Question

Point of arrest for iron correspond to:
A. Stages at which allotropic forms change
B. Stages at which further heating does not increase temperature for some time
C. Stages at which properties do not change with increase in temperature
D. There is nothing like points of arrest

Answer 1

Hint :The beginning temperature of the substance, referred to as the "pouring temperature" in this graph, is at the top of the graph. There is a "thermal arrest" when the phase transition occurs, which means the temperature remains constant. This is due to the fact that matter as a liquid or gas has greater intrinsic energy than matter in the state it is cooling to. Latent heat is the amount of energy required for a phase shift. The slope of the cooling curve at any point is the "cooling rate."

Complete Step By Step Answer:
There are three allotropic forms of iron at atmospheric pressure: alpha iron ($\alpha $-Fe), gamma iron ($\gamma $-Fe), and delta iron ($\delta $-Fe). At extremely high pressures, a fourth form known as epsilon iron ($\varepsilon $-Fe) occurs. According to some contradictory experimental data, there is a fifth high-pressure form that is stable at extremely high pressures and temperatures. Arrest Points are the temperatures at which the shifts occur. Ferrite, or Delta Iron, is formed when iron cools from 1600°C and has a BCC crystal lattice structure. At 1390°C, Iron transforms to an FCC lattice, which is the first of these arrest points. Austenite, or Gamma Iron, is the name given to it currently. The volume of the Iron is reduced somewhat because the FCC Unit Cell is more closely packed. At 910°C, another arrest point occurs, and Iron reverts to a BCC lattice. Both compounds A and B will solidify when the temperature reaches point c, and the liquid phase's composition will stay unchanged. As a result, the temperature will cease fluctuating, resulting in a phenomenon known as the eutectic standstill.
Hence option A is correct.

Note :
Because of variations in carbon solubility, the phases of iron at atmospheric pressure are significant in creating distinct forms of steel. Iron's high-pressure phases are useful models for the solid sections of planetary cores. The Earth's inner core is thought to be mostly made up of a crystalline iron-nickel alloy with $\varepsilon $structure. The liquid iron outer core, which surrounds the solid inner core, is thought to be made up of nickel and trace quantities of lighter metals.