Question

Solid 1 + Solid 2 $\rightarrow$ Solid 3 (during cooling) denotes

• A. Eutectic reaction
• B. Peritectic reaction
• C. Eutectoid reaction
• D. Peritectoid reaction

Hint:

Remember: {Peritectoid = Solid + Solid $\rightarrow$ Solid}.

Answer 1

The Correct Option is D

Step 1: Understanding invariant reactions.
Invariant reactions involve transformation of phases at a fixed temperature and composition in phase diagrams. These reactions may occur in liquid–solid or solid–solid systems.
Step 2: Identifying the given reaction.
The reaction given is: \[ \text{Solid 1} + \text{Solid 2} \rightarrow \text{Solid 3} \] This indicates a solid–solid reaction during cooling.
Step 3: Differentiating between reactions.
Eutectic: Liquid $\rightarrow$ Solid 1 + Solid 2
Peritectic: Liquid + Solid $\rightarrow$ Solid
Eutectoid: Solid $\rightarrow$ Solid 1 + Solid 2
Peritectoid: Solid 1 + Solid 2 $\rightarrow$ Solid 3
Step 4: Conclusion.
Thus, the given reaction represents a peritectoid reaction.

Answer 2

Invariant reactions can be told apart by looking at the phases present before and after the reaction: whether they involve a liquid, and how many solid phases combine or split. Let's check the given reaction, \( \text{Solid 1} + \text{Solid 2} \rightarrow \text{Solid 3} \), against each option.

1. Eutectic reaction: This reaction starts from a single liquid phase and produces two solid phases on cooling, \( \text{Liquid} \rightarrow \text{Solid 1} + \text{Solid 2} \). The given reaction starts from two solids, not a liquid, so it cannot be eutectic.
2. Peritectic reaction: This involves a liquid combining with an existing solid to form a new solid, \( \text{Liquid} + \text{Solid 1} \rightarrow \text{Solid 2} \). Again, a liquid phase is required, which is absent from the given reaction.
3. Eutectoid reaction: This is the solid-state analogue of the eutectic reaction, where one solid phase splits into two different solids on cooling, \( \text{Solid} \rightarrow \text{Solid 1} + \text{Solid 2} \). This is the reverse direction of what's given (splitting, not combining), so it does not match.
4. Peritectoid reaction: This is the solid-state analogue of the peritectic reaction, where two existing solid phases combine on cooling to form a third, new solid phase, \( \text{Solid 1} + \text{Solid 2} \rightarrow \text{Solid 3} \). This matches the given reaction exactly, both in that everything is solid-state and in the direction of combination.

Only the peritectoid reaction has two solids combining into a third with no liquid phase involved anywhere, matching the given transformation exactly.

Therefore, the correct answer is Peritectoid reaction.