves like"). These properties make molecular solids a prime choice for applications requiring low thermal energy or organic compatibility.

• Low melting point
• Weak intermolecular forces
• Typically nonpolar and insoluble in water

Ionic Solids

Ionic solids are defined by the presence of strong ionic bonds between their positively and negatively charged ions. These bonds form due to the electrostatic attraction between ions holding them firmly in place in a crystalline structure.
This strong attraction gives ionic solids a high melting point when in reality they may only conduct electricity when melted or dissolved in water.
Once melted, the ions are free to move, allowing for the crucial conductance of electricity. Understanding the behavior of ionic solids is essential in applications such as construction materials requiring thermal stability and electrical storage systems.

• Strong ionic bonds
• High melting points; conduct electricity when molten
• Made of metal and nonmetal atoms

Metallic Solids

Metallic solids have a unique lattice structure which consists of a sea of delocalized electrons. This structure includes positively charged metal ions surrounded by a cloud of electrons.
This free movement of electrons allows metallic solids to be excellent conductors of electricity and heat.
Unlike ionic or molecular solids, metallic solids are insoluble in water due to the strong metallic bond, which cannot be broken down by water molecules. Metals exhibit properties such as malleability and ductility due to the flexible lattice of ions.

• Great electrical conductivity
• Insoluble in water
• Malleable and ductile

Amorphous Solids

Amorphous solids differ from crystalline solids in that they lack a long-range ordered structure. Instead, their atoms are arranged in a more random, disordered pattern.
A key characteristic of amorphous solids is their lack of a specific melting point. Instead, they melt over a wide range of temperatures, softening gradually as heat is applied.
Aclassic example of an amorphous solid is glass, which can transition from brittle at room temperature to a pliable and moldable state when heated. Amorphous solids are used in a variety of everyday applications, from the glass in windows to various polymers in plastics.

• Lack of long-range order
• Melts over a range of temperatures
• Examples include glass and plastics