Animated Solution for Chemistry - States of Matter: A hard substance melts at high temperature and is an insulator in both solid and in molten state. This solid is most likely to be a/an

Imagine you are a detective trying to identify a mysterious substance based on a set of clues. The question provides us with four distinct physical properties of an unknown solid: 1. It is hard. 2. It has a high melting point. 3. It is an insulator in the solid state. 4. It remains an insulator even in the molten state.

To solve this, we need to systematically evaluate the four main categories of crystalline solids: Ionic, Metallic, Molecular, and Covalent (Network) solids. Let's put each suspect under the microscope.

Suspect 1: Ionic Solids Think of table salt ($NaCl$). Ionic solids are held together by strong electrostatic forces between positively and negatively charged ions. This makes them hard and gives them high melting points. In the solid state, these ions are locked in a rigid lattice, making them insulators. However, there is a catch! When you melt an ionic solid, the lattice breaks down, and the ions become free to move. These mobile ions can carry an electric current, making molten ionic solids good conductors. Since our mystery substance is an insulator in the molten state, ionic solids are eliminated.

Suspect 2: Metallic Solids Visualize a block of iron or copper. Metallic solids consist of positive metal ions surrounded by a "sea" of delocalized electrons. These free electrons are highly mobile, which makes metals excellent conductors of electricity in both their solid and molten states. Because our substance is an insulator, metallic solids are immediately ruled out.

Suspect 3: Molecular Solids Consider ice ($H_{2}O$) or dry ice (solid $C{O}_2$). In molecular solids, discrete molecules are held together by relatively weak intermolecular forces, such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. Because these forces are weak, molecular solids are generally soft and have low melting points. While they are indeed insulators, they fail the hardness and high melting point criteria.

Covalent Network Solids

Suspect 4: Covalent Solids Finally, let's look at covalent or network solids, like diamond or quartz ($Si{O}_2$). In these materials, atoms are bonded to each other by strong, directional covalent bonds, forming a massive, continuous 3D network.

Breaking this giant network requires an immense amount of thermal energy, which explains their extremely high melting points. The rigid tetrahedral geometry (in the case of diamond) makes them exceptionally hard. Furthermore, all the valence electrons are tightly localized within the covalent bonds. There are no free electrons or mobile ions available to carry a charge, making them perfect insulators in both the solid and molten states.

Our mystery substance perfectly matches the profile of a covalent network solid.