It is a physical change used to obtain pure substances.
Explanation
Crystallisation is a separation technique used to obtain a pure solid substance from a solution. It is based on the difference in solubilities of the compound and the impurities in a suitable solvent.
- It is a chemical change resulting in new compounds. is incorrect: Crystallisation is a physical change, not a chemical change. During this process, the chemical composition of the substance remains unaltered; only its physical state changes from dissolved (or liquid) to solid. No new chemical compounds are formed.
- It is a physical change used to obtain pure substances. is correct: It is a physical process used to purify substances. For example, salt obtained from sea water contains many impurities; crystallisation is used to remove these impurities and obtain pure salt crystals. It is often preferred over evaporation because evaporation may degrade heat-sensitive substances or leave impurities mixed with the solid.
- It is an irreversible thermodynamic process. is incorrect: Crystallisation is generally a reversible process. The crystals formed can be re-dissolved in the solvent to form a solution again.
- It is a biological mechanism for mineral deposition. is incorrect: While biological systems can form crystals (biomineralization), crystallisation is fundamentally a physicochemical process governed by thermodynamic principles, not exclusively a biological mechanism.
Key Takeaway: Crystallisation is a physical purification method used to separate pure solids from a solution, ensuring better purity than simple evaporation.