Question:
What happens in a eutectoid reaction on cooling?
What happens in a eutectoid reaction on cooling?
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Eutectoid = solid $\rightarrow$ two solids (pearlite formation in steel).
Updated On: Jun 29, 2026
- Liquid transforms into two solids
- One solid transforms into two different solids
- Two solids combine to form one solid
- Liquid transforms into one solid
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The Correct Option is B
Solution and Explanation
Concept:
A eutectoid reaction is a solid-state phase transformation in which a single solid phase decomposes into two different solid phases upon cooling at a specific temperature and composition.
It is written as:
\[
\gamma \rightarrow \alpha + \beta
\]
Unlike eutectic reactions (liquid → two solids), eutectoid reactions occur entirely in the solid state.
Step 1: Understand the type of reaction.
In eutectoid transformation:
• starting phase is a solid solution
• product phases are two different solid phases
• transformation occurs at a fixed temperature
Step 2: Classical example.
A well-known example is in the iron-carbon system: \[ \gamma\text{-austenite} \rightarrow \alpha\text{-ferrite} + Fe_3C\text{ (cementite)} \] This forms pearlite structure.
Step 3: Why other options are incorrect.
• (A) describes eutectic reaction (liquid → two solids)
• (C) describes reverse or recombination, not eutectoid
• (D) describes melting/solidification, not eutectoid Final Answer: \[ \boxed{\text{One solid transforms into two different solids}} \]
Step 1: Understand the type of reaction.
In eutectoid transformation:
• starting phase is a solid solution
• product phases are two different solid phases
• transformation occurs at a fixed temperature
Step 2: Classical example.
A well-known example is in the iron-carbon system: \[ \gamma\text{-austenite} \rightarrow \alpha\text{-ferrite} + Fe_3C\text{ (cementite)} \] This forms pearlite structure.
Step 3: Why other options are incorrect.
• (A) describes eutectic reaction (liquid → two solids)
• (C) describes reverse or recombination, not eutectoid
• (D) describes melting/solidification, not eutectoid Final Answer: \[ \boxed{\text{One solid transforms into two different solids}} \]
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