Horizontal arrest in a cooling curve represents an invariant reaction.

Explanation:
When a material undergoes a phase change, such as solidification or crystallization, its cooling curve shows a characteristic pattern. This cooling curve represents the relationship between temperature and time during the cooling process.

Cooling Curve:
A cooling curve is a graphical representation of the temperature changes that occur as a material cools from a higher temperature to a lower temperature. It typically consists of a downward-sloping line that represents the cooling process.

Horizontal Arrest:
Horizontal arrest refers to a specific region in the cooling curve where the temperature remains constant for a period of time. This region is characterized by a flat, horizontal line on the cooling curve.

Invariant Reaction:
An invariant reaction is a phase change that occurs at a constant temperature and pressure. It is a reaction in which the composition and properties of the material do not change during the process. In other words, the material undergoes a phase change without any further cooling or heating.

Horizontal Arrest Represents Invariant Reaction:
The presence of a horizontal arrest in a cooling curve indicates that an invariant reaction is occurring. During this phase, the material is undergoing a phase change, such as solidification or crystallization, which occurs at a constant temperature.

Characteristics of Horizontal Arrest:
1. Constant Temperature: The temperature remains constant during the horizontal arrest phase.
2. No Heat Exchange: There is no heat exchange between the material and its surroundings during the horizontal arrest phase.
3. Phase Change: The material is undergoing a phase change, such as the formation of solid crystals or the transformation of a liquid to a solid.
4. Stable Composition: The composition and properties of the material remain constant during the horizontal arrest phase.

Conclusion:
In summary, horizontal arrest in a cooling curve represents an invariant reaction, which is a phase change occurring at a constant temperature. It is characterized by a flat, horizontal line on the cooling curve, indicating a stable composition and no heat exchange during the phase change.