The key difference between eutectic and eutectoid reaction is that in eutectic reactions, a liquid transforms into two solid phases at the same time whereas in a eutectoid reaction, a solid transform into two other solid phases at the same time.
Eutectic and eutectoid reactions are chemical reactions that describe the transformation of one phase into another phase upon the change of temperature of a system.
CONTENTS
1. Overview and Key Difference
2. What is a Eutectic Reaction
3. What is a Eutectoid Reaction
4. Side by Side Comparison – Eutectic vs Eutectoid Reaction in Tabular Form
5. Summary
What is a Eutectic Reaction?
Eutectic reactions are chemical reactions in which a liquid transforms into two solid phases at the same time upon cooling. A eutectic system is a homogenous mixture of substances that can melt or solidify at a temperature that is lower than the melting point of the constituents in that mixture. In fact, the term eutectic temperature describes the lowest possible melting temperature for the all possible mixing ratios that are involved in the formation of the mixture.
Figure 01: Different Eutectic Structures
Upon heating a eutectic mixture, the lattice one component in the mixture will melt first at the eutectic temperature. In contrast, upon cooling the eutectic system, each component in the mixture tends to solidify, forming the lattice of that component at a distinct temperature. This solidification occurs until all materials become solids. Usually, a eutectic system contains two components; thus, at eutectic temperature, the liquid transforms into two solid phases at the same time and at the same temperature. Therefore, we can name this type of reactions as a three-phase reaction. This is a specific type of phase reaction. E.g. a liquid solidifies, forming an alpha and beta solid lattices. Here, the liquid phase and solid phase are in equilibrium with each other; a thermal equilibrium.
What is a Eutectoid Reaction?
A eutectoid reaction is a chemical reaction in which a solid transform into two other solid phases at the same time upon cooling. This is a three-phase reaction because one phase of matter turns into two other phases of matter. It is an isothermal reaction which forms two mixed solid phases. The number of solids in the solid mixture depends on the number of components in the system.
Figure 02: Steel is Formed from Eutectoid Reactions
A eutectoid reaction occurs at the eutectoid point. This reaction is similar to the eutectic reaction; the difference is in the phases that are changing. Eutectoid reaction of iron is an example of this reaction. The eutectoid structure of iron has a special name: pearlite. Pearlite is a mixture of two phases; ferrite and cementite. This structure occurs in many common grades of steel, which is an alloy of iron and carbon.
What is the Difference Between Eutectic and Eutectoid Reaction?
Eutectic and eutectoid reactions are three-phase reactions where one phase of matter converts into two other phases of matter. The key difference between eutectic and eutectoid reaction is that in eutectic reactions, a liquid transforms into two solid phases at the same time whereas in a eutectoid reaction, a solid transform into two other solid phases at the same time.
Below infographic presents the differences between eutectic and eutectoid reaction in tabular form.
Summary – Eutectic vs Eutectoid Reaction
Eutectic and eutectoid reactions are three-phase reactions where one phase of matter converts into two other phases of matter. The key difference between eutectic and eutectoid reaction is that in eutectic reactions, a liquid transforms into two solid phases at the same time whereas in a eutectoid reaction, a solid transform into two other solid phases at the same time.
Reference:
1. Soni, Prem Kumar. “Eutectic, Eutectoid, Peritectoid, Peritectic.” SlideShare, 5 Aug. 2018, Available here.
Image Courtesy:
1. “Various eutectic structures” By Wizard191 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Steel Construction Steel Bar Rebar Free Photo” (CC0) via Needpix