Question

1. What can you conclude if the melting point of the salicylic acid you just synthesized is 152-155

Answer 1

1- if you mix your synthesised salicylic acid with a synthetic (pure) one and you still have the same melting point, this means your synthesised compound is pure. Normally if it's impure you would have a different melting point.

2- your reaction was carried out in water and all reactants were soluble in hot water. Your product was separated as a precipitate from the reaction mixture after careful acidification (which has to be under cold condition as reaction of NaOH and strong acid is exothermic). You only need to filter out your precipitate, which should be pure after washing with water ( to dissolve any remains of reactants). Organic/ aqueous separation is not needed as it's an extra step that would require unnecessary consumption of organic solvents. Also it any acetylsalicylic acid remain unreacted will go to the organic solvent, so your product may not be pure.

3- we rinse it with water to remove any reactants remaining with salicylic acid. So you simply wash out all reactants to get pure salicylic acid. We use ice cold water because salicylic acid has some degree of solubility in water at room temp, so you may lose some of your product on the process of washing.

4- In the lab when we recrystallise a compound, we dissolve it in the minimum amount of the solvent and boil till completely dissolve, then leave to cool to obtain pure crystals that precipitate in a pure form even if it have traces of any impurities. The impurities should remain in the solvent. Don't use excessive solvent or else your compound will not precipitate. Don't dissolve your compound completely at room temperature or it will not recrystallise. So basically, less solvent is needed to dissolve your compound when boiling in comparison with room temperature. The amount of solvent you use must not dissolve your compound at room tem.