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

The four invariant reactions can be summarized by their defining equations.

1. Step 1: Eutectic: \( \text{Liquid} \rightarrow \text{Solid 1} + \text{Solid 2} \); Eutectoid: \( \text{Solid} \rightarrow \text{Solid 1} + \text{Solid 2} \).
2. Step 2: Peritectic: \( \text{Liquid} + \text{Solid 1} \rightarrow \text{Solid 2} \); Peritectoid: \( \text{Solid 1} + \text{Solid 2} \rightarrow \text{Solid 3} \).
3. Step 3: The given reaction, \( \text{Solid 1} + \text{Solid 2} \rightarrow \text{Solid 3} \), matches the peritectoid form exactly, no liquid phase anywhere and two solids combining into one.

\[ \boxed{\text{Peritectoid reaction}} \]

Answer 2

A useful shortcut is to treat "eutectoid" and "peritectoid" as the solid-state versions of "eutectic" and "peritectic," simply replacing the liquid phase in each with a solid.

1. Eutectic reaction: its solid-state analogue is the eutectoid, in which a single solid splits into two on cooling. Neither the eutectic nor eutectoid form matches a reaction that combines two solids into one.
2. Peritectic reaction: here a liquid reacts with a solid to give a new solid. Turning the liquid into a solid gives its analogue, which is exactly two solids combining to give a third.
3. Eutectoid reaction: as noted, this splits one solid into two, the reverse direction of combination described in the question.
4. Peritectoid reaction: this is precisely the solid-only analogue of the peritectic reaction, two solids reacting together to produce a third solid on cooling, exactly the transformation given.

Replacing the liquid in a peritectic reaction with a solid gives exactly the peritectoid reaction described in the question.

Therefore, the correct answer is Peritectoid reaction.