Definite shape and fixed volume due to strong intermolecular forces
Explanation
The physical state of a substance (solid, liquid, or gas) is determined by the balance between intermolecular forces, which tend to keep particles together, and thermal energy, which tends to separate them. A substance exists as a solid when intermolecular forces are dominant.
Detailed Analysis:
- High thermal conductivity is Incorrect: High thermal conductivity is a property specific to certain types of solids (like metals) but is not a defining characteristic of the solid state itself. Many solids, such as insulators (e.g., wood, glass), have low thermal conductivity.
- Definite shape and fixed volume due to strong intermolecular forces is Correct: At room temperature, the intermolecular forces (or interatomic forces in the case of iron) are strong enough to overcome thermal energy. This holds the constituent particles in fixed positions, allowing them only to oscillate about their mean positions. This structural rigidity results in a definite shape and a fixed volume, which are the defining properties of a solid.
- High density relative to water is Incorrect: While solids generally possess high density, density relative to water is not the determinant of the state of matter. For instance, mercury is a liquid at room temperature despite having a very high density.
- Ability to reflect light is Incorrect: The ability to reflect light is an optical property often found in metals (lustre), but it does not dictate whether a substance exists as a solid, liquid, or gas.
Key Takeaway:
Solids are characterized by strong intermolecular forces that restrict particles to fixed positions, thereby conferring rigidity, definite shape, and fixed volume to the substance.