In thermodynamics, supercooling refers to the cooling of a liquid below its normal freezing point without it actually solidifying.

Introduction / Context:
Supercooling is an important concept in thermodynamics and phase change physics. It describes a situation in which a liquid is cooled below its normal freezing point but does not immediately change into a solid. Understanding supercooling helps students connect ideas about freezing, melting, and the role of impurities and disturbances in starting crystallisation. This question checks whether you can distinguish between simple cooling, freezing at the normal freezing point, and the special phenomenon of cooling below that point without solidification.

Given Data / Assumptions:

• A substance is in the liquid state.
• The liquid is cooled gradually.
• The focus is on the temperature region around its freezing or melting point.
• No external disturbance or impurity is assumed that would trigger immediate crystallisation.

Concept / Approach:
Every pure substance has a characteristic freezing point at a given pressure. At this temperature, the liquid normally begins to change into a solid. However, if the liquid is very pure and undisturbed, it can be cooled below this freezing point and still remain liquid. This unusual state is called supercooled. The key idea is that freezing requires not only low temperature but also nucleation sites where crystals can begin to form. Without such nucleation, the liquid can exist in a metastable state below its usual freezing point.

Step-by-Step Solution:
Step 1: Recall that the freezing point is the temperature at which a liquid normally starts to solidify at a given pressure. Step 2: Supercooling describes a liquid that has been cooled below this freezing point but has not yet formed a solid phase. Step 3: The phrase that correctly captures this behaviour is “cooling a liquid below its freezing point without it solidifying”. Step 4: Other descriptions that involve melting point or immediate solidification do not match the precise definition of supercooling.

Verification / Alternative check:
A common classroom demonstration of supercooling is with very pure water. Under ideal conditions, water can be cooled below 0 degree Celsius while still remaining liquid. When the supercooled water is disturbed or a crystal seed is introduced, it suddenly freezes. This confirms that supercooling refers to the liquid state persisting below the freezing point and not simply cooling to or slightly around that temperature.

Why Other Options Are Wrong:

• Cooling a liquid below its melting point and immediately solidifying it: This describes normal freezing, not supercooling, because the liquid does not remain liquid below the freezing point.
• Cooling a liquid exactly at its melting or freezing point: This only refers to the phase change temperature itself, not to temperatures below it while staying liquid.
• Heating a solid above its melting point until it becomes a liquid: This describes melting, the opposite phase change, and is unrelated to supercooling.

Common Pitfalls:
Students often confuse melting point and freezing point, since for a pure substance at standard pressure they have the same numerical value. Another mistake is thinking that as soon as the liquid goes below the freezing point it must instantly become solid. In reality, the formation of a solid phase requires nucleation and growth, and without these, supercooling can occur. It is also important not to mix supercooling with superheating, which refers to heating a liquid above its boiling point without boiling.

Final Answer:
The correct description is cooling a liquid below its freezing point without it solidifying.