Recrystallization and Melting Point Analysis

Learn that recrystallization purifies solids by dissolving impure product in hot solvent, then cooling to selectively precipitate pure crystals while impurities remain dissolved. Gather two beakers, hot plate, stir bar, stirring rod, vacuum filtration setup, powder funnel, watch glass, and disposable pipettes.

Identify distilled water as the recrystallization solvent by comparing solubility data showing benzoic acid is highly soluble at boiling temperature (56.31 g/L) but poorly soluble at room temperature (3.44 g/L). This ensures product dissolves when heated and precipitates when cooled.

Heat 150 ml of distilled water in a 250 ml beaker, then weigh 3 g of impure benzoic acid into a 150 ml beaker. Slowly add hot water using a disposable pipette, stirring with a stir bar on the hot plate until the solid completely dissolves, using approximately 100 ml total.

While the solution is still hot, remove the stirring rod and stir bar, then immediately perform vacuum or gravity filtration to remove any insoluble impurities before the solution cools. This prevents filtering out desired product crystals.

Cover the clear hot solution with a watch glass and allow it to cool slowly at room temperature without disturbance. Observe crystals forming as the solution cools, then place the beaker in an ice bath or refrigerator for 5 minutes to ensure complete precipitation of pure product.

Pour the cooled solution with crystals into a Büchner funnel using vacuum filtration, collecting the solid product. Wash the crystals with pure solvent to remove impurities from the original solution, then run the vacuum for several minutes to dry the crystals.

Insert a sample of dried crystals into a melting point tube, place it in a melting point apparatus, and heat slowly. Record the temperature when melting begins and when it completes to determine the melting point range, comparing it to the original impure sample and literature values to verify purity.

Weigh the purified crystals and divide by the original mass multiplied by 100 to calculate percent recovery, which will be less than 100% due to solubility losses. Address crystallization issues by scraping the beaker bottom with a glass rod or adding seed crystals to promote growth.