Question

Assertion: $6:6$ co-ordination at normal temperature and pressure changes to $8:8$ co-ordination at high pressure.

Reason: Pressure influences the structure of solids.

• A. Both Assertion and Reason are correct and Assertion is the correct explanation of Reason.
  
• B. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
  
• C. Assertion is correct but Reason is wrong.
• D. Assertion is wrong but Reason is correct.

Answer 1

The correct option is A Both Assertion and Reason are correct and Assertion is the correct explanation of Reason.

As pressure increases the close packed structure changes to a less packed structure.For example- NaCl (6:6) structure to CsCl (8:8) structure.

Option A-Both Assertion and Reason are correct and Assertion is the correct explanation of Reason.

$\text{Explanation}:$
As we know that $\alpha$ Iron has a BCC structure and γ-Fe has a FCC structure (refer- Fe-C phase diagram).

As the pressure is increased, the α-Fe $\to$ γ-Fe transition temperature is lowered, whereas γ-Fe$\to$ δ Fe transition temperature is raised. This is true to Le Chateliers principle. As γ-Fe (FCC) is a close-packed structure, it resists the pressure more than α-Fe, or δ-Fe (BCC) which are more open structures (less densely packed), and that is why the area of stabilisation of γ-Fe increases with the increase of pressure. Thus, a FCC changes to BCC structure.