Heat of transition is the heat evolved or absorbed when a substance is converted from

one allotropic form to another allotropic form

The correct answer is option 4. one allotropic form to another allotropic form.

Let us delve deeper into the concept of "heat of transition" and understand its various aspects.

Heat of Transition

The term "heat of transition" refers to the heat (enthalpy change, \(\Delta H\)) absorbed or evolved when a substance undergoes a phase change at constant temperature. Phase changes involve transitions between different states of matter, such as solid, liquid, and gas, or between different allotropes (different forms of the same element).

Types of Phase Transitions and Corresponding Heats of Transition

1. Solid to Liquid (Melting or Fusion):

When a solid substance melts and transitions into a liquid state, heat is absorbed. This heat absorbed is known as the heat of fusion or heat of melting (\(\Delta H_{\text{fusion}}\)). Example: Ice melting into water. The heat absorbed during this process is used to break the intermolecular forces holding the solid lattice together.

2. Solid to Vapor (Sublimation):

Sublimation occurs when a solid substance transitions directly into vapor (gas) phase without passing through the liquid phase. The heat absorbed during this process is called the heat of sublimation (\(\Delta H_{\text{sublimation}}\)).

Example: Dry ice (solid \( CO_2 \)) subliming into \( CO_2 \) gas. The solid \( CO_2 \) absorbs heat to directly convert into gas phase.

3. Liquid to Vapor (Vaporization or Boiling):

Vaporization is the process where a liquid substance changes into vapor (gas) phase at its boiling point. The heat absorbed during this process is called the heat of vaporization (\(\Delta H_{\text{vaporization}}\)).

Example: Water boiling into steam. The liquid water absorbs heat to overcome intermolecular forces and transform into water vapor.

4. One Allotropic Form to Another (Allotropic Transitions):

Some elements exist in multiple forms known as allotropes under different conditions (e.g., temperature and pressure). Allotropic transitions involve a transformation between different allotropes of the same substance. The heat absorbed or evolved during such transitions is also referred to as a heat of transition.

Example: Graphite (a form of carbon) transforming into diamond (another form of carbon). This transition involves rearrangement of carbon atoms into a different crystal structure, accompanied by heat changes.

Importance and Application

Thermodynamic Significance: Heat of transition is crucial in thermodynamics as it reflects the energy required or released during phase changes or allotrope transitions.

Measurement and Calculation: These heats of transition can be measured experimentally using calorimetry or calculated using Hess's law by combining known heats of related reactions.

Industrial and Practical Applications: Understanding heat of transition is essential in various industries such as pharmaceuticals (for crystallization processes), food processing (freezing and thawing), and material sciences (for studying phase behavior).

Conclusion

The term "heat of transition" encompasses the heat absorbed or released during phase changes or allotrope transitions of substances. Each type of transition (solid to liquid, solid to vapor, liquid to vapor, and allotrope transitions) has a specific heat of transition associated with it, reflecting the energy changes involved in these processes. Understanding these concepts helps in predicting and controlling physical transformations of substances under different conditions.

Therefore, the correct and comprehensive explanation aligns with option (4) One allotropic form to another allotropic form.

This option correctly describes heat of transition as encompassing the transformation between different allotropes of a substance, in addition to phase changes.