To determine the type of solid described in the question, we need to analyze the given properties:
Property: Hard Substance
Typically, covalent solids are known for their hardness. They are composed of atoms linked by covalent bonds, forming a rigid, lattice-like structure.
Property: High Melting Point
Covalent solids generally possess high melting points because breaking the covalent bonds requires significant energy. For instance, diamond, a well-known covalent solid, has a very high melting point.
Property: Insulator in Both Solid and Molten State
Covalent solids are poor conductors of electricity in both solid and molten states, since they lack free-moving charged particles. In contrast, ionic solids conduct electricity in their molten state, and metallic solids conduct in both states.
Based on these characteristics, the substance described fits the properties of a covalent solid. Let's examine the other options to confirm this classification:
Ionic Solid: Typically conducts electricity in the molten state. Also, while hard, they tend to be brittle and are not insulators when molten.
Metallic Solid: Good conductors of electricity in both solid and molten states, which contradicts the insulating property.
Molecular Solid: Generally have lower melting points and are held together by weaker forces like van der Waals forces, not exhibiting the described hardness.
Therefore, the most likely type of solid based on the described properties is a covalent solid. These solids possess all the mentioned attributes: they are hard, have high melting points, and act as insulators in both solid and molten states.
