In basic physical chemistry, crystallisation of a dissolved solid from its solution is an example of which type of change?

Introduction / Context:
Crystallisation is a common purification technique in chemistry and also a natural process in geology and everyday life. When a solute crystallises from a solution, it separates out as a solid with an ordered crystal lattice. Determining whether this is a physical or chemical change is a standard question in introductory chemistry, helping learners distinguish between processes that form new substances and those that simply change physical state or arrangement.

Given Data / Assumptions:
- The process described is crystallisation of a dissolved solid from its solution, such as salt crystallising from seawater.
- The solute before and after crystallisation is chemically the same substance.
- No new chemical species are formed during simple crystallisation under normal conditions.
- The options include physical and chemical changes and unrelated processes like galvanisation and nuclear change.

Concept / Approach:
A physical change involves a change in physical state or form without altering the chemical composition of the substance. In crystallisation, the solute molecules or ions come together to form a solid crystal from a liquid solution, but their chemical identity remains unchanged. For example, dissolved sodium chloride and crystalline sodium chloride are both NaCl. Chemical changes, by contrast, involve breaking and forming chemical bonds to produce new substances. Since crystallisation does not produce new substances, it is classified as a physical change.

Step-by-Step Solution:
Step 1: Consider a typical example, such as common salt dissolving in water and then crystallising when the solution is evaporated or cooled. Step 2: Note that the substance in solution and the substance in the crystal are chemically identical; both are sodium chloride. Step 3: Recognise that the process involves molecules or ions simply arranging themselves into an ordered solid lattice, not changing into a different compound. Step 4: Recall the definition of a physical change as one where the composition remains the same but the state or arrangement changes. Step 5: Conclude that crystallisation is an example of a physical change.

Verification / Alternative check:
If crystallisation were a chemical change, the original solute could not be recovered by simply redissolving the crystals in the solvent. In practice, crystals formed by crystallisation can be dissolved again to reproduce the original solution, showing that the substance is unchanged. This reversibility under suitable conditions is characteristic of physical changes. Textbooks that discuss separation techniques such as evaporation, distillation, filtration and crystallisation group them as physical methods that do not alter chemical identity, reinforcing this classification.

Why Other Options Are Wrong:
Chemical change implies the formation of new substances with different properties, such as when iron rusts to form iron oxide. Crystallisation does not involve such transformations. Chemical reaction with the solvent would produce new compounds, but simple crystallisation is just removal of solvent and ordering of the solute. Galvanisation is a specific process of coating iron with zinc and has nothing to do with crystallisation. Nuclear change involves alterations in the nucleus and is far removed from physical processes like crystallisation. None of these descriptions fit the basic crystallisation process.

Common Pitfalls:
Students may mistake the formation of crystals, which looks dramatic and permanent, for a chemical change, especially if they associate change of appearance with chemical reactions. Another pitfall is to overlook the fact that the same solute can be recovered unchanged by redissolving the crystals. Keeping in mind that no new substance is formed and that the process is reversible under suitable conditions helps classify crystallisation correctly as a physical change.

Final Answer:
Crystallisation of a dissolved solid from its solution is an example of a Physical change.