Crystallization

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Best method to obtain pure crystals from an impure solid soluble in hot water and less soluble in cold water is:

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1. **Recrystallization Principle**: Based on the difference in solubilities of the compound and its impurities in a suitable solvent at different temperatures.
2. **The Solvent**: A solvent is chosen such that the substance is highly soluble in it at **high temperatures** and sparingly soluble at **room temperature**.
3. **Process**: 
   - Dissolve the impure solid in the minimum amount of hot solvent.
   - Filter to remove insoluble impurities while hot.
   - Allow the filtrate to cool slowly.
4. **Crystallization**: Pure crystals grow while the concentrated solution cools. The soluble impurities remain in the 'mother liquor'.
5. **Efficiency**: This is the most common technique for purifying solids.

Crystallization

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