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Recrystallisation of benzoic acid Questions and Answers

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Recrystallisation of benzoic acid Questions and Answers Define Recrystallisation Process of repeated crystallisation in order to purify a solid or to obtain more satisfactory crystals of a solid that is already pure. (You start with a crystalline solid, remove the impurities, and then make new...

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  • September 30, 2024
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Recrystallisation of benzoic acid
Questions and Answers
Define Recrystallisation - answer Process of repeated crystallisation in order to purify
a solid or to obtain more satisfactory crystals of a solid that is already pure.

(You start with a crystalline solid, remove the impurities, and then make new crystals.)

Recrystallisation procedure. - answer 1. Dissolve in a minimum amount of hot water
2. Hot filtration
3. Cool - recrystallisation - ice bath
4. Cold filtration
5. Dry

Important point about the solubility of benzoic acid - answer Benzoic acid has a
*high* solubility in hot water and a *low* solubility in cold water.

Therefore, when a saturated solution of benzoic acid is cooled, crystals are formed.

Purpose of dissolving in a minimum of hot water - answer 1. When the benzoic acid
is dissolved in the minimum amount of hot water, then that means that the hot water
contains the maximum amount of benzoic acid.
2. This minimises the amount of benzoic acid that remains dissolved in the cold water
(benzoic acid is slightly soluble in cold water).

Benzoic acid has a *high* solubility in hot water and a *low* solubility in cold water.

Purpose of hot filtration - answer To remove insoluble impurities.
−When the benzoic acid is dissolved, the impurities that are insoluble in water
precipitate out while the pure benzoic acid and the soluble impurities become dissolved
in the water.

Hot filtration:
1. Why does it have to be hot?
2. Why do we use fluted filter paper? - answer 1. To prevent it recrystallising on the
filter paper.
2. It allows the filtration to occur faster.

Purpose of the cooling - answer To cause the benzoic acid to recrystallise

How are the soluble impurities removed? - answer When the benzoic acid
recrystallises, the soluble impurities remain in solution because they are much less
concentrated than the benzoic acid solution.

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