Consider the below statements with respect to asbestos and identify the correct option.
Statement A: When asbestos is heated below 550°C, asbestos loses its elasticity and strength and becomes brittle but restores its properties on cooling.
Statement B: Asbestos molecules are strongly bound together only in one direction, whereas the lateral bond with adjacent molecules is quite weak.
🔬 Detailed Analysis of the Statements
This question tests the understanding of the fundamental chemical and physical properties of asbestos, a naturally occurring fibrous silicate mineral.
Analysis of Statement A
"When asbestos is heated below 550°C, asbestos loses its elasticity and strength and becomes brittle but restores its properties on cooling."
This statement is INCORRECT. The loss of properties in asbestos due to heat is an irreversible chemical change. Asbestos minerals are hydrated silicates, meaning they have water molecules chemically bound within their crystal structure. When heated to high temperatures (typically starting around 300-500°C and becoming significant above 550°C), this chemically bound water is driven off. This process, called dehydroxylation, permanently alters the crystal structure. The asbestos loses its fibrous nature, strength, and elasticity and becomes brittle. Because this is a chemical decomposition, the original properties cannot be restored upon cooling.
Analysis of Statement B
"Asbestos molecules are strongly bound together only in one direction, whereas the lateral bond with adjacent molecules is quite weak."
This statement is CORRECT. This describes the very essence of a fibrous material. The atoms within the long silicate chains or sheets of asbestos are held together by very strong covalent bonds. This gives the individual fibers extremely high tensile strength along their length. However, the bonds holding these parallel chains or sheets together (the lateral bonds) are much weaker van der Waals forces. This weakness is what allows the bulk mineral to be easily separated into fine, flexible fibers.
✅ Conclusion and Study Tips
Based on the analysis, Statement A is incorrect because the damage from heat is permanent. Statement B is correct as it accurately describes the anisotropic bonding that gives asbestos its fibrous character. Therefore, option (b) is the correct choice.
💡 Study Tips for Asbestos Properties
- Heat Damage is Permanent: Remember that heating asbestos causes an irreversible chemical reaction (loss of water), destroying its useful properties forever.
- Strong Along, Weak Across: This is the definition of a fiber. Think of a rope: it's very strong when you pull on its ends (along the fibers), but you can easily separate the individual strands from the side (weak lateral bonds). Asbestos is the same at a microscopic level.
- Asbestos is a Hydrated Mineral: The "hydrated" part is key. Driving off the water with heat is what destroys it.