Ethanedioic acid is an organic solid that can be purified by

Step 1: Analyze the question Ethanedioic acid (oxalic acid, HOOC-COOH) is a solid organic compound. We need the method used to purify a solid that is soluble in a solvent such as water.

Step 2: Review the concept Purification methods depend on the type of mixture:

• Decantation: separates an insoluble solid that has settled from a liquid. It does not purify a dissolved solid.
• Distillation: separates liquids by differences in boiling point, or recovers a solvent from a solution.
• Filtration: separates an insoluble solid from a liquid. It removes insoluble impurities but does not purify a soluble solid.
• Crystallization: purifies a soluble solid. The impure solid is dissolved in a hot solvent, the solution is filtered to remove insoluble impurities, then cooled. Pure crystals form while the soluble impurities stay in solution.

Step 3: Apply to ethanedioic acid Ethanedioic acid is a solid that dissolves in water, and its solubility rises sharply with temperature. This is exactly what recrystallization needs. Dissolve the impure acid in hot water, filter, cool, and pure crystals of hydrated ethanedioic acid (H₂C₂O₄·2H₂O) separate out.

Decantation and filtration only separate insoluble solids from liquids, and distillation is for liquids, so none of these purifies the solid itself.

Step 4: Answer The correct option is C. crystallization.

Tip: Remember "Solid + soluble = crystallize." For purifying a soluble solid, think of crystallization. For an insoluble solid in a liquid, think of filtration or decantation. For separating liquids, think of distillation.