The process in which a liquid on cooling turns into a solid with the formation of crystals is called its ___________.

Introduction / Context:
This question belongs to basic physical chemistry and changes of state. It focuses on the process by which a liquid becomes a solid, forming an ordered crystalline structure. Understanding terms such as melting, freezing, crystallisation, evaporation and condensation is important for describing everyday phenomena like formation of sugar crystals, freezing of water, and purification of substances by crystallisation.

Given Data / Assumptions:

• The process considered is a liquid turning into a solid on cooling.
• Crystals of the solid are formed during this process.
• We need to select the correct scientific term that describes this change.
• Options include crystallisation, melting point, evaporation, sublimation and condensation.

Concept / Approach:
Crystallisation is the process by which a substance forms a crystalline solid from a liquid or solution. When a hot saturated solution of a solute is cooled, the solubility decreases and excess solute separates as pure crystals. Similarly, when a pure liquid solidifies in an ordered arrangement, this process can also be described as crystallisation. The term melting point is a temperature, not a process. Evaporation is conversion of a liquid to gas, sublimation is direct conversion of solid to gas and condensation is gas to liquid. None of these fit the description of liquid to crystalline solid on cooling.

Step-by-Step Solution:
Step 1: Identify the direction of change in state: the question clearly describes a liquid becoming a solid when cooled. Step 2: Recall that the general term for liquid to solid change is freezing or solidification, and when the solid forms an ordered structure, it is often called crystallisation. Step 3: Crystallisation is specifically used when pure crystals form from a solution or melt, which matches the description of crystals forming as the liquid cools. Step 4: Examine the option melting point. Melting point is the temperature at which a solid becomes a liquid; it is a property, not a process in the reverse direction. Step 5: Evaporation is the process of a liquid changing into vapour, which is not involved here. Step 6: Sublimation is the change from solid directly to vapour, skipping the liquid state; again, this does not match the given description. Step 7: Condensation is gas to liquid, also unrelated to liquid forming a solid. Step 8: Therefore, the only option that correctly names the process is crystallisation.

Verification / Alternative check:
In laboratory practice, crystallisation is a common method of purification. A hot saturated solution of a solid like copper sulphate is cooled, and as it cools, blue crystals separate out. This is exactly a case of a liquid solution forming a solid crystalline phase. The word crystallisation is used consistently in school chemistry for this process, confirming that it is the correct term when a liquid or solution yields a crystalline solid upon cooling.

Why Other Options Are Wrong:
Melting point is a temperature value where solid becomes liquid; it is not a process in which liquid becomes solid. Evaporation describes liquid to gas, sublimation describes solid to gas and condensation describes gas to liquid. Since the question clearly states that a liquid turns into a solid, these three options cannot be correct. They involve different pairs of states and different directions of change.

Common Pitfalls:
Students sometimes confuse melting and freezing terminology because they are reverse processes. It is important to note that melting point and freezing point are often the same numerical temperature, but the process names depend on the direction of change. Another pitfall is to think of crystallisation only as something that happens in industry, forgetting that it is the basic scientific term for crystal formation when a liquid or solution cools.

Final Answer:
The process in which a liquid on cooling turns into a solid with the formation of crystals is called crystallisation.